The Parochial History of Cornwall/Volume 1/Introduction to the Geology of Cornwall
AN INTRODUCTION
TO
THE GEOLOGY OF CORNWALL.
BY Dr. BOASE.
The geological notices inserted in this Work, are necessarily so brief and so unconnected, on account of the alphabetical arrangement of the parishes, that it is desirable that a few introductory remarks should be made that this defect may be in some measure obviated.
Cornwall does not possess a great variety of rock formations, being composed of primary and transition or intermediate groups, covered here and there with deposits of gravel, sand, and clay, which belong to the modern epoch. But this limited range of formations is more than compensated for by the great facilities which the geologist here enjoys in his investigations; the very extended line of cliffs which deeply indent the Cornish shores proffer numerous and instructive sections; and the vast mining operations have brought to light most important phenomena. Indeed, no country of equal extent can, in these combined advantages, be compared with Cornwall.
On a general view of this country, the surface exhibits two systems of valleys; one running parallel with the central ridge, which is highest near Launceston, and gradually, but irregularly, declines till it terminates at the Land's End, in cliffs about a hundred feet in height; the other intersecting the longitudinal valleys at right angles; and as all the intermediate hills are more or less rounded, the country has an undulating appearance in both directions, not unaptly compared to the waves of the sea. Through these two systems of valleys, the rivers flow, seeking outlets into the sea by the nearest continuous descent; sometimes they effect this along the longitudinal, at others through the transverse valleys, receiving tributary streams on either hand from the lateral valleys, or curved hollows which they intersect. But sometimes the river is diverted in its course more than once, in consequence of the concavity of a cross curve rising above its level or by that of the other system descending below this point by which irregularity the stream is compelled to flow along the more favourable drainage; thus the Camel rises about four miles from the sea, in the moors near Davidstow, and flows more than twenty-five before it empties itself into the sea at Padstow. In the hollows of these valleys the drainage is often sufficiently obstructed to produce marshy ground; but seldom of such extent as to deserve the name of a lake.
As regards the soil, on the high grounds it is frequently very shallow and barren; but in the valleys it is very productive, and here and there is well wooded, more particularly on the southern coast. On the northern coast, in the vicinity of the sea, it is very sandy, owing to the light testaceous sand of the shore being carried inland by high winds; in this manner no inconsiderable tracts of fertile land have been devastated.
Let us now turn our attention to the internal or geological structure of Cornwall. The highest parts of the central ridge, already alluded to, are composed of granite, which occurs in the form of four large insulated patches, so disposed at nearly equal distances from each other as to resemble a chain of islands extending from Launceston to the Land's End, that is in the direction of N.E. and S.W. On the same ridge, but rather parallel than continuous thereto, is the great granitic group of Dartmoor in Devon, the whole of which is sometimes called the Ocrynian Range.
The granitic patch of Dartmoor is by far the most extensive, being nearly twenty miles in diameter; that of Launceston is ten miles in length by six or seven in breadth at its widest part; and its most elevated hills, Rough-tor and Brown-Willy, do not much exceed 1,300 feet in height. The granitic rocks of this patch, like that of Dartmoor, are not much exposed by artificial excavations, so that their varieties cannot be easily examined; the weathered blocks, which on the summit and sides of the hills form tors and detached masses, consist of the hard or siliceous varieties of the common and fine-grained granites, such as have withstood the action of the elements. One of the most curious of these tors is the Cheese Wring, near Liskeard, a pile of single blocks, each being larger than the one immediately beneath. Proceeding along the ridge towards the Land's End, the next, or St. Austel patch of granite will be found, much less than the last, hut more interesting both in a scientific and a commercial point of view. In addition to the kinds of granite already noticed, it contains beds of talcose granite, or protogine, which by its decomposition furnishes that valuable substance china clay or kaolin, many thousand tons of which are annually exported for the potteries. The third, or Redruth patch of granite, affords many varieties of this rock, and has been well explored by numerous mines which have been productive in both tin and copper ores, affording also to the mineralogist a great variety of rare specimens. But the fourth, or Land's End granite, is by far the most important to the geologist, for the land becoming here very narrow, the sea has produced cliff-sections, both in the granite, and also at its point of junction with the slate, exhibiting many interesting phenomena. Among these, the veins of granite in the slate are beautifully displayed, and have long been a great attraction to geologists; but the modes in which these rocks meet and unite, are not less deserving of notice. But for a detailed description of these curious facts, the reader may be referred to my "Treatise on Primary Geology."
Besides these four principal patches of granite, there are four others: 1st. that of Kitt Hill, near Callington; 2nd. that of Tregonning and Godolphin, near Helston; 3d. that of Cligga Point, near St. Agnes; and lastly, that of the celebrated St. Michael's Mount. The two first are of some size, and at the surface are both subdivided near the middle, by the overlapping of the slate: the other two are small, but very interesting to the geologist.
It is worthy of remark that the fertility of these granitic groups gradually increases as they diminish in elevation; and it is a curious but not surprising coincidence, that the number of parish churches thereon follows the same order: thus on the eastern and most extensive tract of granite, near Launceston, there is only one church; on the next there are three; on the Redruth patch six; and on the Land's End granite no less than nine, within a space considerably less than that of the eastern tract.
Each of these insulated groups of granite is surrounded by schistose rocks, the layers of which, on all sides, incline from the granite at various angles, from 20° to 40°. Although these groups are thus separated from each other by the slate at the surface, yet it is the general opinion that they gradually approach beneath, until they are all united into one and the same mass—the intermediate hollow spaces (the valleys, as it were, between the granitic mountains) being occupied by the slate. It might, however, be maintained that the granite is imbedded in the slate, in large rounded masses, which would also account for the former rock underlying the other, within the limits of mining operations: and such an opinion would derive some support from the fact, that small insulated masses of granite in the slate are not of unfrequent occurrence.
The slate formation consists of two very distinct groups, when the most characteristic rocks are alone regarded; but it is not easy to trace the boundary between them, as the contiguous rocks appear to pass gradually into each other. These groups have one character in common: viz. that they consist of several distinct kinds of rocks, each genus of which respectively may be subdivided into schistose and compact rocks; the latter are imbedded in the former are more crystalline and often contain clusters of their constituent minerals: so that the complicated composition of these rocks is made known by Nature's own analysis.
The group next the granite is primary, the more remote one is transition, or as they are termed in the following pages the porphyritic and calcareous series of the slate formation. The series next the granite is characterised by its beds of porphyry, and by its abounding in veins of tin and copper ores; the other series by the frequent occurrence of calcareous spar and strata of limestone, with occasional organic remains, and by its being sparingly metalliferous, containing no tin-ore, but being productive of lead and antimony. It is within the last series that the magnesian or serpentine rocks occur, and which in the Lizard district are developed to a large extent, forming one of the most interesting geological features of Cornwall. Those who are desirous of more information concerning this slate formation, will find a copious account in the fourth volume of the Transactions of the Geological Society of Cornwall. It is there proposed to give these rocks, whenever they possess well-marked characters, distinct names, and not to refer a great variety of rocks to the vague and indefinite genera of clay-slate and greywacké. An accurate and more ex-tended nomenclature would have rendered the following notices much more intelligible.
In describing the general features of Cornwall, we must not omit to mention the metalliferous veins the great source of its commercial prosperity and the channel through which much curious information has been obtained, concerning the structure of the earth. These veins traverse indifferently both the granite and the slate, but are most abundant in the latter rock, in the vicinity of the granite. The general direction of the tin and copper veins (or lodes, as they are provincially called) is nearly E.N.E. and W.S.W., and they are crossed by another system of veins, nearly at right angles, which are not commonly metalliferous, and when they do contain ores these are often of lead, antimony, silver, and other metals. This is not however a general rule, for in the parish of St. Just, Penwith, the tin and copper ores occur in the cross veins. The course of the veins is not straight, but they are always more or less undulating, both in their direction and in their dip or underlie. Various interesting phenomena result from their meeting with and intersecting each other, known to the miner under the names of heaves, throws, and slides. This subject is replete with curious facts, but would require considerable space for their enumeration; the reader must therefore be again referred to the work already alluded to on "Primary Geology."
Before concluding this brief sketch, a few words must be said on the beds of clay, sand, and gravel which cover the low grounds, both on the granite and on the slate. These may be resolved into two kinds: 1st. those resulting from the decomposition of the rocks, and which are in their original position; and 2nd. those which are not in situ, affording by their appearance, composition, and position, evidence of their having been transported. These deposits belong to three distinct periods, and alternate with terrestrial and marine remains, according to their situation in respect to the sea-level at their respective epochs. Each of these transports was accompanied, or followed by, a change in the sea-level: the last corresponds with that now existing; the previous one, by the elevated ancient beaches, indicates that the sea was about fifty feet above its present level; and the earliest transport, being covered with sub-marine forests, shows that the sea was at that epoch at least fifty feet lower than it now is. (See 4th vol. Geol. Trans. of Cornwall, p. 466, et seq.) In the lowest or oldest deposit the tin-ore (stream-tin) is found in the form of sand and gravel mixed with earthy substances: it affords the purest kind of tin, known in the market by the name of grain-tin; and appears to be the source from which the ancients derived all their tin. The notices under each parish will furnish more particulars concerning these interesting deposits.