Transactions of the Geological Society, 1st series, vol. 4/On the Hill of Kinnoul
[Read March 4th, 1814.]
By the J. Mac Culloch, M.D. F.L.S. president of the geological society, Chemist to the Ordnance, Lecturer on Chemistry at the Royal Military Academy, and Geologist to the Trigonometrical Survey
IN transmitting to the Society the specimens from the hill of Kinnoul which accompany this paper, I have thought it necessary to enter into a description somewhat detailed, of appearances attended with considerable interest, and involving some difficulties. We are yet, it is to be feared, in want of a theory capable of solving all the cases which the increased activity of geological research is daily bringing to light. It is among difficult and unexplained phenomena that we are to seek for the stimulus which will lead us to pursue those researches on the multiplication of which alone we can hope to found a true system; and it is to a salutary distrust of the all-sufficiency of any hypothesis, that we must look for protection from its paralyzing effects.
The hill of Kinnoul, from which the specimens now before the Society were selected, has been frequently visited by geologists and mineralogists, more perhaps with a view to the minerals which the rock contains than for the purpose of examining those remarkable geological phenomena which it exhibits. Except the account of it in the travels of Faujas de St. Fond, I know not that any description of this hill has been laid before the public. The peculiar opinions of that author are well known, and I believe that in this country it is not necessary to enter into any refutation of his conclusions. As far indeed as the appearance of the trap rocks and their peculiar mineralogical character are concerned, the Huttonian theory offers an explanation better able to fulfil the requisite conditions than his hypothesis. I shall therefore decline entering into a comparative statement of systems so well known, or investigating the solidity of the examples which the French geologist has adduced from Great Britain. Although never formally discussed, the arguments and objections are familiar to most of those who have engaged in geological researches, and a full examination of his individual cases would lead into a dissertation foreign to the purpose of this notice, I may therefore briefly remark that the chief part of the hill in question consists of trap, which, like the other rocks of this family that alternate with secondary strata, is known in the Wernerian nomenclature, by the term of flœtz trap. I do not pretend to name its rank in the several formations enumerated under this general title, as I much doubt if the assigned characteristics are constant. If they are universal it will not be difficult to give it its place in the system when I have described its features and connections.
The phenomena which are most interesting in this hill render it necessary to take a range somewhat wide before describing the rock itself, without which we should be unable to trace its connection with the neighbouring rocks and the surrounding country.
Those who are acquainted with the mineral geography of Scotland know that a great portion of its northern district is separated, in many places very accurately, by a tract of breccia from the secondary rocks which occupy its middle portion. An irregular line drawn through Troup head, Dunnotter, Blair gowrie, Delvin, Creiff, Tillycoultry, Callander, Aberfoyle, Drymen, and other almost nameless places to the westward, marks the range of this breccia, leaving on the north side all those rocks distinguished by the name of primary, with many of those which bear the name of transition, and being followed to a certain distance southward by the usual series of sandstones and other secondary strata. In the middle of this secondary tract arises the hill of Kinnoul, forming the westernmost part of a long irregular ridge which extends from the north of Dundee to Perth, where it terminates. Through part of this course it exhibits an abrupt elevation to the south, subsiding northward by a more gentle declivity into the great plain of Stormont and Strathmore. Its visible boundary to the south is the alluvial plain of Gowrie, while to the north the red sandstone and that breccia which accompanies or precedes the sandstone, form the only rock for a distance of many miles, till we arrive at the mountain schistus. I am not acquainted with the connection of this ridge at its eastern end.
The height of Kinnoul, (that part of the ridge which I purpose to describe) is 600 feet above the plain of the Tay, and it occupies a length of a mile or thereabouts, exhibiting many abrupt faces in a state of constant ruin and degradation, which have thus formed a rapid slope at the feet of the precipices.
The rock itself contains many of the most remarkable varieties belonging to the tribe. It will be sufficient to give a general description of them, as no purpose could be served by an attempt to define rigidly either the spaces which they respectively occupy, or the order which they follow, circumstances which are subject to such variations as to obey no general rules. In some places a black basalt may be observed, but it is every where amorphous and approaches here and there to the most ordinary kinds of compact greenstone. This variety is the least abundant. It occasionally assumes a brownish or reddish colour, retaining its original compactness and uniformity of texture, and thus forming a member of the trap family, to which no distinct name has been assigned. This becomes at times porphyritic, containing numerous minute crystals of felspar imbedded in a blackish or brown basis, and forming a showy variety of trap porphyry. The felspar, as far as I have observed, is always opaque. Sometimes the rock is both porphyritic and amygdaloidal, but perhaps the greater part of the whole hill consists of a rock merely amygdaloidal, the base of which is either the well characterized basalt above described, or that variety which is less marked, or else a rock intermediate between this latter and indurated clay.
The character of the amygdaloid varies considerably according to the substances which it contains, and these are green earth, calcareous spar, quartz, and chalcedony.
Green earth or chlorite, is the substance which perhaps occurs most frequently, and it is indeed that by which the trap of Kinnoul is particularly distinguished from almost all the rocks of this tribe in Scotland. The nodules vary but little in size, and although they are occasionally met with of the bulk of a filbert, they are generally little larger than a grain of mustard. But they vary much in colour, assuming all shades from greenish black to the brightest verdegris green. They differ equally in texture, being sometimes minutely powdery, at other times of a large scaly fracture, like the chlorite which occurs in primary rocks. It is this substance which Faujas has called steatite; no real steatite being found in the trap of Kinnoul, although it is common in that of the island of Sky.
When calcareous spar forms the amygdaloidal nodule, it is commonly invested with a coating of chlorite. These nodules are generally of a small size, and of a spheroidal or ellipsoidal form. Their fracture is crystalline, and they are often perfectly transparent, but in all cases where they are in immediate contact with the trap they possess no external crystallized form. It is only when in contact with quartz or agate that they exhibit an external as well as an internal appearance of crystallization. In other words, they appear to bear the impression of the basalt when they are in immediate contact with it, while on the contrary, when in contact with quartz or agate, they impress their forms on these. Calcareous spar thus assists in forming a compound nodule not uncommon in these rocks. The spar is sometimes crystallized at liberty in a cavity of quartz or agate; at other times it is closely invested by quartz, forming the center of a solid nodule, or is dispersed in small crystals irregularly scattered through the whole pebble. Both these substances occasionally retain their perfect characters, even when in immediate contact; while in other instances the quartz for some small distance in the vicinity of the calcareous crystal is converted into chert or into agate.
Quartz is found among these rocks in the usual variety which it exhibits when it is an inmate of trap. In its simplest form it is a crust investing a cavity, and terminating interiorly in assemblages of crystals of various sizes and of various colours, white, brown, and amethystine. The exterior quartzy crust often puts on the character of chert, sometimes that of chalcedony; not infrequently the chalcedony appears in the form of a stalactite, of which the several icicles are encrusted with beautiful and crowded assemblages of crystals. These stalactites of crystals depend, as is usual in stalactites, from the upper parts of the cavity. The nodules of this description are of various magnitudes and often of a foot or more in diameter, and, like the smaller ones, are shut in and surrounded by the solid rock on all sides. It is more common for the boundary of the quartz in the immediate vicinity of the trap to be formed of various zones of coloured chalcedony. The quartz in this case assumes a peculiar well-known aspect, and is called in the Wernerian nomenclature, amethyst, although most commonly of a white or watery appearance. Different zones of chalcedony and quartz will even at times succeed each other in the same nodule.
But the imbedded mineral from which this place has acquired its greatest celebrity, is the agate, or coloured chalcedony, with which it abounds, but which it possesses only in common with many other places in Scotland. The nodules of this substance vary exceedingly both in size and colour, and their general aspect is much too well known to need any description; yet a few circumstances respecting them deserve to be considered, as they involve difficulties which it is incumbent on any general theory of the formation of these rocks to explain. Their external surfaces, I believe invariably, bear those marks of indentation by the surrounding rock which determines their posteriority of formation, or at least their posteriority of induration, to that of the rock in which they are imbedded. Their internal structure is also most commonly zoned, with irregularities corresponding to those of the external boundary; but in some cases they exhibit a complication of structure, which as it cannot be well described in words, I have ventured to represent in the accompanying sketches. In the first example, a stalactite may be observed occupying a portion of a hollow cavity, marking as in the case of the larger quartz cavities described above, the gradual deposition of siliceous matter by infiltration. The change of disposition in the zones in the figures N° 2 & 3, seems to be the result of a similar process, the horizontal parallel lines appearing to be the stalagmite formed on the bottom of the cavity, and proceeding from below upwards in successive additions, in a way similar to that which may be observed in the complicated chalcedonies of Faroe. The peculiarity observable in the fourth example can be explained only by a similar supposition.
Such are the circumstances of chief importance which occur in the trap nodules, and which, although by no means limited to this place, appear deserving of notice, on account of the intimate relation they bear to any general theory. It is incumbent on that hypothesis which explains one of these difficulties, to explain the others also, or at least to require nothing which shall exclude the means of explaining them. It is almost needless to say that I here allude to the different explanations which the aqueous and the igneous theories of the origin of trap have given of the imbedded nodules. Each hypothesis has its difficulties when it refuses to yield ground to its antagonist; yet, is each perhaps incapable of exclusively and clearly explaining the appearances in question. The stalactitical forms which line the hollow cavities, can perhaps have resulted only from watery infiltration, yet the existence of the previous cavities can scarcely be accounted for by that hypothesis which considers trap as a deposit crystallized from an aqueous solution of earths.
Different substances are found occupying veins as well as nodules in this rock; among these, calcareous spar and quartz are the most common. Sulphate of barytes occurs more rarely, as do chert and agate, the latter of which often exhibits the same zoned appearance in the veins which it does in the nodules.
The great abundance of chlorite in this trap might have rendered it probable that the rare mineral heliotrope was an inmate of it: accordingly I have laid before the Society some small specimens found there by Lord Gray's workmen in removing earth at the foot of one of the precipices. It forms veins of different dimensions in the fragments of rock, with which it is intimately united: it is of a finer green colour than that which is found in Rum, but none of the specimens which I procured contain the red spots for which this mineral is principally valued. It sometimes is associated with a green quartz, coloured apparently by the same material, the more transparent parts having the aspect of plasma, which not improbably owes its colour to chlorite. I did not succeed in finding this mineral in the face of the rock from which these specimens appear to have been detached, but among the agate pebbles there to be seen I obtained some which present appearances illustrative of its composition. These pebbles are frequently incrusted with a coating of chlorite, and in those to which I allude the chlorite penetrates the external part of the agate to the eighth of an inch, so as to convert the outer crust into heliotrope. I may here also add that I have observed this mineral among the agates found in Ayrshire, and that it occurs in the island of Mull, where it forms spheroidal nodules in basalt, so that it is not rare in Scotland. When I say that zeolites are not found in Kinnoul, I ought to add, that I picked up one loose and had specimen of red stilbite.
On the top of the great mass of trap which I have now described, there is to be seen a portion of a bed of conglomerate, consisting of trap pebbles imbedded in a cement of the same nature, a rock improperly designated by the name of trap tuff. The origin of this rock is not easily explained, but I must defer the remarks that might be offered on this subject to some future opportunity.
The last and most remarkable circumstance occurring in Kinnoul is that of an extensive range of those junctions with other rocks which are supposed by many persons to demonstrate the igneous origin of trap.
About the middle of the broken face of the hill a portion of a stratum of sandstone is to be observed entangled in the mass of the rock, and at the same time much broken and bent. But there is nothing in its appearance so very different from the other instances of this fact which have been frequently described, as to call for a particular detail.
Proceeding from this point eastward another set of junctions is seen, of which I have not met with any resemblance in other places, and which offers some anomalous appearances. The stratified rock which is here conjoined with the trap, is perfectly similar to the best characterized graywacké slate, of a fine grain and greenish hue. It is easily separated into flakes, and on being broken, exhibits slender scales of mica. A remarkable change of its aspect may be seen where it approaches the trap, and before it comes into actual contact with it. If the weathered surface only be examined, it shews a series of solid laminæ alternating with, or rather graduating into other laminæ which have a spongy appearance, or abound with open cavities similar to those of weathered amygdaloids. These cavities increase in number and magnitude towards the middle of the spongy laminæ. The same appearance of pores or cavities is to be observed at most of the points of contact where the graywacké is much contorted and mixed with the trap. A good illustration of this appearance may be given by comparing it with that assumed by the burnt micaceous schist sometimes found in the walls of the vitrified forts, and which by the action of the fire on some of its laminæ is swelled up and thickened, apparently from the disengagement of aeriform matter. When, however, this stone is broken, the cavities are found to be filled by calcareous spar, and exactly resemble those which occur in the trap rocks themselves.
I remarked above, that the schist is often much contorted and mixed with the trap. This mixture affords a great variety of appearances, a variety so great that no limited series of specimens, nor any drawing less than that of the whole face of the hill itself, could give an adequate idea of them. The accompanying sketch exhibits one of the most general. In many parts innumerable detached fragments may be seen imbedded in the trap, the whole of the schist displaying a greater disorder and confusion than the sandstone does in any of the instances hitherto described. In a few places another singularity may be remarked. Veins of different dimensions, and ramifying in different directions, are to be found traversing considerable portions of the rock, and in some instances terminating in the schist, with which they are continuous. As the schist graduates into the vein, its laminated texture disappears, but in other respects there is an identity of composition between the vein and the laminated schist, at least for a considerable space. The same loss of the laminated texture of the schist takes place wherever, from its proximity and intermixture with the trap, it is materially perverted from its original even direction. In other respects the identity of substance is here, as in the other case, preserved, nor is any decided line drawn to determine the discontinuation of the laminated structure, either in the progress to contortion or ramification. However contradictory it may appear at first sight, that the same substance should exhibit both the character of a laminated and bedded rock and that of a vein, the state of the incurvated and contorted masses may perhaps by analogy assist in solving the difficulty. It is not an unreasonable supposition, that a rock in a condition to be bent and incurvated, should by a peculiar application of external force admit of that more continued prolongation which in certain circumstances would produce a vein. The analogous incurvations indeed, and the evident prolongations which take place among the laminæ of mica slate, and in the contorted veins of granite and quartz which traverse them, offer cases of parallel difficulty. If the solution which I have offered be incompetent to this purpose, it is only one more added to the numerous unexplained phenomena which are to be found attending the subject of geology wherever we turn our regards: for we can then look to neither of the prevalent hypotheses for an adequate explanation of this case; the mechanical structure of the schist as indicated by the parallel disposition of the mica, combined with the want of similar mechanical arrangement in the vein, offering a difficulty to the one as great as it is to the other. I speak of a mechanical arrangement in the schistose rocks as if it were admitted by all, because it appears a circumstance attending on many of these rocks, as perfectly demonstrated as any thing of which we have not actually witnessed the creation, can be demonstrated to our senses.
For the same reasons I speak without hesitation of the displacement, fracture, and incurvation of the graywacké which is imbedded in the trap, and in so doing it is not my wish to speak the language of an hypothesis, but to describe a fact, in such terms as can alone convey an adequate notion of the appearances to a mind divested of all hypothesis. If Nature has really produced imitations of mechanical arrangement by processes unknown to us, it is to be wished that the mode in which they have been produced may be shown, either by means of experiments, or by analogies drawn from that science of which the laws regulate the great proceedings of Nature as they do the narrow operations of our own laboratories. Till that be done, it is not only legitimate, but it is indispensable for the purposes of accurate reasoning, to describe facts by their most obvious analogies where we cannot pronounce on their nature, to call that a mechanical arrangement in Nature which bears a resemblance to mechanical arrangement in the products of art, and to consider that only as the result of chemical action in Nature, which is imitable by chemistry under the direction of art, or has been demonstrated to be in other cases the result of chemical laws.
It appears then that at Kinnoul portions of schist are found presenting an obscure appearance of connection with the red sandstone. It also appears that this rock exhibits but a small portion of a bed, instead of that great and extensive continuity in which it is generally disposed. And it is further seen that this portion is entangled and almost surrounded by a mass of rock of a peculiar aspect, which bears no mark of stratification or of mechanical arrangement, and that it is much bent and contorted, so as to be irregularly intermixed with the unstratified mass. Lastly, it is to be remarked, that appearances of fracture as well as contortion occur in the stratified rock; that veins pass from it, and that fragments of it are dispersed throughout the unstratified one; as far at least as a judgment can be formed from the only view we can obtain of the imbedded portions.
It is difficult to see on what other ground these and similar appearances can be explained than on that of motion; the action of the unstratified on the stratified mass, and that stratified mass existing in two different conditions, a state of softness capable of extension, and a state of hardness capable of fracture. Further than this the facts visible at Kinnoul do not perhaps bear us out, and beyond this point it is not my design to venture, since the simplest record of facts is perhaps the most important service which can now be rendered to geology.
Before concluding this paper I must add, that at the eastern end of Kinnoul a sandstone breccia may be observed, apparently lying below the trap, and similar to that which is the lowermost of the secondary strata throughout the whole of this district.
- Pl. 10. N° 1.
- Pl. 10. N° 2 & 3.
- Pl. 10. N° 4.
- Vide Plate, 11.