Quarterly Journal of the Geological Society of London/Volume 33/On the Serpentine and associated Rocks of the Lizard District
1. Literature of the subject and Introductory matter.
2. General description of the west Coast.
3. General description of the east Coast.
(a) The coast south of Coverack Cove.
(b) The Igneous Rocks of Coverack Cove.
4. Some inland sections.
5. Summary of inferences.
6. Microscopic examination of the Serpentine.
8. Analyses of the rocks.
Literature of the Subject.
The following papers occur in the 'Transactions of the Royal Geological Society of Cornwall:'—
"Sketch of the Geology of the Lizard District," by A. Majendie, i. p. 32.
"Hints on the Geology of Cornwall," by Sir H. Davy, i. p. 38.
"On the Serpentine District of Cornwall," by the Rev. J. Rogers, ii. p. 416.
"Contributions towards a Knowledge of the Geology of Cornwall," by H. S. Boase, iv. p. 160.
The subject is also treated in the following works:—
"On the Physical Structure of the Lizard District," by Professor Sedgwick (Trans. Cambridge Phil. Soc. i. p. 291).
"Report on the Geology of Cornwall and Devon," by H. T. De la Beche (published 1839).
"On the Serpentinite of the Lizard," by Profs. W. King and T. H. Rowney (Phil. Mag. ser. 5, vol. i. p. 280).
In the last of these papers the authors maintain the metamorphic character of the serpentine, but consider it probably an altered pyroxenic rock, like that of Bufaure in the Fassa-Thal. The paper appears to have been mainly written with the view of calling attention to imitative organic, especially Foraminiferal structures. So far as my experience goes, any thing of this kind is very rare.
The great work of Sir H. De la Beche, and the shorter paper of Professor Sedgwick, are models of careful observation; and several things mentioned hereafter have been already noticed by them. The former author strongly inclines to the belief that the serpentine is of igneous origin, though he admits some difficulties. The latter is of the same opinion, but suggests that the gabbro, greenstone (including some of what we now should call hornblende schist), and serpentine may be portions of the same igneous mass, which varied in mineral composition. As neither of these works is now very readily accessible, and I wish to make my paper as complete as possible, I have occasionally described phenomena noticed by them, although it has added slightly to the length of the communication. The microscope has, I hope, enabled me to explain several of those phenomena with which the appliances at their command could not deal. Where I differ from them, as I do occasionally, it is with diffidence, and only because, after careful consideration, I am unable to adopt any other conclusion. The remaining works are generally too vague in their terminology to be of much use at the present day.
For the general features of the Lizard peninsula I must refer, for the sake of brevity, to Sir H. De la Beche's admirable memoir; suffice it to say here that the district of which I more particularly treat may be described as a plateau, partly cultivated, partly wild moorland, which, as a rule, descends precipitously to the sea, except where it is furrowed by small coves and gullies. The cliffs often rise vertically from 100 to 200 feet above the sea; of beach beyond the high-tide mark there is but little, the base of the cliffs being then often washed by the waves for hundreds of yards together. In not a few places the cliffs are totally impracticable from above, and could only be examined (without a hope of landing) from a boat below in the calmest weather. This, of course, adds greatly to the difficulty of investigating the district; for, even at low water, progress at the base of the cliffs, where possible, is often laborious, and the state of the tide has to be carefully watched.
The district described in this paper forms the southern part of the Lizard peninsula. I have examined the coast of this from Lizard Head to Mullion Cove on the west side, and from the same to Manacle Point on the east, as carefully as circumstances admitted, and have also traversed the interior in two or three directions.
The following rocks, the positions of which may be seen on the geological map, occur in the above district:—(1) hornblende schist; (2) serpentine; (3) gabbro; (4) granite (restricted, as will be shown, to the west coast); (5) greenstone, i. e. dark augitic or hornblendic traps, described hereafter in detail, restricted, so far as I know, to the east coast.
The most convenient arrangement will be to commence with a few remarks on the hornblende schist, next to describe the petrology and stratigraphy of the two coast-sections in detail, and then the inland sections, reserving to the end all details of the microscopic structure of the serpentine.
The Hornblende Schist.—Under this title are included an extensive group of rocks which I do not profess to have minutely investigated, as I was chiefly occupied with the rocks of presumed igneous origin. The following description, however, will be fairly accurate:—This rock varies in the hand specimen from a black, schistose, but not very fissile rock, which appears to consist mainly of hornblende, to a dark greyish granitoid rock, in which the hornblende is not conspicuous, and the prevailing minerals are quartz and felspar in variable quantities, so that the rock sometimes might almost be called a quartzite; at others it resembles a vein granite. The more schistose hornblendic varieties, on a closer examination, show, generally, fine white specks of felspar. Now and then we find porphyritic varieties, sometimes with felspar crystals about 1 inch long, sometimes with hornblende about the same size; occasionally also the rock appears to contain a talcose or chloritic mineral in large quantities, especially when it shows signs of decomposition. I have examined microscopically three varieties of this rock—one (1) a typical specimen of the prevalent black hornblende schist, another (2) a dark grey granitoid variety, and the third (3) a greyish quartzo-felspathic rock, very difficult to distinguish from true vein granite.
1. About two thirds or more of the field is occupied by crystalline grains of hornblende of rather irregular outline, with very characteristic cleavage, of a green colour, and strongly dichroic. There are many small rounded or vermicular grains of a black mineral, probably magnetite, often included in the hornblende, occasional small rounded or subangular grains of clear quartz, and a few small acicular hexagonal crystals, probably apatite. The rest of the field is occupied by a kaolinized or altered felspathic mineral, which seems, in parts, as if it had never been perfectly crystallized.
2. Taken from a junction with a gabbro vein, and shows traces of foliation parallel to the common surface. Exhibits an imperfectly crystallized and decomposed groundmass, as in the last, though much more abundant, as well as numerous fairly defined felspar crystals, generally plagioclase; quartz rare; magnetite less abundant than in the last, as is, of course, the hornblende; the crystals of this are also more irregular, with less-distinct cleavage-planes, often somewhat acicular, platy or fibrous. There are microliths—some, probably, apatite; others, shorter and strongly dichroic, may be tourmaline.
3. A rarer variety, closely resembling a vein granite, being highly crystalline, and consisting of quartz, felspar, and a little mica. A microscopic description of one of these will be given further on.
In general the hornblende schist, except in the darkest and most compact varieties, shows distinct signs of stratification; sometimes thin felspathic (or quartzose) and hornblendic bands alternate, occasionally exhibiting current-bedding; and sometimes quartzo-felspathic strata, from an inch to a few feet thick, alternate with more hornblendic, chloritic, or earthy layers. Epidote is occasionally present in minute quantities; and veins of quartz, felspar, and ferruginous matter occur. Foliation seems generally almost, or quite, parallel with the stratification; there are many extremely beautiful small contortions. The rock is rather sharply jointed, and weathers into bold headlands, dark in the wash of the waves, grey and lichen-covered where exposed to the blasts.
At the south-west corner of the Lizard peninsula is a mass of talco-micaceous shales, described by De la Beche (p. 29), and separated in his map. As far, however, as I can make out, they only form a zone with some slight lithological peculiarities in the hornblendic schist, into which they seem to pass almost insensibly. These may be well studied in the descent to the shore at Polpeor, and in the base of the cliffs there.
The Western Coast.
Following the cliffs from Polpeor for about a mile in a straight line, we come to the first junction with the serpentine, at the south end of the narrow strip of sand called Pentreath Beach; a little chine runs almost along the line of junction. To make out the relations of the two rocks here is no easy task; both are extremely decomposed for some yards, and traversed by numerous cracks, which are filled with calcite, often stained red with hæmatite, and project like a network from the weathered ground. The rocks are thus almost brecciated in situ. The difficulty is caused by the close resemblance of the two rocks in their extremest decomposition, so that it is sometimes almost impossible to separate them. After two or three visits and a most minute examination, I think I have succeeded. The cliff on the north bank of the little chine is all serpentine; on the south the headland is all hornblende schist; but after a few yards the serpentine rises from the shore and forms the lower part of the cliff, its boundary curving gradually upwards. Thus there is not really a passage from the one rock to the other here, or a faulted junction, but the serpentine is intrusive. The serpentine then forms the cliffs as far as they can be followed; above them, near the upper end of the beach, and some 50 feet above it, a granite vein breaks in two or three places through the serpentine, which is cracked and altered at the junction; the granite is finely crystalline, chiefly composed of quartz and felspar, with only a little mica; it is of a pinkish grey colour, becoming red and friable in weathering. One mass has carried up some irregular fragments of hornblendic schist. Close by is another mass of hornblendic schist, included in the serpentine, possibly forced into it by the granite; and another included mass may be seen on the beach below. On the hillside, a short distance beyond, is a serpentine-quarry. Two of the varieties of the rock obtained here are very pretty (no. 1):—one, of a dull red colour, irregularly mottled with a waxy-looking dull green mineral, with occasional flakes of greenish bronzite, rather hard and irregular in fracture; the other, a dull purplish red, veined with greyish green, the latter generally fringing a thin dark line, like a crack, and forming a sort of polyhedral network. It is probably only the result of decomposition, but produces a very pretty effect. At the north-east angle is a bifurcating granite vein, about a foot wide, colour and texture much as before; the serpentine adjoining is much baked. In a small excavation on the east side of the quarry, a piece of hornblende schist may be seen included in the serpentine.
Passing on northwards we find a granite vein exposed on the north side of the little cove just before reaching Kynance, and another at the zigzag of the road descending to that cove. Serpentine forms the cliffs on the mainland, the large island, the Steeple rock, and some of the smaller skerries; it is generally of a dull red, mottled with dark green, which often coats the joints (no. 2). Much of it shows the sharp but rather irregular jointing so common in the Lizard serpentine; one of the nearer skerries, however, exhibits a very distinct jointing in a series of parallel curves, such as we not seldom see in igneous rocks. But three or four of the reefs that rise from the sandy spit joining the island to the mainland are highly crystalline hornblende schist; and about the same number are little bosses of vein granite: two small bosses of this also lie just opposite to the opening of the cove. Of this there is a larger boss in the middle of the serpentine, above the "Drawing-room " cave, and another on Asparagus Island, just beyond and above the "Post Office;" these two bake and crack the adjoining serpentine, and resemble that already described.
By scrambling over the boulders along the beach to the north of the Steeple rock we come (in about 100 yards) to a most interesting and difficult section. At first sight it seems rather to confirm the idea of a passage from hornblende schist into serpentine, the two rocks being apparently interstratified, almost vertically, in a low terrace-like step at the foot of the cliff which is of serpentine. As we face this, we have on the south (1) serpentine, (2) a mass of grey, rather sandy "hornblende schist," about 8 feet thick, with apparently many thin laminæ of red serpentine, (3) red serpentine (no. 3), rather fissile in structure, two and a half feet, (4) a dark brownish grey rock with crystals rather resembling diallage, two feet, (5) red serpentine, four and a half feet, divided by a thin band of the schist (2), then (6) bedded schist like (2), with the apparent layers of serpentine, for about six feet. Here a branching granite vein breaks very irregularly through the schist on the top of the terrace, and shows again in three places in the shore just at the foot. When we examine carefully the back of this terrace, we see that all this schistose mass is really included in the main mass of serpentine. A great fragment of schist has been caught up by that rock when molten; some of the beds composing it have been forced asunder and parted by tongues of serpentine. The apparent interstratification of schist and serpentine on a smaller scale is due to the fact that a serpentinous mineral has been deposited by infiltration in the schist (as is commonly the case near a junction); and, further, the staining of certain layers by red peroxide of iron makes them simulate a rather decomposed serpentine. In two or three cases these may be the ends of thin tongues of intruded serpentine; but inmost the red streak is certainly not true serpentine. Finally, a granite vein has cut irregularly through all these rocks.
The rock (4) is not easy to determine; its texture is coarser on the face of the crag than it is along the outcrop a few feet back; I have had slides prepared from each part. The finer variety consists of an interwoven mass of rather acicular crystals of hornblende (actinolite), in a clear base, which, with crossed prisms, either remains dark or exhibits an obscure microcrystalline structure. This in parts seems to resemble steatite; in others it is more like one of the pseudomorphic products after felspar, which will be noticed below. Granules of magnetite are scattered about; and there are some very irregular grains or plates of a brown hornblende, full in many parts of a black dust. This mineral appears, from its cleavage, to have a platy structure. In the coarser specimen there appears still more actinolite, and the brown hornblende grains are much larger; they appear in some cases to have been broken or partly destroyed after they had been formed. The mineral to the eye has a clove-brown colour; it much resembles anthophyllite, but is certainly not an orthorhombic mineral; probably it is the variety of hornblende noticed by Rosenbusch (Mikr. Phys. p. 264). I believe that this rock was originally a hornblende schist, that its entanglement with the intrusive rock affected it to some extent, and that since then it has undergone further changes, chiefly by the action of water.
Nearly above this place, at the top of the cliff (the Rill), is a landslip, or a deserted quarry, of considerable size. Here are two granite veins in the serpentine—one about 4 feet wide, forming a curved dyke running up the face of the higher cliff, the other showing in it a short distance towards the south. Both are much decomposed and of a red colour; they crack the serpentine in contact considerably; and in places it is so much altered that for a few inches it might be taken for a talcose schist. We have thus an irregular line of veins and small bosses of granite extending pretty continuously over more than half a mile. The serpentine forming the cliff just mentioned is compact in texture, of a dull purplish colour; the bronzite crystals are few and small (no. 4). When examined carefully the rock shows a sort of parallel streaky structure, indicated by darker lines; the faces of the joints are coated by films of green or whitish steatite; and old surfaces weather dark rusty brown. The parallel structure becomes much developed by the weathering, and might easily induce the supposition that the rock was really stratified. It is particularly well exhibited along the down to the north of Kynance Cove, though it may also be observed to the south of it, as in other places. In the coarser varieties the structure appears to be formed by bronzite crystals, which have resisted weathering better than the matrix in which they are imbedded.
We pass now along the edge of the cliffs northward; these are not seldom so precipitous as to make a close examination impossible. The serpentine varies in character, being sometimes dull and compact, sometimes redder in colour, and containing larger bronzite crystals. The headland called the Horse is remarkable for the boldness of the jagged rocks that form its crest, which in form, colour, and even in aspect, recall memories of the gabbro of the Cuchullin Hills; the rock, however, is an ordinary serpentine.
Nearly a mile in a direct line north-west from Kynance is a cove called Gue Graze. A gully on the north side exhibits a granite vein; it resembles that already seen. On descending to the beach we find the serpentine is in places much brecciated and cemented by steatite, which is here abundant. Much of the serpentine on the beach is rather peculiar in appearance (no. 5), being of a dull red colour, with obscure dark greenish lines, a slightly rougher fracture, and (under the lens) more granular texture than common; it is also remarkable for hardness and the absence of conspicuous bronzite. In many places it is sand-polished. The joints are often coated, as usual, with films of green steatite.
Returning to the higher ground and passing Vellan Head, we continue to observe the apparently stratified structure in the serpentine, which sometimes even seems to mimic current-bedding. Near the Head its dip is fairly persistent, about 50° south; but this does not continue for long; I observed, however, that it was often parallel to one of the leading systems of joints; and hereabouts fairly defined tabular jointing is not unusual. The end of the serpentine is reached at a spot called Ugethawr, on one side of George's Cove, to the south of the place where a well-marked valley descends to the sea; thus the breadth of this practically uninterrupted tract of serpentine, measured in a straight line from sea to sea, is about 21 miles. I could not find the actual junction of the hornblende schist and serpentine on the rocky slope; but it is possible to scramble down to the water's edge, and there it can be discovered in a little sea-cave. It is in many respects an interesting one. The hornblende schist is rather compact and very dark; so also is the serpentine, especially near the junction—so compact, dark, and hard, indeed, and with so conchoidal a fracture, that at first sight it might readily be mistaken for Lydian stone; it overlies the hornblende schist, which here dips rather evenly about 60° W.N.W., and seems to be closely welded to it. One or two thin tongues of serpentine are thrust into the schist within a foot or two of the junction. The serpentine is therefore intrusive; but the junction is a remarkably clean one, and the close correspondence in colour adds to the difficulty of seeing it. The serpentine, in places, near the junction exhibits very regular, thin, parallel bands, of a greyish green mineral, and of very lustrous chrysotile, and is in all respects a remarkable variety (no. 6).
The serpentine is not again seen on the coast till the other side of the bold headland of Pradanack Point, rather more than a mile away in a straight line. The boundary line between it and the hornblende schist, according to the geological map, curves inland, the greatest distance from the sea being a good half mile. I have not traced it, but have examined the serpentine in a small pit, not far from the boundary, on the left bank of the valley mentioned above. Here the rock is rather decomposed and traversed by many joints, coated by a white steatitic film; in parts it shows some indications of a streaky structure; the colour is a dull earthy red to brown. The rock is full of minute scales, of a talcose aspect, which gives a glimmering lustre to the broken surfaces. Fracture rather uneven (no. 7).
The dark crags of hornblende schist which form the northern side of Pradanack headland are singularly grand, as are the cliffs of the next mass of serpentine. The junction here is difficult, to examine. A small gully seaming the face of the cliff marks it, as is often the case; but though we spent some time in careful search over the accessible parts, we could find no actual contact, though we traced the two up to within a yard of each other. This mass of serpentine is also dark; a specimen obtained a few yards from the gully is of a dull olive, approaching black (streaky structure clearly indicated by paler lines), rather unequal fracture, and lustreless surface; joints coated with paler greenish films. The hornblende schist is much broken and disturbed near the junction, and looks as if in the vicinity of an intrusive rock. The dip, in a glen a little south of the above spot, was 55° E.N.E.
The celebrated Mullion Cove is the end of a valley which very nearly defines the northern limit of this mass of serpentine; it is, however, cut everywhere through the hornblende schist. The actual junction is masked by debris; so this also is inconclusive.
The serpentine at the northern end varies: most of it is dark dull green to black; but in one place it assumes a redder tint. A common variety has a dark, almost black, matrix, full of small scales of glittering mineral, similar to, but darker than, those at Pradanack (no. 8). A fine sea-cave here well repays a visit.
A very short distance beyond the cove is another mass of serpentine, not marked in the map, dull in colour, streaky in structure, resembling that in the southern part of that which has just been mentioned. There is sometimes a little difficulty in tracing the exact junction of this and the hornblende schist; but there can be no doubt that the serpentine is intrusive; on the northern side, against the cliff, is an included fragment of the schist. Again, the headland to the west of the above is serpentine, with two included fragments of schist by the track which ascends it; the adjoining islands appear to be one serpentine, the other schist.
The hornblende schist on each side of the cove has a fairly regular strike, of about S. 23° W., running straight at the main mass of serpentine, and dips at a high angle (about 86°) on the western side; north of the above spot I could not find any more serpentine.
The Eastern Coast.
(a) The Coast to Coverack.—The sections on this coast are, on the whole, more complicated than those on the western. Commencing at the narrow cove of Perranvose near Landewednack, we find this cut down to the shore through hornblende schist; but by turning aside along a track before the steepest part of the descent, we are in a few minutes brought to a serpentine-quarry at the top of the cliffs above the sea.
By the side of the road leading into this, the junction of the serpentine with the hornblende schist is well seen. The former overlies the latter, following very nearly the plane of its bedding, which here dips at about 40°. The last foot or so of the serpentine is extremely rotten, crumbling into dust under the fingers; the hornblende slate is also rotten, and stained red; hence, as is often the case, the junction may be easily overlooked. Its nature, however, is clear; for careful search will detect two small tongues of serpentine a few yards distant in the hornblende slate—the larger about 2 feet in diameter. They are extremely rotten, but undoubtedly are serpentine.
The quarry furnishes some handsome varieties of this rock, generally a compact purplish or reddish brown groundmass, mottled and veined with pistachio green, in which are small groups of bronzite crystals of greenish hue and submetallic lustre, and sometimes veins of brightish red hæmatite (no. 9). Some masses are almost wholly of a grey-green colour; but this is only the result of decomposition; in some, minute crystals of magnetite are common. The lower part often exhibits a curious fissile structure, the cracks being filled by calcareous films. Sometimes the latter are about 1 inch thick and the mineral is aragonite.
By descending the "tip" of the quarry to the beach and walking to the end of a little headland, we have further proof of the intrusive character of the serpentine. The headland consists of hornblende schist, resting on serpentine which forms a little isthmus; the slope on the south of the "tip" shews hornblende schist with intrusive tongues of serpentine; then two or three more masses of hornblende schist crop out in the slope; and finally there is a small headland of that rock. On the other side, beyond the debris, is serpentine curving round to another little headland of the same rock.Rounding this, we enter a second little recess, and passing two fragments of schist included in the serpentine, come, on the opposite side, to a third of the most singular shape. Any one looking at the outline only, would take it for a dyke (see fig. 1). The bedding however, and mineral character of the rock are perfectly distinct. Other smaller masses, included in the serpentine, will be noticed on the shore, in some cases indicating by their peculiar jointing that they Fig. 1.—Irregular Strip of Hornblende Schist caught up by Serpentine.
A. Hornblende schist. (The parts shaded by fine lines are serpentine.)
Fig. 2.—Foliated Gabbro Vein in Hornblende Schist at the Balk.
A gabbro vein in the hornblende schist should be noticed; it is about half a yard thick, and in form a rude rhomboid (fig. 2). It exhibits a marked foliated structure, the felspar and the pyroxenic constituent being to a great extent separated, and the latter running in seams roughly parallel to the longer sides of the vein, except towards the top, where it tends to become parallel to the upper surface. This structure is more conspicuous near the lower surface. The felspar, at any rate on the exterior, is a dull yellowish white; the other constituent, as is very common here, is chiefly made up of minute crystals of hornblende, which, as will presently be explained, have almost entirely replaced the diallage. Some six feet above the end of this vein a tongue of serpentine, about 11 foot wide, is exposed in the schist.
The bay is bounded by a small headland; and the shore is strewn with fallen blocks of schist and coarse gabbro. I will first describe their general relations, then discuss their lithological character.
The gabbro and hornblende schist are here mixed up in the most extraordinary way; the gabbro has penetrated again and again through the latter, crumpling up pieces of it in places so much that it is difficult to believe they come from a sedimentary rock. The best example of this intricate intrusion can be seen from a narrow track just above the headland. Here some of the veins of gabbro are only an inch or two wide and about a quarter thick; they thin away to mere strings, but remain rather coarsely crystalline to the last. This seems to indicate that the whole mass of the rock was at a pretty uniform high temperature at time of the intrusion. The so-called granite vein is a grey bed of a highly altered rock: a crystalline granular compound of quartz, felspar, and a little mica or hornblende. At the base it is most difficult to distinguish from vein granite. Still, after several very long and careful examinations of this part of the coast, I am quite convinced that it is merely a case of extreme alteration, and that there is no granite here. Below this is another intruded mass of gabbro, terminating in a broad broken tongue, the root of which rests upon a prominence of serpentine. Hence both these rocks are here intrusive in the schist. On the northern side of the headland we have a large mass of gabbro intrusive in and enclosing blocks of hornblende schist—and three masses of serpentine, one of large size.
The hornblende schist is here rather variable in character, having much less hornblende than in the ordinary black variety, and a considerable quantity of felspar and quartz. The gabbro usually consists of an opaque white or pale cream-coloured mineral of rather granular fracture, and of diallage which is often replaced wholly, or to a great extent, by a green mineral, something like chlorite. The former under the microscope appears irregular in outline, partly semitransparent, partly occupied by more or less opaque dotted aggregates of dark grey dust. Indications of cleavage-planes may be sometimes traced in this, shewn by fine parallel more transparent lines. With crossed prisms this mineral is, as might be expected, almost, or quite, dark, the clearer portion appearing as an aggregate of minute crystalline granules and microliths of ill-defined form showing faint colours, with occasional small irregular interspaces of a pale translucent bluish grey. Now and then there is a portion still showing the colour and striping of plagioclase felspar. This alteration is very common in the gabbros of this district. Some from Coverack Cove, which I shall presently describe, show it in an early stage; some from Karak Clews in a later; a specimen also from this very headland, cut to shew the junction of the gabbro with the hornblende schist, still retains its plagioclase in many parts unaltered. A similar alteration has taken place in a gabbro which I have collected from Mont Colon, in the Pennine Alps; and I have observed it in not a few other cases. I regard the mineral therefore as a kind of pseudomorph, the result of the alteration of labradorite or some plagioclase felspar. It is the mineral often called saussurite, and is quite as hard as, sometimes a little harder than ordinary felspar.Fig. 3.—Hornblende in Gabbro Vein from the Balk.
The part left white is altered feldspar
The other mineral is sometimes diallage, but in others a rather dark green mineral, something resembling chlorite at first sight; microscopic examination proves this to be hornblende. The larger patches are found to be composed of irregular aggregates of small prismatic crystals and grains (or possibly occasionally folia) of that mineral; these are generally pale green in colour, fairly dichroic, changing from a strong dull green to a sort of straw-green, and now and then showing very distinctly the characteristic cleavage along ∞P (fig. 3). On examining a series of specimens, both macroscopically and microscopically, this change, which we shall find to be very common in these Cornish gabbros, is seen to take place as follows:—The aggregated hornblende crystals form as a kind of border to the diallage (fig. 7, p. 912), when the latter generally becomes rather opaque under the microscope and loses its brilliant colours with polarized light, and its metallic lustre with reflected light, assuming a greenish colour and silky aspect. Small crystals of hornblende also appear here and there in the body of the crystal, inserting themselves, as it were, between the planes of principal cleavage; until at last the whole crystal is converted into an aggregate of small crystals of hornblende. From the general appearance of the mineral I take it to be actinolite, which as a non-aluminous variety of hornblende would most readily be formed from ordinary diallage. In many cases it is almost fibrous in structure; then it is paler in colour and feebly dichroic. The specimens which I have examined have not shown me any olivine; yet, as we shall presently see, this mineral abounds in the gabbro further north. This absence seems strange: I have, however, some reason to think that it, too, in this case, has been replaced by actinolite.
The "granite vein" headland is a prominence on a rather larger one; beyond this is a little chine and another small headland. Over this space serpentine predominates; but fragments of hornblende schist are included, and intrusive dykes and veins of gabbro are common. Every step shows something new and interesting. At one place the gabbro on the left, and the serpentine on the right, make an almost vertical junction in the cliff. The former includes a long strip of hornblende schist in an upright position; the latter assumes near the junction a rather fissile character. Often it would be hard to say whether the serpentine or the gabbro were the intruder; but here and there may be found conclusive evidence that the latter is the newer. The gabbro is coarse, the diallage crystals being some- times very large, one composite specimen being about 6″ × 2″ × 2″. The foliated structure mentioned above is often seen; and I observed that, as a rule, it was best developed where the gabbro intersected the hornblende schist, especially where it had passed between two masses along the plane of bedding. As the whole mass cooled, these already solid schists would doubtless produce a definite pressure on the crystallizing rock between them at right angles to their bounding surfaces, and so determine its structure. I have seen the mica crystals in a granite vein which had broken through angular fragments of a schist lying at right angles to normals from their surfaces, and have often observed that on the outside of a granite vein the mica plates tend to lie parallel to the surface.
The next headland exhibits both serpentine and gabbro intrusive in schist, with a large felspar vein. But it is needless to carry these details further; so I will select one more section for description, the last which can be reached from the shore. Here a lofty cliff of serpentine is shattered by veins of gabbro, one of which, about ten feet thick, forms a sort of terrace, some height above the shore, which can be reached with a little difficulty; the gabbro is extremely coarse, the diallage crystals being often two or three inches long; and some of the largest occur in a vein only a few inches thick. Just on the left there appears to be a small fault in the serpentine; on the right is a large included mass of schist forming a headland, down to which descends a dyke of dark compact trap about a foot wide. This can be seen to cut through two sheets of gabbro and the serpentine between them, and ends abruptly against the schist. The examination of this part of the coast leads to the following conclusions:—
1. That the serpentine is an intrusive rock.
2. That probably the hornblende schist was metamorphosed prior to its intrusion.
3. That the gabbro was probably intruded when the serpentine had arrived at its present condition.
4. That the black trap dyke was intruded last of all.Fig. 4.—Gabbro intrusive in Serpentine and Hornblende Schist north of the Balk.
C. Gabbro showing a foliated structure. D. Hornblende schist.
Under the microscope the trap is found to consist of a groundmass generally microcrystalline, consisting probably of some pseudomorphic product after plagioclase, with perhaps minute crystals of actinolite enlarged, and a large number of small hornblende crystals, merely rather platy in structure, but varying from the normal to the actinolitic form. There are, as usual, grains of magnetite, and a number of microliths, sometimes acicular, sometimes rather irregular in form, which are commonly included in the larger hornblende crystals, lying with their longer axes in the planes of principal cleavage. I have not been able to satisfy myself as to the nature of these. At present we must call the rock a diorite; but whether it has always been hornblendic is by no means certain.
From the above headland a walk of about a quarter of a mile along the edge of the cliffs leads to Polbarrow Cove. The cliffs, which appear impracticable, consist of serpentine; but some of the jutting reefs below resemble hornblende schist.
Descending into Polbarrow Cove by a narrow track, we find that the southern part of it consists mainly of hornblende schist; but there are intrusive veins of serpentine in it close to a natural arch, as well as in the little ridge of schist, on the middle of which stands a small boathouse. Serpentine may also be seen in a quarry above the cliff south of the boathouse. Just north of this is a junction of the schist with the serpentine, which runs obliquely in this direction up the cliff. It now continues for some time; but two low headlands which bound the cove are capped by hornblende schist; and the next one to north is wholly of that rock. There is a gabbro vein in the schist, near the junction with the serpentine, about on a level with the boathouse, and another much higher up the cliff in the latter rock.
Returning to the cliff we come to a small quarry showing a junction of hornblende schist and serpentine. The former rock dips about 27° E.N.E., and in 1873 could be seen to be overlain irregularly by the serpentine. In one part of the quarry a piece of the schist was intercalated between serpentine. In 1876 the relations were less clear. The schist is very dark and full of hornblende. The specimen described above was collected here. The cliffs, as may be seen from the geological map, consist of hornblende schist from this spot to near Cadgwith, when we come to another junction in the celebrated Devil's Frying-pan, obviously an old sea-cave whose roof has fallen in.
The relations of the serpentine with the schist, as seen by descending into this hollow, leave no doubt that the former rock is intrusive. On reaching the shore we find ourselves at the bottom of a funnel-shaped pit communicating with the sea by a natural archway. This, and the greater part of the two adjoining sides, is of hornblende schist, which dips westward at an angle of 40° to 50°, increasing gradually to about 70° towards the west on the southern side, and rolling over considerably on the northern. The remaining side is serpentine.
The junction at the S.W. corner is masked by debris; but on the other it is clearly an intrusive one, a large fragment of schist being entangled between two masses of serpentine. I think that the pre- sent summit of the entrance archway also marks a former junction with the serpentine; for the schist of this looks much disturbed and slickensided, and has a "junction" aspect. Also an overlying block apparently in situ is serpentine. The serpentine is dull mottled blackish brown and red in colour, full of small glittering scale-like crystals with a rather silvery lustre, as at Mullion and at Pradanack Quarry.
After climbing back to the pathway leading to Cadgwith, the descent into that village is mainly over serpentine. But the rocks exposed in the little cove forming the harbour are very characteristic hornblende schists, dipping about 35° N.N.W.
On reaching the summit of the steep ascent which leads from Cadgwith village to the level of the main plateau above the sea, we again find the serpentine. A quarry here (in 1876) showed the junction with the schist very well. The former rock overlay the latter (which had a dip of some 30°) with a rather irregular junction; but here, as in other cases, it seems to have generally forced its way fairly evenly along the plane of bedding. The serpentine (no. 6) is very compact in texture, and varies from a claret-colour to an olive green, both varieties being veined and mottled, the former with dull green and bright red, the latter with a purplish tint; occasional layers of greyish-green steatite occur.
The serpentine in the cove beyond is mottled light and dark green. This rock now continues to form the cliffs above the sea for some distance. About half mile from Cadgwith (measured in a straight line) the shore is strewn with huge boulders and overhung by cliffs of a dark serpentine. This rock is almost as black and compact as that of George Cove; but the surface is varied by a vein- like mottling of very dark olive-green, and by bronzite crystals about 0⋅2 inch wide with a bright submetallic lustre (no. 10); it weathers a dull earthy green.Some 100 yards from this spot, along the shore, we reached an apparently faulted junction of the serpentine and hornblende schists. Directly after, the serpentine sets in again, overlying the other rock, following nearly the plane of bedding. The hornblende schist is extremely altered; some bands in it are in colour and texture very like ordinary vein granite; but, after most careful examination, I feel convinced there is no true granite here. The ends of the strata are bent over on the southern side; and in the lower part a coarse breccia of schist and serpentine separates the two masses of these rocks. There is a similar breccia, though less irregular, on the other side, where the base of the serpentine is greatly decomposed, the rock assuming an earthy aspect, of a mottled dull greenish or reddish grey colour, veined with indian-red, having scarce any resemblance to normal serpentine. A little further on are four masses of hornblende schist, one after another along the shore, caught up in the serpentine (fig. 5). Parts, again, of these are Fig. 5.—Masses of Hornblende Schist included in Serpentine.
(The serpentine indicated by fine wavy lines.)
b. Schist, bedding nearly vertical. c. Tongue of serpentine.
d. Schist, bedding nearly horizontal, highly altered.
I have had a section made of the heart of the dyke; and my friend Mr. Allport has kindly lent me two of his own cutting, one being from the outside. The former shows that the rock consists of a much decomposed plagioclase felspar in long narrow crystals. The interstices are occupied by a pale-coloured (generally greenish) hornblende of rather fibrous or filmy aspect. There is also some magnetite. The aspect of this rock is so like a typical dolerite, and so unlike a diorite, that I suspect the hornblende to be a secondary product, as in the gabbro. The other section is totally different, and closely resembles the hornblende schist above described. I examined the rock at the time to see whether a piece of schist had been caught up; but if it was, I failed to detect it.
Some of the quarries belonging to the Poltesco works are up the valley inland; but these I have not been able to visit. From Caerleon Cove a steep ascent leads us up again to the main plateau.
Hornblende schist may be seen on the shore of the next little cove, and on its left bank, above the path, intrusive gabbro, coarse, but showing a rather foliated structure, may be seen on the rough grassy slope. I observed three exposures; near to the furthest there appears also to be a little schist, the rock generally on each side of Caerleon Cove being serpentine.
This rock, on the beach beyond the next headland, includes many large fragments of quartzose rock, some of which very closely resemble granite veins. After careful examination, however, I am of opinion that they are only bands in the schist. The more quartzose and harder layers have been forced among the softer, so as to mimic intrusion.
Beyond this, in cliffs of a dark serpentine, is a greenstone dyke, the lower part of which bifurcates; and just beyond this is an included fragment of schist of singular form. The dyke in the mass has a slightly serpentinous aspect. It is a finely granular, almost compact, very dark grey rock, faintly variegated with minute white specks. Under the microscope its appearance is as follows:—There is a tolerably clear homogeneous-looking base containing a large number of small prismatic crystals and folia of greenish hornblende with fairly marked dichroism, and a good many small grains of magnetite. With the two Nicols the base exhibits the microcrystalline pseudomorph after felspar, and the hornblende shows brilliant colours. A little has a rather fibrous structure, and on rotating the stage be- haves as an orthorhombic mineral; it is strongly dichroic, showing a brown tint (? anthophyllite); there is, however, not enough to enable one to be certain about it. A vein is filled by asbestiform mineral, possibly a variety of chrysotile.
Proceeding along the shore we pass in quick succession some other narrow dykes (three, I think) of very similar appearance. The last but one (2 to 3 feet thick) cuts through a vein of coarse gabbro about 10 inches thick, which shows again in one or two places in the face of the cliff. A little to the north it is apparently cut by another dyke; but after carefully examining the latter, especially where it is exposed on the shore, I believe it to be only an included fragment of a peculiar compact variety of the schist, highly altered. About 30 yards to the north is another included mass of the schist, standing upright in the serpentine cliff so as to look wonderfully like a dyke.
Just where the sandy beach of Kennack Cove commences is another large mass of included schist, occasionally resembling vein granite, which may be traced some way inland. Two or three intrusive tongues of highly decomposed serpentine may be seen in this mass.
Kennack Cove is a sandy tract at the embouchure of two flattish valleys, divided by a low headland of serpentine. In this is a small dyke about a foot (or rather less) thick, closely resembling those described above, but perhaps even more compact.
Crossing the second stretch of sand we come again to cliffs of dark serpentine, and find almost at the first point a dyke, generally from 4 to 5 feet thick, which bifurcates above. The appearance of this rock is very similar to those already described. Under the microscope it is found to consist of longish, rather irregularly outlined, plagioclase crystals, and a quantity of aggregated grains or imperfectly shaped small crystals of hornblende, green-coloured, strongly dichroic, and showing bright colours with polarized light. There are also some irregular grains of magnetite and a few needles of apatite.
A short distance further along the shore a mass of rock, forming a group of low reefs, bears, at first sight, a close resemblance to a granite vein. This is heightened by the extraordinary way in which, in one place, it inserts itself in thin tongues into the adjoining rock, which is a crumbling, dull, greenish to reddish substance, not unlike a decomposed serpentine. Careful examination, however, shows that we have here, highly metamorphosed and entangled in the serpentine, another mass of sedimentary rock, which has once consisted of lenticular bands of a more sandy character, in a mud whose mineral composition somewhat resembled that of hornblende.
The first stage has been the conversion of the former into a kind of granulite, the latter, probably, into a hornblende schist. Torn off and squeezed by the igneous mass (now serpentine), the harder bands have been crumpled up, and in some cases forced into the softer, which at last, by slow action of water, have been converted into a rotten chloritoid and rather serpentinous schist. The serpentine around is also rotten near the junction. In some places the two rocks are so altered by addition and subtraction of mineral constituents that it is almost impossible to fix their precise boundary; still I am convinced that the above explanation is the correct one.
I have examined a slice from the most granitoid part of this rock; and, though highly altered, it quite confirms my view. It consists of quartz, felspar, orthoclase, and some plagioclase, with a little of some variety of magnesia mica. The felspar is full of microlithic alteration products. In the quartz are a good many minute cavities and shapeless microliths—also some microliths of larger size, which may be apatite. Here and there a piece of the felspar (it is not very characteristic) is full of minute branching empty cavities or microliths (I rather think the latter), which would certainly be quoted as canal systems by the opponents of Eozoon. I have seen something similar in a granitoid rock from Holsteinborg (Greenland), but at present can do no more than record the occurrence, hoping to return to the subject on a future occasion. It is the nearest approach to an organic structure that I have ever seen.
From Kennack Cove we proceeded along the cliffs to the headland of Karak Clews, about a mile distant. So far as I saw, serpentine continues all the way; and just before reaching the point called Carn Sparnack a small quarry affords some very pretty varieties (no. 11)—one a rich red mottled with dark olive green, the other claret-colour with similar markings, both containing in the green part small crystals of greenish bronzite with a submetallic lustre. Karak Clews is a bold headland formed by the extremity of a great elongated mass or broad dyke of coarse gabbro, which, according to the map, is about two miles long and a furlong broad. The headland terminates in a narrow ridge leading down to a precipitous mass of rock running some little way out to sea; the general direction is nearly N. and S., the mass further inland for most of its course having a N.W. strike.
There are very few places more instructive than this to the student of igneous rocks. Broadly speaking, the ridge consists of gabbro, in which so many pieces of serpentine are entangled that it would often be easy to suppose the latter intrusive in the former. Careful examination, however, will show that there is no doubt as to their true relations. The serpentine is generally much decomposed.
On the western side of the headland the gabbro extends for some distance along the shore. I was unable to examine closely its junction with the main mass of the serpentine; it appeared, however, to be, as usual, intrusive. A small shallow gully just on this side of the actual headland affords the best study of the gabbro, which might be mistaken for a metamorphic rock. The schistose structure strikes about N. and S., extending in considerable perfection over a space about 5–6 yards broad, and dips to the eastern side at an angle of about 80°. It is, however, quite impossible to draw any line of demarcation between the foliated and the ordinary (rather coarsely) crystalline gabbro. This consists of a purplish-grey plagioclase felspar, probably labradorite, often mixed up with a dead-yellowish-white felspar (the saussuritic variety already mentioned), which of course predominates on exposed surfaces, crystals of brownish diallage, often about 1 inch across, and having a metalloidal lustre, and a considerable, but variable, quantity of the minute rather dark green hornblende already described. In short, the process of alteration from an augitic to a hornblendic rock has taken place here as at the Balk; and specimens may be found in almost every stage. Not seldom the diallage seems to be entirely replaced by these pseudomorphs. In some of the most schistose varieties the dark "eyes" of this hornblende remind one in appearance of the spots in the Knotenschiefer.
The two minerals (the felspar and diallage, or hornblende) are often quite separated in alternating bands, those of felspar being from nearly 1 inch downwards to mere lines. Not seldom the diallage predominates, felspar only occurring in very thin threads, with occasional "eyes" as described above. From such specimens we pass to normal coarse gabbro—a variety in which the plates of the diallagoid mineral are wavy in outline, and tend to be parallel, being very common.
Among the most schistose varieties lenticular pieces and long slab-like masses of included serpentine are very abundant, and may not improbably have contributed to the development of the structure, as at the Balk.
I have had two sections cut from the gabbro of Karak Clews. The normal rock consists now chiefly of short broad rather irregular crystals of partly altered plagioclase, with numerous microliths and aggregated small crystals of pseudomorphous actinolite. Here and there, however, portions of plagioclase crystals still remain but little altered, as well as crystals of diallage in which the change to hornblende has not been completed. Every stage of the change from plagioclase to the saussuritic mineral can be traced in various parts of the slide. There are occasional microliths and larger grains of magnetite; and one or two of the diallage crystals are filled with an opaque black dust, the result of decomposition. These have some resemblance to certain hypersthenes; but the mineral does not belong to the orthorhrombic system. There is a general banded structure shown in the arrangement of the minerals.
The second slide is cut from a pale-coloured, apparently imperfectly crystallized rock, consisting chiefly of two minerals—the principal a whitish or pale pinkish felspathic one, the other a pale green mineral. The former, under the microscope, when examined with polarized and analyzed light, assumes the characteristic granular aspect of the saussuritic pseudomorph after plagioclase. The other mineral appears to be an almost colourless augite, containing a large number of very minute microiiths. There are but slight indications of secondary hornblende. Some small roundish white specks, just visible to the eye, in the felspathic portion of the rock, appear, under the microscope, as rather oval blotches, often opaque and brownish, but in other cases showing aggregate polarization of a somewhat fibrous mineral, with colours from a rather orange yellow to a purplish blue, probably one of the zeolites. This rock also shows a banded structure.
On the eastern side of the headland we come at once on the serpentine as the predominant rock, though it is broken through by many veins of coarse gabbro. The mineral composition of these resembles that just described. Some are of considerable thickness; but others die away as mere strings. The material of the gabbro, therefore, must have been injected in a very fluid condition, and, as even the strings are coarsely crystalline, must have made its way into rock of high temperature, and have cooled down very slowly. The larger masses more frequently exhibit a schistose structure than these finer veins, though they are sometimes quite without it. Every stage may be noticed here as on the other side of the headland. Here, also, the "eyes" of hornblende are frequently seen in the schistose varieties.
Between the headland and Lankidden Cove are several gabbro veins. The serpentine is a red variety, much resembling one presently to be described. On approaching the Cove two or three narrow greenstone dykes are seen in the cliffs, and one in a skerry projecting from the sand. These cut both the serpentine and the gabbro, and closely resemble those already described near Kennack Cove. A section from one exhibits plagioclase felspar in fair preservation (the crystals commonly five or six times as long as broad, characteristically twinned, and mostly well defined), augite (often well preserved), and some magnetite. Besides this there is in the interval of the larger plagioclase crystals, a good deal of an aggregate of a fibrous transparent mineral, changing, with polarized light, from bright blue to yellowish or occasionally pinkish colours—doubtless a pseudomorph after felspar, perhaps replacing a magma. Here and there the augite changes to pale green rather fibrous hornblende, showing marked dichroism, and rather rich colours with analyzer and polarizer. There is also a brown dichroic mineral: some of this is probably little more than a ferruginous staining; but in one case it is certainly a distinct mineral, probably an iron-mica. There is no unaltered olivine; but I think I have detected a few pseudomorphs. This rock, then, is a basalt only slightly altered.
The gabbro veins continue to the eastern side of the cove. One, which shows a marked schistose structure, contains, as usual, many long strips of serpentine. Close by it is another gabbro vein, only a few inches thick, but quite unique in character. It consists mainly of felspar, a whitish to bluish-grey labradorite, with crystals often from an inch to an inch and a half long, in excellent preservation. In the interstices of the felspar are aggregated minute crystals of a dull green mineral. The latter under the microscope proves to be actinolite. The aggregated clusters of crystals are very beautiful objects with polarizer and analyzer; and the felspar crystals (which are a little decomposed) contain, especially near their edges, many small acicular crystals of the same mineral.
Close to this vein is a small dyke (6 to 12 inches wide) of a very slaggy-looking, compact, dark rock, which, I have no doubt, is an old basalt. Some of the serpentine near this weathers to a dull green colour, and contains distinct crystalline grains of magnetite. Beyond this point is about a mile of coast, coloured as serpentine in the map, which I have had to pass over almost unvisited; and the steep cliffs will, I think, render detailed examination no easy task.
(b) Coverack Cove.—We then come to Coverack Cove, one of the most interesting localities on the coast. Without a regular survey and large-scale map it is not possible to give very precise details; but the following general description may render clearer those which I can furnish.
The cove terminates in a widish valley. On the right bank is a headland of serpentine, on the left the great gabbro mass which rises from the sea to the upland of Crousa Down.
The village stands on a low cliff (a raised beach, traces of which are also conspicuous along the edge of the gabbro mass) on the right bank of the cove. Beneath it is a rocky foreshore.
The principal rock beneath the village is serpentine; but this is broken up by a network of dykes and intrusive veins. As will be seen from what follows, we have here the following association of rocks in order of age:—
(1) Serpentine (no. 12).
(2) A gabbro, generally of a dull reddish green colour, which I shall refer to as the Older Gabbro.
(3) A gabbro of more normal aspect, which I shall call the Newer Gabbro.
(4) Some dark trap dykes, similar in general appearance to those described at Kennack and Lankidden.
At the eastern end of the village is a small harbour bounded by a pier, which starts from a little headland. I commenced my examination on the further side of this. The rock here is serpentine, much jointed and cracked, and often rather decomposed. This is also found on the other side of the harbour, where at the base of a high wall a vein of gabbro is exposed, about 4 feet thick and with a schistose structure. The top and bottom of the rock shows the usual mineral changes. The serpentine in contact is much cracked and burnt. The projection of rock, which forms the western limit of the port is cut by a dyke of newer gabbro; and the same rock, a few yards further on, appears at the base of the cliff on which the village stands; an isolated patch also breaks through the serpentine on the shore. A few yards from this is the first patch of the older gabbro. This older gabbro forms the base of the cliff for a short distance, while the shore consists mainly of newer gabbro, veins of which cut the older. Just where a projecting angle of the cliff begins is an interesting junction. The older gabbro is cut by a dyke of newer gabbro about 18 inches wide, and both again cut by a dark trap dyke about 12 inches wide (fig. 6). The two intrusive dykes are displaced by a fault of a few inches. This last dyke (1) may be traced for some distance towards the sea; its general direction is N.N.E.–S.S.W. At the angle named above, serpentine replaces the newer gabbro, both on the shore and in the cliff, and on the whole is continued to beyond another small projection.
Here another trap dyke (2), generally about 5 inches wide, may be traced about 20 yards along the shore in a northerly direction. Beyond this veins of the newer gabbro break repeatedly through the serpentine. Then comes a dyke of compact trap (3), about 4 inches wide, cutting into the cliff and running N.N.W.–S.S.E. Beyond we find serpentine with intrusions of newer gabbro, till we again find the older gabbro beneath the cliff, broken into by the newer, and both cut by another compact trap dyke about 1 foot wide. A few yards further, over serpentine and newer gabbro, is another trap dyke cutting both, exposed up the face of the cliff for 7 or 8 feet. It is about 3 feet wide, and rather coarser in texture than the others. Well-marked horizontal joints give it a slightly columnar aspect. The shore for some little distance further consists of serpentine with some intrusive newer gabbro, and at least one more dyke of compact trap.
It will now be convenient to describe more precisely the lithological characteristics of these rocks, omitting at present the serpentine.
The Older Gabbro.—This might easily be mistaken for a mere variety of serpentine. It has a compact, dark, dull red or purple groundmass, often mottled with a brighter red, in which are imbedded crystalline grains of a greyish white felspar, perhaps about 0⋅2 inch in diameter, and some rather smaller crystals of diallage, generally about 1 to 1 inch apart. The matrix for a quarter of an inch square or more is often unbroken by any crystals visible to the eye; occasionally, however, the felspar predominates. The rock does not vary much in texture, the smaller veins being about as coarse as the larger masses. One small boss has the groundmass a dark green instead of red. It is, however, the same rock, though as a rule it is rather more finely crystallized than the average of the red variety; for towards the outside it exhibits perfectly the change into the red variety. I have specimens about 4 inches long, red at one end and green at the other. Hence the alteration may not be so deep-seated as from its uniformity I should have supposed.
I have examined three slides of the red, cut from slightly different varieties of the rock, and one of the green. I will refer to them by numbers, taking the most normal specimen first.
On placing slide I. beneath the microscope, we find it to consist of colourless or nearly colourless felspar, in rather irregular to roundish oblong grains, occasionally showing lines indicative of twinning, traversed often by cracks and in places partly kaolinized. Associated with this, in about equal quantities, are a large number of irregular grains of olivine; these in parts are almost unaltered, in other parts entirely converted into serpentine. This is occasionally translucent and of a greenish yellow, occasionally opaque, from the presence of a muddy brown peroxide of iron, and showing every grade of intermediate staining. The process of conversion of the olivine into serpentine will be described below.Fig. 6.—Shore below Coverack Cove.
Newer gabbro intrusive in older gabbro, and both cut by dyke of dark trap.
a. Older gabbro. b. Newer gabbro. c. Dark trap.
With polarized light (crossed prisms) the felspar is seen to be crystallized in irregular grains, many of which show characteristic plagioclase twinning; bright colours, however, are rare, shades of light and dark milky grey being commonest. In parts the felspar is almost opaque from decomposition; in other parts it presents the usual finely granular "saussuritic" aspect. The olivine, when unchanged, shows its characteristic rich colouring. The process of conversion into serpentine, best examined by rotating the polarizer, is as follows (see figs. 8 & 9). In the cracks of the olivine a dark ferruginous stain is deposited; then on either side of this a layer of fibrous serpentine of pale golden colour (probably chrysotile) is formed; thus the olivine grains seem traversed by an irregular network of associated dark and light strings. The interspaces then seem to be attacked; and they also are converted into serpentine; but in them the mineral is usually in an isotropic or noncrystalline state, and the peroxide of iron either forms a dark clot in the middle (sometimes rendering nearly the whole opaque), or clouded granules disseminated in various ways. At last when the whole grain is converted into serpentine, the black strings marking the original cracks become less definite, being interrupted and disturbed, probably from the action of molecular forces, so that their significance might readily be overlooked. The chrysotile strings also become less conspicuous; but still they may often be traced when the prisms are crossed. It is worth noting that, as a rule, the formation of chrysotile appears to proceed from the surface of a crack inwards, and is generally arrested at a fairly constant distance from the outside; but the conversion of the remainder of the grain into non-crystalline serpentine appears to be nearly simultaneous over the whole. The process very closely corresponds with that which I have already described in the Ariége lherzolite (see fig. 9, p. 916), except that here, as the olivine is apparently ferruginous, there is a more marked discoloration. The augitic constituent is less abundant than the other two minerals. Most of it is diallage; but grains, and sometimes even parts of a crystal, seem rather to be normal augite. Occasionally the serpentine appears to have been deposited in a crack in them: but as a rule there is not the slightest advance towards a conversion of these minerals into serpentine.
The diallage occasionally assumes a dusky, finely granulated, stained aspect along the principal cleavage-planes, which I have observed in cases where a change to hornblende is commencing; and one crystal is bordered by an aggregate of minute crystals, which I fully believe to be actinolite. There are few included microliths, excluding these ferruginous clots; but there seem to be one or two grains of magnetite, and in one of the diallage crystals are two needles which I think are apatite.
Slide II. (cut from a rather duller-coloured specimen) does not materially differ, except that the iron stains in the olivine are more uniformly dark. Some of the diallage (which mineral is fairly abundant in this specimen) contains small opaque belonitic microliths lying parallel to the plane of principal cleavage.
In slide III. the felspar decidedly predominates over the other minerals, the olivine coming next. Some of the grains here are wholly converted into serpentine; one shows, with polarized light, a single speck of the original mineral alone remaining in a good-sized grain. There are both diallage and augite; and here, as in the other slides, there are appearances of a passage from one variety to the other. One crystal shows in parts the ordinary imperfect cleavages of augite parallel to ∞P, with an occasionally marked clino-diagonal cleavage; then, in places, the last cleavage dominates over the other, forming very definite parallel lines of weak cohesion; and, lastly, the inner (and major) portion of the crystal is filled by a fine parallel cleavage, giving it an almost granular-linear aspect, in which, I think, occasional cracks indicate traces of a cleavage parallel to ∞P. The absorptive powers of the two minerals seem to differ. By rotating the stage the unaltered part of the crystal becomes (with crossed prisms) as nearly as possible black after it has been moved through 291° from parallelism with a vertical line; but the other part offers no approach to complete darkness, and, though darkest when near the above position, is not all uniformly at its darkest then. This, however, may be only due to imperfect cohesion of the cleavage-planes.
Slide IV. is cut from the green-coloured variety of the older gabbro. The principal differences of this from I. and II. are:—that the conversion of the olivine is about as complete as in III., but is unattended by any red colour; the serpentine is of a pale yellowish- green tint; the dark dust is formed in the cracks of the olivine, as described above, but appears to recombine, or in some way or other is made partially to disappear at a further stage of the process; for some of the most completely changed greenish grains are very fairly clear. The augite or diallage is scarce, and, as a rule, occurs in very small grains. The resemblance of this rock, macroscopic and microscopic, to the troktolite of Volpersdorf is very remarkable, the only difference being that the felspar in the latter shows brighter colours and the serpentinous part is a shade darker in its green. The cracks traversing the felspar in all these rocks are noteworthy, as they appear to radiate from the serpentinized olivine granules and to imply some strains caused by them in the process of alteration. I may also call attention to the fact that the pyroxenic constituent seldom, if ever, shows any signs of being converted into serpentine.
The Newer Gabbro.—This is generally much coarser in texture than the others, and more decomposed. It pierces the adjacent rock with very irregular veins, which sometimes thin away to mere strings not half an inch thick, but are fairly coarse to the last. The plagioclase, often in crystals nearly an inch across, varies from a dull purplish tint to white. The diallage crystals are frequently from 1 to 3 inch across. In places red spots may be observed; these are evidently altered olivine, as in the last rock, but they are often larger, more friable, and irregular in their occurrence. Included fragments of serpentine are also seen. Spots of the green hornblendic mineral are frequent towards the exterior. As might be expected the weathered surface of the coarser varieties is much rougher than in the older gabbro. The latter rock also has a perfectly close junction with the serpentine, as if it were welded to it; while between that and the newer gabbro is more or less of a crevice.
The result of microscopic examination of a slide of one of the dark purple and green specimens is interesting. It contains abundant plagioclase, apparently labradorite, resembling that in the other gabbros, in fair condition but dull in colour, which here and there seems partially replaced by a clear isotropic pseudomorph. The green spots prove to be altered diallage: here and there small portions of the original mineral with characteristic cleavage &c. remain, but the greater part is converted into a mass of pale green actinolite, the crystals being crowded together like interlacing leaves, and sometimes forming fringes almost like tufts of grass; they make their way sometimes up cracks in the felspar, and appeal even to form as endomorphs. The difference in dichroism between these and the yet unchanged diallage is well exhibited on rotating the polarizer without the analyzer. I find no olivine or grains of serpentine; but a speck here and there resembles the latter; the outline, however, of some of the bunches of actinolite resembles rather that of olivine than diallage. As there certainly has been olivine in the rock, this confirms my suspicion that this mineral sometimes, instead of forming serpentine, contributes to the actinolite. When the latter mineral is formed, it is remarkable how greatly the cohesion of the constituents of the rock seem to have been disturbed; for it not only invades the edges of the felspar in needle-like fringes, but also occurs in blades and tufts anywhere in the heart of that mineral. In fact, where this process has been carried far, it is only now and then that the outline of the original crystals can be traced.
Lastly, as to the trap-dykes, I regret to say that I cannot give precise information with regard to all of these, as I have found it difficult in one or two cases to identify those which I had observed in different visits. The notes made in 1873 and 1875 were taken with a rising tide; and so, as other matters were of still greater interest, I rather hurried over these dykes. In 1876 the tide was very favourable, and I observed more dykes than I had noticed previously; but still, as there was so much to do that I did not collect a specimen from every one, I have a little difficulty in identifying those collected on former occasions with the dykes noted on this. However, the following results are fairly correct. There are two varieties of rock in the dykes:—one, coarse enough to be obviously a crystalline rock, much like an anamesite; the other very compact, of a dark purplish colour, looking almost like a dull serpentine. Thin veins of serpentine often show in the latter; and films of the same not seldom coat joint-faces. The dyke seen in the cliff may be taken as a type of the former. On microscopic examination it proves to be a felspar basalt,—the plagioclase being, as a rule, clear, though with polarized light dull in colour, the augite little changed, and dull green spots replacing probably the olivine. There are also grains of magnetite, with brown stains from one of the other ferric oxides; and cracks are overspread by an infiltrated serpentinous mineral.
The second specimen (collected in 1873 from a dyke "cutting gabbro and serpentine") is quite different. It has an isotropic matrix, which remains black, with crossed prisms, as the stage is rotated, full of rather acicular crystals of actinolite, very pale clear green with ordinary light, slightly dichroic with the polarizer, and often showing brilliant colours with the analyzer. Here and there are patches of the brown hornblende granulated with magnetite (?), like that described from the shore north of Kynance (p. 889). Some of these occur in such a way as to produce a strong impression that they too have been partly replaced by actinolite. One or two portions of unchanged plagioclase crystals remain in the matrix; and probably some of the microliths in it are the pseudomorphs already mentioned. As a type of the other variety, I take a specimen from a dyke about four inches wide (just opposite the north end of a row of white cottages), which cuts through two rather thin veins of newer gabbro to a slight hummock of serpentine (I believe, no. 3 in my description above). A hand specimen might very easily be taken for a dull-coloured serpentine, were it not for its greater hardness and its behaviour under the hammer. Microscopic examination shows it to be an altered basalt, infiltrated here and there with serpentinous veins, full of minute crystals of plagioclase (or probably a pseudomorph after it), occasionally about 0⋅02 inch long. Here and there occur pale brownish films replacing the ordinary dusky granulated matrix. Examination with polarized light shows that actinolite is present here; and minute bright specks over the whole slide suggest the presence of the same mineral. The grains of magnetite (perhaps also of ilmenite) are much decomposed. Here, then, is a basalt, probably a magma basalt, greatly altered, but still not in any true sense of the word converted into serpentine. Another specimen, broken in 1873 from a very similar dyke, shows a still greater change, the felspar crystals being still just discernible, but the actinolite more characteristic and perfectly formed.
Near the western end of the village, in the little open glen which comes down to the cove, and about 100 yards from the sea, is a quarry in serpentine. This (no. 13) is rather a dull dark variety of an aluminous aspect, and is much cracked and jointed. This is probably due to some small gabbro veins (coarsely crystalline and much decomposed) which have been exposed in opening the quarry. On the shore, east of a small row of houses, a mass of the newer gabbro is exposed, very schistose in the upper part. Gabbro veins may be seen on the hill-side behind the village.Proceeding onwards, we observe that the shore, where free from sand, consists of serpentine with intrusive veins of the newer gabbro. These become more numerous as we approach the main mass of gabbro, which is reached about a quarter of a mile from the end of the village. In plan this is an irregular oval, a little more than four miles long and two wide, which forms the elevated upland of of Crousa Down, some 300 feet above the sea. The line of the raised beach is well marked by a low cliff, and in one place by a projecting rocky plateau of considerable extent a short distance above sea-level. The rock at the southern edge of the massif varies greatly in texture, being sometimes moderately coarse (often, owing to the prevailing greyish-blue tint, seeming from a short distance quite compact), sometimes very coarse, with crystals of felspar and diallage nearly an inch across. In the latter the felspar generally weathers to a dull white; the diallage is more or less metallic, commonly with a greenish tinge. Some of the more compact varieties also weather to a decided mottled white and dull green, like the gabbro of Mont Colon and the Matterhorn (Pennine Alps); but others (for what reason I cannot say) assume a slightly rusty tinge on their surface as the felspar weathers out, leaving the diallage crystals projecting, as happens with the gabbro of the Cuchullin Hills, Skye. I have had slides cut from one of the dark bluish unweathered specimens, and from one of the exterior white and green. The former shows the rock to be an olivine gabbro. There is plagioclase felspar, generally in rather short irregularly oblong crystals, showing bright colours and twinning. Some of these exhibit a peculiar strongly marked cleavage (or minute twinning, with one set of crystals dominating), which gives them a general resemblance to the structure of diallage. The felspar is in places rather decomposed. There is a fair amount of diallage, and a few crystals of common augite. These with ordinary light are as nearly as possible colourless, and are in good preservation. The olivine is rather rough in texture and much cracked; the cracks are marked out by a deposit of granular opaque mineral, probably magnetite, which in some cases appears to penetrate the intermediate spaces (which are commonly fairly translucent), rendering them almost opaque; now and then it assumes a browner tinge, as from hæmatite or limonite, and the grains are slightly stained with brown or green. Most of the olivine grains have a finely granulated aspect at the edges, and are sometimes bordered by a finely fibrous mineral, probably serpentinous and a secondary product; the grains, however, show very little trace of conversion into serpentine. Except the minute granules described above, there is very little magnetite or other iron-oxide visible. The above appearance would lead us to conclude that the olivine is a rather ferruginous variety. The other slide (fig. 7), Fig. 7.—Diallage partly altered into Hornblende, from outer part of the great Gabbro mass at Coverack.
A. Decomposing felspar. B. Diallage. C. Hornblende.
There are some dykes in this mass of gabbro. Soon after getting on it we come, as we clamber along the shore, to two small dykes, running rather irregularly, of a sharp-jointed, rather splintery, very compact dull-greyish or greenish rock, of a rather serpentinous aspect, the larger generally about 18 inches, and the smaller about 15 inches wide. I have had a slide cut from the former. It proves to be a basalt, chiefly consisting of small crystals of plagioclase and augite, both rather altered, and rather poor in magnetite. I suspect that much of the plagioclase is now a pseudomorph; we have the irregular low-tinted granular aspect in part of the field. A good deal of the augite has undergone change, and should, perhaps, rather be called uralite; but that the rock has been a basalt I have no doubt. There is a little apatite. A short distance further is a dyke from one to four yards wide, which forms a little headland. This is in parts very like an ordinary basalt or anamesite, with a glistening surface and weathering brown. Parts of it are rather porphyritic, having very white felspar crystals up to about 1 inch long. I have had slides cut from this; and on examination it proves to be a basalt, the plagioclase being fairly well preserved, the augite sometimes very characteristic, and a good deal of olivine with a granulated dusky aspect like (though on a smaller scale) that described above; there is also a fair quantity of magnetite. The most porphyritic variety of this rock, macroscopically, so closely resembles the most porphyritic variety of the "diorite" at Poltesco, that the specimens might easily be confused.
Beyond this I observed two other dykes:—one resembling a basalt, not more than a foot wide, in a little headland; the other some quarter of a mile further on, also like a basalt, very compact, splintery in fracture, much cracked and jointed. The above-described dykes lie well in the first mile of the gabbro. After following the shore for about that distance—a slow and rather laborious process—I was obliged, by want of time, to take a path along the low cliff above, as I was anxious to examine the greenstone of St. Keverne.
This I was not able to do as completely as I had hoped; for where from the map I had expected to come well into the mass (here coloured over a space nearly half a mile across), I was still on gabbro, pierced by frequent veins of greenstone; and I had not time to go as far as Manacle Point, a promontory, according to the map, formed of the greenstone. This rock is, in the veins, very compact, of a dull greyish green colour, extremely hard and tough, weathering brown; in general appearance it is not unlike the 18-inch dyke described above. Under the microscope it is found to be a finely crystalline rock, closely resembling the aforesaid specimen—the augitic constituent now appearing generally as a pale greyish or brownish rather fibrous mineral, very feebly dichroic, changing with polarized and analyzed light from a pale or golden yellow to a blue or puce-brown tint. The felspathic constituent also only alters from light to dark milky grey; hence I have no doubt that it too originally was a finely crystallized basalt.
To the north of this there is a little serpentine associated with gabbro (schistose in places), in the neighbourhood of Polkerris Point. These have been briefly described by Prof. Sedgwick and Sir H. De la Beche; and I have examined specimens in the Woodwardian and Mr. S. Allport's collections, but have not yet been able to visit the district. I hope, should they prove of sufficient interest, to make them the subject of a future communication.
Some Inland Sections.
In conclusion, I must briefly mention two places inland where I have more particulary examined the serpentine:—one, at its first appearance, on the road from Helston to Llandewednack. Here, so far as I know, the junction is concealed by vegetation; but a little stream running through a ravine must be very close to it. On the north side of this is seen altered Devonian rock, apparently shattered and baked. On the south the serpentine rises in some picturesque knolls by the roadside. It is a dark-coloured homogeneous rock (no. 14), resembling that at the south end of the Mullion mass, weathering a brownish grey, and showing, more especially in the weathered surfaces, the banded structure already noticed.The other instance is on Goomhilly Downs, on the road from Helston to Coverack Cove. In the first serpentine-pit which I passed (about half a mile beyond Sir L. Vyvyan's lodge), that rock is found to be dull and dark in colour (no. 15.) with but few bronzite crystals, homogeneous in aspect, but showing the banded structure remarkably well, so that on weathered surfaces it quite mimics stratification. Another quarry, to the right of the road, about seven miles from Helston, shows a similar structure; but here the serpentine varied much in different parts of the pit, some specimens barely indicating it. Many of the latter were extremely compact in texture, almost conchoidal in fracture, and very beautiful, having a dull-reddish to greenish groundmass, with veins of bright red and of yellowish steatite; the bronzite crystals are small and inconspicuous. The serpentine weathers a brownish grey, often with a very rugged surface.
Summary of Inferences.
From the details, somewhat lengthy I fear, given above, we are, I think, entitled to draw the following conclusions:—
(1) That the serpentine of the Lizard was originally an intrusive igneous rock.
(2) That its intrusion was posterior to the metamorphism of the hornblende schist, the fragments of that rock included in it not differing materially from the main mass, though, of course, a little more altered.
(3) That on the western coast the serpentine has been broken through by several granite dykes.
(4) That on the eastern coast it has been broken through by the following igneous rocks:—
(5) That the gabbros appear liable to three forms of mineral change.
(b) The conversion of their diallage into hornblende by pseudomorphism, or rather by a recrystallization, not generally by paramorphism, and that in certain cases some olivine disappears in this process.(c) The conversion, more or less complete, of the olivine into serpentine, in which case the diallage appears little changed.
(6) That these trap dykes were very probably once all dolerites or basalts, and that the hornblende, which undoubtedly characterizes many of them, is a secondary product due, as above, to metamorphism of the original pyroxenic constituent.
(7) That the metamorphism of the serpentine was probably complete before the intrusion of any of the above rocks.
(8) That the serpentinous aspect of a rock is often rather illusory, being due to the presence of an extremely small proportion of that mineral; hence that statements about the conversion of ordinary pyroxenic or hornblendic rocks into serpentine require confirmation from microscopic examination.
Microscopic Examination of the Serpentine.
With the view of ascertaining, if possible, the original character of the rock which now constitutes the serpentine of the Lizard, I have had altogether twenty-one slides prepared from different localities; and Mr. S. Allport has kindly permitted me to use others from his collection, admirably cut by himself. I have also procured, for comparison, several others from foreign serpentines. A suspicion of the true nature crossed my mind in 1874, when examining a slide from the black serpentine near Cadgwith (no. 10). A specimen collected in 1875 at Coverack Cove rendered the conjecture a certainty; and a further confirmation was given by my visit to the Ariége, and by specimens collected at other Cornish localities, in 1876. In the following description I shall not take the slides in the order in which their localities are mentioned above, but group them so as best to tell their story.
|Fig. 8.—Portion of a Slide of Serpentine from Coverack Cove.
||Fig. 9.—Portion of a Slide of Lherzolite (Etang de Lherz, Ariège), showing the formation of Serpentine.|
The granulated part unchanged olivine; the rest rather fibrous serpentine.
(Magnified 50 diameters, about.)
Coverack Cove (no. 12).—This serpentine is often considerably decomposed; but my slide was cut from a well-preserved specimen. To the eye it is a dull mottled red-and-green rock, with ill-defined flakes in the latter part of a silky bronzitic mineral. With the microscope, we see that about two-thirds of the rock consists of a clear transparent mineral of a texture rather like frosted glass, traversed by reticulated veins of a golden-coloured (varying to greenish and reddish-brown) serpentine. These serpentine veins run together sometimes like matted roots, and apparently coalesce like the pseudopodia of foraminifera (fig. 8). As described in the gabbro above, there is the same deposit of iron peroxide (here reddish) parallel to the sides of the fibres, which in the larger patches of serpentine seem either to disappear or to be aggegated in dusky patches. On applying polarized light, the clear mineral filling the interspaces of the serpentine network proves, as we should anticipate, to be olivine, often very well preserved, and showing brilliant colours, while the serpentine (with crossed prisms) is dull milky white, with an indistinct fibrous structure in the strings, and often almost or quite dark in the larger patches. The other mineral, which is colourless with ordinary light, is now seen to be of more than one species. One part exhibits the characteristic cleavage of augite; this occurs in somewhat rounded grains; very thin veins of serpentine frequently traverse the crystal, following the lines of its cleavage-planes. This mineral does not seem the result of change of the augite, but to have formed in the cracks; and the generally open condition of the cleavage-planes rather bears out the idea. The colours of the augite are generally dull; but parts of a crystal occasionally show the usual rich colours, as if the present low tints were due to some subsequent chemical change. The other mineral, also colourless, and showing much the same tints with polarized light, has a peculiar silky aspect, and one well-developed set of close and slightly wavy cleavage- planes. I was at once struck with the resemblance of this mineral to the enstatite in my specimens of lherzolite, and on testing it find it to be orthorhombic, and so true enstatite. I found, however, a crystal or two of ordinary diallage. Besides the above ferruginous microliths, doubtless secondary products, there are a few larger opaque grains of iron peroxide, probably original constituents. I searched the slide for picotite, but could not be certain of any specimen, though one or two grains resembled an opaque variety of this mineral. One or two cracks, filled with fibrous serpentine, traverse the slide.
Mullion Cove (no. 8).—With transmitted light the slide appears to be composed of a great number of small subangular grains of a clear mineral, often associated with aggregated black or brownish dust, and generally clear, rather irregularly oblong crystals, about ⋅02 inch in greatest length, showing a prismatic cleavage, one set of planes being rather strongly developed, all lying in a base of yellowish green serpentine. On applying polarized light we find that, as before, the grains are olivine, only that the process of conversion has here advanced a stage further than in the last slide. The other crystals show moderately bright colours: many of them are rather dusky in parts, as if somewhat decomposed; and the patchy change of the colours with polarized light confirms this. As in the last slide, they seem to have been cracked after crystallization, as though they had been subjected to a strain; and serpentine has been deposited in the cracks. Some specimens resemble normal augite; others are nearer to diallage; other small crystals are enstatite. This quite bears out the macroscopic appearance of the rock, which is full of a mineral with a rather silvery lustre, but not exactly like ordinary diallage. Here, too, a few small grains very much resemble picotite. In the augitic mineral the planes of principal cleavage are approximately parallel in many of the crystals, and there are other indications of a flow or pressure structure. This is still more conspicuous in two specimens lent to me by Mr. Allport, one of which is cut transversely to the streaky structure. From these, especially the latter, it is quite clear that the structure is produced, not only by a general parallelism in the longer diameters of crystals of a prismatic outline, but also by a partial separation of the constituent minerals—the augitic constituent, the olivine with strings of serpentine, and the magnetite being each, more abundant in particular layers.
Gue Graze (no. 5).—The greater part of the slide closely resembles that of no. 8, except that there is a slightly larger proportion of serpentine, and the rock has not been quite so coarsely crystalline. The augitic constituent, however, is less conspicuous, and there are a number of small patches of a semitransparent, rather earthy-looking mineral from about ⋅01 to ⋅03 inch in greatest length, most of which seem to be coated externally by a reddish, film of iron peroxide. The augitic constituent appears, as at Coverack, to be partly normal augite, with some diallage, and partly enstatite. The crystals, however, are much smaller here; so that it is more difficult to determine them. The granular mineral, with polarized light, very closely resembles the pseudomorphic product after the felspars of the gabbros described above; and on close examination, some indications of an irregularly oblong crystal-like form may be detected. This rock, therefore, probably differs from the others in containing a small quantity of felspar; and I may state that it is the only case in which I have detected that mineral in the serpentines of the Lizard.
Lower Pradanack Quarry (no. 7).—The slide from this quarry, as might be expected from the general resemblance of the rock, corresponds very closely with that described above from Mullion Cove; the olivine is in much the same condition of conversion into serpentine. It contains also little dark brown and greenish brown semitransparent grains, which I feel certain are picotite. Here also a banded structure is indicated by a partial separation of the component minerals; enstatite, however, in this rock, rather predominates over the augitic constituent.
Rill Quarry (no. 4).—This slide shows a number of subangular transparent grains, generally edged with black, of clear, colourless, and rather fibrous crystals of longish oblong outline, and of scattered granules of magnetite, set in a very pale yellowish green serpentine, now and then stained brown. A few larger, irregular, imperfectly transparent granules seem to be picotite. On applying polarized light the subangular grains prove to be olivine, and the fibrous crystals enstatite (lengthened in the direction of the macrodiagonal). There is also a crystal about 0⋅2 inch long, of rather rhomboidal shape, which proves to be this mineral; it exhibits the usual cleavages, and is rather cracked and separated along those parallel to ∞ P. This rock contains small quantities also of an augitic mineral, but much less than the other.
Helston Road (no. 14).—This slide shows a very pale greenish serpentine, here and there a little clouded with a pale olive tint, with colourless granules of olivine, a good deal of a rather fibrous mineral in irregular aggregated grains, granules and dust of magnetite, and larger grains of dark picotite. One or two pale brownish grains show dichroism and a cleavage like that of hornblende.
On applying polarized light the serpentine shows the usual arrangement of doubly refracting meshes on an isotropic ground, not much of the unchanged olivine remaining to show colours. By testing the fibrous-looking mineral I have found some to be enstatite; but with the greater part the optic axial plane seems certainly not to coincide with the plane of principal cleavage, but to make an angle of about 10° with it; I believe it to be hornblende, not diallage. The bedded structure is indicated by a tendency to a banded arrangement in the component minerals and a frequent approach to parallelism in the longer diameters of prismatic-shaped crystals, and of the principal cleavage-planes. Part of the slide is traversed by an irregular vein filled with a steatitic mineral, which is almost transparent with ordinary light, and with crossed prisms shows a feebly doubly refracting granular structure. I regard it as a secondary product.
Goomhilly Downs (no. 15).—Another dull-coloured serpentine with a decided banded structure. Groundmass of very pale light-greenish serpentine, with numerous small angular or subangular grains scattered in it rather irregularly, many aggregated dusky clots, streaks and grains of magnetite, several clear brown crystals with a prismatic cleavage rather like hornblende, in parts somewhat dusky, with several semiopaque dusky patches of greyish and also greenish colour. Small grains of picotite occur, dull olive-brown and subtranslucent. With crossed prisms the field appears partly dark, partly occupied by a slightly fibrous, feebly doubly refracting variety of serpentine, of a dull bluish-grey colour,—chrysotile or some allied variety. The grains of olivine show the usual clear bright colours; the hornblendic mineral is not brilliant; and enstatite is seen. The dusky spots show a granular structure, something like the felspar pseudomorphs described above; but of their true nature I cannot be certain. The streaky structure is indicated both by a tendency to grouping in parallel bands on the part of the minerals, and by a parallelism in the longer diameters of the prismatic crystals and the streaks of magnetite.
Kynance Cove (no. 2).—A fine specimen kindly lent to me by Mr. Allport, intermediate in character between that of Gue Graze and Mullion. Some unaltered olivine, a good deal of augite or diallage (it seems generally, as at Mullion, nearer the former) and perhaps a little enstatite; there are also a few semi-opaque patches which may be altered felspar as at Gue Graze. With ordinary light the serpentine is of a yellowish colour; and there is a good deal of staining with a dark ruddy-brown iron peroxide. There is a steatitic band containing enclosures of a doubly refracting variety, some of which, are more like the pseudo-organic forms of Messrs. King and Rowney than I have elsewhere seen, and many augitic crystals.
Serpentine from Quarry behind Coverack Village (no. 13).—This rock, as might be expected from its appearance, is, when examined microscopically, a peculiar one: rather clear strings of pale gold chrysotile, including not unfrequently small granules of magnetite, divide the field into a number of irregular grains; each of these is subdivided by a network of yellower strings of the same mineral; these are crowded with an exceedingly fine black dust composed of microliths of various forms, mostly rather granular; and the interspaces, often roughly quadrangular, are occupied by a colourless mineral also partly filled with dust, probably magnetite. This, commonly, is aggregated, as described above, on the outer edge of the interspace; also, by bisecting the string, it often shows the position of the original crack. There is no enstatite or diallage recognizable; but some fair-sized scales of a very dark brown and all but opaque mineral, whose cracks are filled by colourless serpentine, may have been some such mineral. With crossed prisms the larger strings vary from opaque to a pale milky grey; the smaller, commonly, have a distinct golden tinge, and the interspaces are dark or very pale dull milky grey.
Dyke of Serpentine, base of Cliffs north of Kynance (no. 3).—This may bo regarded as an exceptional condition of the serpentine of this district; it is very compact in structure, streaky, and red. On placing it under the microscope we find that the slide consists partly of roughly parallel wavy branching bands of nearly clear serpentine, with a number of roundish grains, something like the eyes in a piece of knotted wood, of a ruddy brown tint, inclining now and then to purple; there appears to be little or no unaltered olivine, but a fair sprinkling of magnetite. On examining it with the two Nicols, we find that the clearer serpentine is the feebly doubly refracting variety described above, and that in most cases the browner spots are slightly doubly refracting. Hardly any of the olivine has escaped alteration; but any one who has made himself familiar with the other slides will have no difficulty in recognizing the characteristic structure in the replacing serpentine. I havo no doubt the rock shows a true flow structure.
Specimen from Junction near George Cove (no. 6).—This is also an exceptional variety, characterized by its exceeding compactness and parallel bands of chrysotile. Under the microscope both this mineral and the serpentine appear nearly colourless; but the latter contains many streaks and fibre-like aggregates of magnetite, while the former has only occasional minute microliths included between the fibres of chrysotile, and so roughly at right angles to the sides of the bands. Some dark brown subtransluccnt grains are probably picotite. In one band of the slide these are rather abundant. The banded structure, very conspicuous throughout this specimen, is parallel to the junction-face of the rock; and I regard it, like that in the Karak-Clews gabbro, as a pressure rather than a flow structure. On applying the two Nicols, the major part of the slide is seen to consist of the usual bluish-grey fibrous mineral; and the only difference between that which makes up the mass of the rock and that in the pale golden band is the greater regularity of crystallization in the latter. A few minute crystals show a more brilliant tint.
Carn Sparnack (no. 11).—Obviously an altered olivine rock, though but little of the olivine remains unchanged; the general aspect of the groundmass resembles no. 3, except that there is no tendency to a parallel structure, the irregular network of fine serpentine strings with the granules bordered within by iron peroxide being very distinct. The olive-green mineral, with submetallic lustre, which forms the green marbling, is all or almost all enstatite, though here and there probably is a little diallage; but alterations and staining make it difficult to be sure.
Blade Serpentine from near Cadgwith (no. 10).—The ground of the slide is a pale green serpentine, traversed by numerous strings forming the usual network; these are colourless, except that they are often darkened by the abundant deposit of fine magnetite dust. Several small grains of picotite occur, and a good many crystals of rounded exterior and platy structure—the same which is so conspicuous to the eye by its bright metallic lustre. The rock is clearly an altered olivine rock; and on applying polarized light we see that a few grains of that mineral yet remain unchanged. The metallic mineral is all or almost all enstatite, though part of it is rather altered, and converted into a kind of serpentine; and on comparing some of the specimens with that which I have described in lherzolite from Sem (Ariége), one cannot fail to be struck with the correspondence.
The Balk Serpentine (no. 9, two varieties).—These have the same general character as the Carn-Sparnack rock, both being mottled with olive-green; but the ground of the one is more or less red, of the other rather pale claret-colour. A detailed description is needless; both obviously are altered olivine rock, the chief distinction being that in the former the iron occurs rather in little patches and lines, often fringing the included grains (which are clear but isotropic, and so serpentine also), and is the red peroxide; while in the other it is more generally disseminated, giving a dusty look to the slide, and appears to be mostly the black oxide, while the grains are generally a dull pale green: both contain some picotite. The green mineral in both is wholly or almost wholly enstatite, though often a little altered into a serpentinous mineral.
Mr. Allport has lent me two specimens from the Balk, collected from a lower level than my own: one shows the banded structure very well, and contains many augitic crystals, with one which I think is certainly enstatite; the other has some olivine still unchanged, and a good deal of enstatite partially converted into serpentine.
A Specimen from the Quarry, south of Kynance (no. 1).—The dull claret-coloured rock with many greenish grey strings, can be now identified as an olivine rock, though very highly altered, and much more difficult to recognize than the others. The metamorphic process has been carried very far; the grains between the strings are stained a very pale greyish brown; and there is a faint concentric banding like that of agate. Some of the iron remains as black dust; the rest is the red peroxide. The enstatite or diallage (in small crystals always) is almost replaced by an opaque brown mineral deposited between its cleavage-planes, probably some oxide of iron. Polarized light, as usual, shows the strings to be doubly refracting, the grains isotropic.
I possess four other slides, cut from specimens from the east coast, two from the Balk and two from near Poltesco; of the precise localities I am less certain than in the case of the above. It is, however, needless to describe them, as they would add nothing to the above evidence, being more highly altered than the majority. Still it is clear they have been olivine rocks; and the glistening mineral also appears to be enstatite.
That the Lizard serpentine is an intrusive rock I have already shown in the earlier part of this paper; the microscopic examination confirms the idea, which both a priori chemical considerations and the general aspect of the rock suggest, that it is an altered olivine rock. The process, as is shown by what we have seen above and what I have described in my paper on the "Lherzolite of the Ariége," apparently consists in the gradual decomposition of the olivine by the action of slowly infiltrated water, during which the hydrous compound serpentine is formed, and the iron thus liberated is thrown down as either Fe2O3 or Fe3 O4, commonly the latter. Mr. Macpherson considers the reaction as a " loss of one fourth part of its (magnesia) base .... replaced by two molecules of water," a reaction which can be expressed by the formula
(4MgO 2SiO2) + 2HO = 3MgO 2SiO2 + 2HO + MgO.
The magnesia he conceives to have been removed. He does not, however, take account of the iron, which must have entered into the composition of the original olivine, replacing some of the magnesia; as this seems to remain, all that would be required would be the decomposition of the original mineral, and hydration of some of its constituents. Mr. Macpherson's paper, which I did not see till I had nearly completed my own, quite confirms my observations; and some of his figures might have been taken from my Cornish slides, so great is the resemblance. As, then, the Lizard serpentine also contains enstatite, an augitic mineral, and even picotite, all constituents of typical lherzolite, we are, I think, justified in regarding it as the result of an alteration (by the action of water, not necessarily at a high temperature) of a great mass of that rock.
The results of my examination of the older gabbro and dark trap dykes of Coverack, and of these Lizard serpentines, render me rather suspicious of the common statements about the metamorphism of ordinary pyroxenic and hornblendic rocks (i.e. those also containing a fair proportion of felspar) into serpentine. An olivine constituent, as a rule, changes readily into that mineral; enstatite also alters, though more slowly, as we have seen, and as is shown in Brögger and Vom Hath's description of the great enstatite crystals from Norway (Monatsber. d. k. Akad. der Wissensch. Berlin, 1876, p. 549). The augite yields also, though, I think, generally subsequently to the enstatite; but if it is an aluminous variety, I believe the silicate of alumina remains to form the rather shapeless dirty-looking microliths which I have often noticed under these circumstances. To remove a felspathic constituent of a rock or convert it into a magnesian pseudomorph would, I think, not be a common operation in nature.
In conclusion we may ask the question, Is there any clue to the age of the various igneous rocks noticed above? From what has been stated, we are, I think, justified in concluding:—
(a) That the sedimentary rock had been metamorphosed before the intrusion of the lherzolite.
(b) That the lherzolite had become serpentine before the intrusion of the gabbro and of the granite.
(c) That the dykes of dark trap are the latest rocks in this part of the peninsula.
I am not aware that there is any proof whether the granite or gabbro is the older rock. Sir H. De la Beche (Report, pp. 99, 173) speaks of the granite as cutting the gabbro; but, as the only evidence seems to be the supposed vein at the Balk, which I am convinced is not true granite, but only an altered sedimentary rock, we have, so far as I know, no clue.
The sedimentary rocks of the Lizard peninsula are probably about Lower-Devonian age. The great granite masses further north are probably late Carboniferous, or at any rate pre-Triassic. Doleritic outbreaks, such as might have been fed by the great gabbro massif, the form of which seems to suggest the probability that it was once deep below a volcanic cone, occur both of Devonian and Carboniferous age; and the latest known igneous action in "West Wales" is recorded by the Triassic basalts in and about Exeter. We may then venture to conjecture that the intrusion of the lherzolite was of later Devonian age, and that both the gabbro and the granite may belong to some part of that immense period when the Coal-measures of Central and Northern England were being deposited and afterwards denuded before the "New Red" series was formed. Possibly the dark traps may record the slight sporadic igneous action indicated by the Exeter basalts, and so be of Triassic age. The aspect, however, of some of these seems indeed to suggest that they solidified at no great distance from the surface, and so under conditions not very different from those at present existing. Hence it is possible that these veins, together with the phonolite of the Wolf Rock, are remains of a yet later period, the Miocene, during which volcanic action was so rife in Scotland, Central France, and Germany.
- Mr. Smyth, in his Presidential Address, Quart. Journ. Geol. Soc. xxiii. p. lxiii, calls attention to a distinct case of intrusion of the serpentine, and doubts the evidence of a passage into gabbro. He also mentions some interesting facts supporting the theory of a peridotic origin for the serpentine.
- See Mr. Hudleston's analysis, p. 928.
- These numbers are for reference in the microscopic descriptions.
- Until I come to the microscopic examination of the serpentine, I shall use this term as generic, to include either metalloidal diallage or enstatite.
- To save time, I shall use this term for the opaque mineral, obviously an oxide of iron, common in many of the rocks described below. As, however, it generally occurs in small rounded grains, I cannot be sure that it is always magnetite, since it may be ilmenite. Chromic iron has been found in serpentine near Cadgwith (Trans. R. G. S. Corn. ix. 99).
- The granite of Gue Graze may be regarded as fairly typical of the veins on the west coast; so that it is the only one which I have examined microscopically. It consists of quartz and felspar, both orthoclase and plagioclase. There is a scale or two of magnesia mica, a good many small clusters of minute granules of magnetite, with some fine needles and asbestiform fibres, some of which I think are tourmaline. There is an orthorhombic mineral of secondary origin here and there in the felspar, probably prehnite. One rather abundant microlith seems to be apatite; but there is probably another mineral also present. Altogether there are a good many included microliths and some minute cavities.
- Its hardness varies slightly, but is about 5.
- See Mr. Hudleston's analysis, p. 927.
- See Mr. Hudleston's analysis, p. 927.
- I have good reason to believe that this replacement of augite or diallage by a form of hornblende has taken place in several of the Welsh "green- stones." It is not precisely a paramorphic process like the formation of uralite, nor a pseudomorphic, because the form of the original crystal is often lost, but a replacement of a mineral by another, which, if not really a dimorphous form of the first, is very closely allied to it. I conceive the change to have been mainly brought about in the "wet way." I have seen the same change in the gabbros from Mont Colon (mentioned above), and the Matterhorn, also in the hypersthenite of Penig (Saxony). MM. Poussin and Renard have observed it in the Ardennes rocks (Roches Plutoniennes de la Belgique, &c. P . 69).
- Analyzed by Mr. Hudleston, see p. 927,
- Perhaps the structure erroneously described as a Laurentian organism, 'Nature,' xiv. 8. 68. may have been this.
- As it is described by most writers who have noticed it.
- Geol. Mag. Dec. 2, vol. iv. p. 59.
- These, with the others used in this paper, have been excellently prepared for me by Mr. Cuttell, 52 New Compton Street, Soho.
- My friend Mr. Main, our Lecturer in Chemistry, kindly ascertained for me the amount of Al2O3 in this rock, and informs me that the amount is only 0⋅4 per cent. This would mean rather more than 1 per cent, felspar; but then this amount of Al2O3 might be present in the pyroxenic constituents. So the determination is uncertain.
- It is obvious that very similar structures may result from tension and pressure; thus the fissile structure of certain igneous rocks (as I have shown in Quart. Journ. Greol. Soc. xxxii. p. 140) is like true cleavage. Professor J. D, Forbes regarded the banded structure of glacier-ice as the result of differential motion, like that of slags. Professor Tyndall considers it a pressure structure; while in some igneous rocks a pseudo-foliation seems to result, now from pressure, now from tension.
- See Mr. Hudleston's analysis of this rock, p. 925.
- It may be interesting to mention that I have had a fragment sliced from that very dark serpentine with calcareous veins called Genoa marble, which has been used in the decoration of the Hall of the Fitzwilliam Museum, Cambridge. It, too, is a highly altered olivine rock, with small crystals of rather altered enstatite, and veins of dolomite (?).
- "On the Origin of the Serpentine of the Rhonda Mountains," Ann. Soc. Espan. Hist. Nat. vol. iv. pt. 1.
- From what has been said it will be seen that most of this is the variety with a metallic lustre, or true bronzite, though it has often been somewhat altered by the action of water, and is now a hydrous bronzite. I have never seen true bastite in the Lizard serpentine, but believe I have detected it in the Clicker-Tor rock.
- In addition to the rocks mentioned above, the change of olivine into serpentine is well exhibited by the serpentine of Elba. One also, a serpentine with bastite from Sta. Catarina, is very like the Clicker-Tor rock described by Mr. Allport, Quart. Journ. Geol. Soc. vol. xxxiii. p. 422.