Illustrations of the Huttonian Theory of the Earth

by John Playfair

Note xii. § 43.

Elevation and inflection of the strata.

187. The evidence of the different formation of the primary and secondary strata, and of the changes which the former have undergone, is best seen at the points where those strata come into contact with one another. Dr Hutton was not the first who observed these junctions, though the first who rightly interpreted the appearances which they exhibit. He has mentioned observations of this sort by De Luc on the confines of the Hartz; by the author of the Tableau de la Suisse, at the pass of Yetz; by Voight, in Thuringia; and Schreiber, at the mountain of Gardette.[94]

[94] Theory of the Earth, vol. i. p. 410 to 453.

The leading facts to be remarked, are,

I. The vertical or very upright position of the primary or lower strata.

II. The superstratification of the secondary, in a position nearly horizontal, so as to be at right angles to those on which they rest.

III. The interposition of a breccia between them or, as happens in many cases, the transition of the lowest of the secondary beds into a breccia, containing fragments sometimes worn, sometimes angular, of the primary rock.

This last is a phenomenon extremely general, and all our subsequent information confirms Dr Hutton's anticipations concerning it. "It will be very remarkable," he says, "if similar appearances, (such as those of the breccia described by Voight,) are always found upon the junction of the Alpine with the level countries."[95] Saussure, in a part of his work, not published when Dr Hutton wrote this passage, has attested the generality of the fact with respect to the whole Alps, from the Tyrol to the Mediterranean: "Un sait que l'on observe sans aucune exception, ce sont les amas de débris, sous la forme de blocs, de breches, de poudingues, de grès, de sable, ou amoncelés, et formant des montagnes, ou des collines, dispersés sur le bord exterieur, ou même dans les plaines qui bordent la chaine des Alpes."[96]

[95] Theory of the Earth, vol. i. p. 448.

[96] Voyages aux Alpes, tom. iv. § 2330.

This passage is perfectly decisive as to the generality of the fact, that the Alps, from the Tyrol to the Mediterranean, are bordered all round by pudding-stones or breccias. At the same time, it is necessary to remark, that M. Saussure, by enumerating loose blocks and sand, along with pudding-stones, breccias and grit, confounds together things which are extremely different, and which have had their origin at periods extremely remote from one another. The consolidated rocks of breccia, pudding-stone and grit, though they are indications of waste, have received their present character at the bottom of the sea: the loose blocks of stone, the sand and gravel, on the other hand, are the effects of the waste now going forward on the surface of the land, and are the materials out of which rocks of the three kinds just mentioned may hereafter be composed. If so skilful a mineralogist as Saussure is guilty of such inaccuracy, it must be ascribed to the confusion necessarily arising from the system which he followed, and not to his own want of discrimination.

188. The same phenomenon, of a breccia circumscribing the primary mountains, is met with in Scotland; and the Grampians, wherever they are bounded by secondary strata, whether on the south or north, afford examples of it. The breccia generally consists of the fragments of the primary rock, most commonly rounded, but sometimes also angular, united by a cement of secondary formation, and the whole disposed in horizontal beds. It was on the constancy of this accompaniment of the primary strata, and on the great quantity of highly polished gravel often included in these breccias, that Dr Hutton grounded the hypothesis of the double raising up and letting down of the ancient strata. See § 43.

189. As the spots where the primary and secondary rocks may be seen in contact with one another are of great importance in geology, and present to the senses the most striking monuments of the high antiquity and great revolutions of the globe, it may be useful to point out such of them as have been observed in this island. To those which Dr Hutton has described, I have a few more to add, the result of some geological excursions, which I made in company with the Right Honourable Lord Webb Seymour, to whose assistance I have been much indebted in the prosecution of these inquiries.

190. The most southern junction which we observed is at Torbay, where the ancient schistus which prevails along the coast, from the Land's End to that point, receives a covering of red horizontal sandstone, the same which composes the greater part of Devonshire. The spot where the immediate contact is visible, is on the shore, a little to the south of Paynton; and one circumstance, which among many others serves to distinguish the different formation of the two kinds of rock, is, that the schistus, which is elevated here at an angle of about 45°, is full of quartz veins, which veins are entirely confined to it, and do not, in as far as we could observe, penetrate into the sandstone, in a single instance. It is probable, that on the north shore of the bay, the same line of junction is visible: we saw it at Babicomb Bay, still more to the northward.

191. From this place, the secondary strata of different kinds prevail without interruption, along the coast of the British Channel, and of the German Ocean, as far as Berwick upon Tweed, and for some miles beyond it. The sea coast then intersects a primary ridge, the Lammermuir Hills, which traverses Scotland from east to west, uniting, near the centre of the country, with the metalliferous range of Leadhills, and afterwards with the mountains of Galloway. The section which the sea coast makes of the eastern extremity of this ridge, is highly instructive, from the great disturbance of the primary strata, and the variety of their inflections. The junction of these strata with the secondary, on the south side, is near the little sea-port of Eyemouth, but the immediate contact is not visible.

On the north side of the ridge, the junction is at a point called the Siccar, not far from Dunglass, the seat of Sir James Hall, Baronet. By being well laid open, and dissected by the working of the sea, the rock here displays the relation between the two orders of strata to great advantage. Dr Hutton himself has described this junction; Theory of the Earth, vol. i. p. 464.

192. From the point just mentioned, the secondary strata continue as far as Stonehaven, where the southern chain of the Grampian mountains is intersected by the sea-coast. Here a great mass of pudding-stone appears to lie on the primary strata, but their immediate contact has not been observed.

193. Going along the coast toward the north, the next junctions which we saw were on the shore, one near Gardenston, and another near Cullen, in Banffshire. The latter is very distinct; it is about a mile to the westward of the rocks called The Three Kings, where a red sandstone, the lower beds of which involve much quartzy gravel, lies horizontally upon very regular, upright, and highly indurated strata. Some of these strata are micaceous, and others of the granulated quartz, mentioned in § 153.

194. This last is, I believe, the most northern junction which has been observed in our island. The western coast furnishes several more, which however are not all visible. The line of separation, between the primary schistus of the Grampians and the sandstone which covers it, is intersected at its western extremity by the Frith of Clyde, not far from Ardencaple in Dunbartonshire. The two kinds of stone can be traced within a few yards of each other, but not to the actual contact: the beds of sandstone nearest the schistus form as usual a breccia, loaded with fragments of the primary rock. The secondary rock, which begins here, continues for about fifty miles south, to Girvan in Ayrshire, where the primary schistus again rises up, but is not seen in contact with the secondary. It extends to the Mull of Galloway and the shores of the Solway Frith.

The Isle of Arran, however, not far distant from this part of the coast, contains a junction at its northern extremity, where secondary strata of limestone lie immediately on a primary micaceous schistus. This is described by Dr Hutton, and was the first phenomenon of the kind which he had an opportunity of examining.[97] The junction is visible but at one spot, and is not seen so distinctly as in some of the instances just mentioned; but the great quantity of pudding-stone near it, renders it more interesting than it would be otherwise. As the greater part of this little island is surrounded by secondary strata, other junctions might be expected to be visible.

[97] Theory of the Earth, vol. i. p. 429.

195. On the coast of England and Wales, from the Solway Frith to the Land's End, though there are several alternations from secondary to primary strata, I know not that any of them have been observed. At St Bride's Bay, in Pembrokeshire, the primary and secondary strata are seen very near their junction; but the precise line I believe is not visible. The coal-pits in the secondary strata, approach here within a few hundred yards of the primary. The secondary strata which commence at this place, occupy both sides of the Bristol Channel, and meet the Cornish schistus, which extends across the north of Devonshire to the Quantock Hills, in a line that may be looked for on the sea coast, some where between Watchett and Minehead.

196. Besides the sea coast, the beds of rivers may be expected to afford information on this subject. To the instances I have mentioned, I have accordingly two others from the inland country to be added. One of them is from the river Jed, a little way above Jedburgh, where the secondary strata are seen lying horizontally on the primary, a section of both being made by the bed of the river. The phenomena here are very distinct, and strongly marked; Dr Hutton has described and represented them in a plate.[98] He has mentioned another junction, not far from this, which he saw in the Tiviot. Both these belong to the same primary ridge with the Siccar point.

[98] Theory of the Earth, vol. i. p. 430; also plate 3.

197. I shall mention only one other, which was discovered by Lord Webb Seymour and myself, at the foot of the high mountain of Ingleborough, in Yorkshire. As we went along the Askrig road from Ingleton, about a mile and a half from the latter, an opening appeared in the side of the hill, on the right, about one hundred yards from the road, formed by a large stone, which lay horizontally, and was supported by two others, standing upright. On going up to the spot, we found it was the mouth of a small cave, the stone lying horizontally, being part of a limestone bed, and the two upright stones, vertical plates of a primary argillaceous schistus. The limestone bed, which formed the roof of the cave, was nearly horizontal, declining to the south-east; the schistus nearly vertical, stretching from north-west by west, to south-east by east. The schistus, though close in contact with the limestone, seemed to contain nothing calcareous, and did not effervesce with acids in the slightest degree.

As this cave is at the foot of Ingleborough, a cold wind, 24° below the temperature of the external air, which issued from the mouth of it, might very well be supposed to come from the inmost recesses of that mountain. Ingleborough, which consists entirely of strata of limestone and grit, nearly horizontal, and alternating with one another, rises to the height of 1800 or 2000 feet above the spot where we now stood. This, I believe, is the greatest thickness of secondary strata that has ever been observed incumbent on the primary, and it is therefore a geological fact highly deserving of attention. The country all round, to a very great extent, is composed of limestone, with a few beds of grit interposed, and forming, beside Ingleborough, some other high mountains, such as Wharnside and Pennigant, all resting, it is probable, on the same foundation.

At the spot just described, no breccia appeared to be interposed between the primitive and secondary rock; but we found a breccia at another point of the same junction, not far distant. This was at a cascade, in the river Greta, called Thornton Force, about two miles and a half from the place just mentioned. The Greta here precipitates itself from a horizontal rock of limestone; and, after a fall of about eighteen or twenty feet, is received into a bason which it has worked out in the primary schistus. This schistus is in beds almost perpendicular; it exactly resembles that which has Just been described, and stretches nearly in the same direction. On the south side of the river a breccia was seen, lying upon the schistus, or rather, it might be said, that the lowest beds of limestone contained in them many rounded fragments of stone, which, on comparison, resembled exactly the schistus underneath. The primary rock itself is here seven or eight hundred feet above the level of the sea.

The same schistus, somewhat lower down the valley, and nearer to Ingleton, appears in large quantities, and is quarried for slate. Here, however, the immediate junction of the limestone and schistus does not appear.

I have dwelt longer on the description of these appearances than on any others of the same kind, because, from the great mass of secondary strata which here covers the primary, the circumstances are such as we cannot expect to see very often exemplified.

198. The Lakes of Cumberland are much visited by travellers; and it may be worth remarking, on that account, that, as the site of these lakes is a patch of primary country, bounded on all sides by secondary, so, in the rivers that run from the lakes, such junctions as we are now treating of may be expected to be found. Under Dun-Mallet, on the side toward Ulles Water, we observed a breccia, which was in horizontal layers, and seemed to lie on the primary schistus, so that the whole hill is perhaps a piece of more indurated breccia, or secondary rock, which has resisted the wearing and washing down of the rivers better than the rest.

199. After ascertaining the fact of the disturbance of the strata, and their removal from their original position, it is of consequence to inquire into the direction of the force by which these changes have been produced. Now, if the disturbed or elevated strata, were every where in planes, without bending or sinuosity, it might perhaps be hard to determine, whether that force had acted in the direction of gravity, or in the opposite. Either supposition would account for the appearances; and, as gravity is a known force, providing we can find some place fit to receive the matter impelled downward by it, its action would furnish the most probable solution of the difficulty.

It is on this principle that the Neptunian system proceeds, imagining, that certain great caverns or vacuities having been opened in the interior of the globe, a great part of the waters which formerly covered its surface, retired into them, and much of the solid rock also sunk down at the same time. In this way, one extremity of a stratum has been elevated, while the other has been depressed, and a certain inclination to the horizon has been given to the whole of it. Thus one cause serves two purposes; the vacuities in the interior of the earth account, both for the depression of the sea, and the elevation of the land; and the Neptunists, if the phenomena were all such as have been now stated, might boast of a felicity of explanation, not very usual in their system.

But this appearance of success vanishes, when the elevation and disturbance of the strata are more minutely examined, and are found to include waving and inflection, in a great variety of forms. It then becomes evident, that the beds of rock, at the time when they were disturbed from their horizontal position, had not their present hardness and rigidity, but were, in a certain degree at least, soft and flexible. Without these qualities, they could not have received, as they have often done, the curvature of a circle, not many feet, nay, not many inches, in diameter; nor could they have been bent into superficies, with their curvature in opposite directions, so that the same surface is in one part convex, and in another concave, on the same side, with a line of contrary flexure interposed. These are appearances, not reconcilable with the mere falling in, and breaking down of indurated rocks.

200. The inflections and wavings that we are here speaking of, though not peculiar to the primary strata, are found most frequently among them, and are perfectly familiar to every one who his travelled among mountains with any view to the study of geology. The following are a few instances of this phenomenon out of a great number which might be produced.

Saussure, in describing the route from Geneva to Chamouni, mentions many remarkable instances of the bending of the strata, and particularly where the small stream of Nant d'Arpenaz forms a cascade, by falling over the face of a perpendicular limestone rock. The strata of this rock are bent into circular arches, extremely regular, and with their concavity turned to the left. What deserves particularly to be remarked, is, that a mountain behind the cascade has its strata bent in a direction opposite to the former, or with their concavity to the right. There is no doubt that the strata of both rocks are the same, so that a vertical section of them would give a curve, in the figure of an S.[99] These circumstances are mentioned by Saussure, and from them we may infer this other property of these strata, that their section by a horizontal plane, must exhibit a system of straight lines, probably all parallel to one another.

[99] Voyages aux Alpes, vol i. § 472; also, Theory of the Earth, vol. ii. p. 30.

The same mineralogist describes the calcareous strata which compose the mountain Axenberg, on the side of the Lake of Lucerne, as having from top to bottom of the mountain the form of the letter S compressed, (ecrasée) with their curvature in some places very great. These inflections are repeated several times, and often in contrary directions; the layers are sometimes broken, where their curvature is greatest.[100]

[100] Voyages aux Alpes, tom. iv. § 1935.

On the side of the same lake, is another instance of bent strata, in a mountain, of which the beds are horizontal in the lower part, but are bent at one end upwards, in the form of the letter C. The horizontal part is of great extent, and the rock is also calcareous,[101]

[101] Ibid. § 337.

The Montagne de la Tuile, near Montmelian, receives its name from the beds of rock being incurvated in form of a tyle.[102] Among secondary mountains, the same kind of phenomena are observed, though less frequently, and with less variety of inflection. The chain of Jura is secondary, and the beds which compose it are of limestone, or of grit: they are bent in such a manner, that in a transverse section of the mountain, each layer would have the figure of a parabola.[103]

[102] Ibid. vol. iii. § 1182, and plate i.

[103] Ibid. tom. i. § 334.

201. The Pyrenees furnish abundance of phenomena of the same kind, as we learn from the Essai sur la Mineralogie des Pyrenées. The calcareous strata of the valley of Aspe, represented plate v. of that work, deserve particularly to be remarked.

202. Our own island abounds with examples of the bending and inflection of the strata, especially the primary, and many of them very much resembling those in the Alps and Pyrenees. On the top of the mountain of Ben-Lawers, in Perthshire, there is a rock, the face of which exhibits a section of a great number of thin equidistant layers, bent backwards and forwards like those described by Saussure; and this unequivocal proof of the rock having once existed in the state of a flexible and tenacious paste, is rendered more striking, by the great elevation of the spot, and the ruggedness and induration, both of the stone itself, and of every thing that surrounds it. Many other mountains in this tract consist of a schistus, which is talcose rather than micaceous, and subject, in a remarkable degree, to the sort of sinuosity and inflection here treated of.

The appearances of the primary strata on the coast of Berwickshire, have been already mentioned, as affording much valuable instruction in geology. They also exemplify the waving and inflection of the strata on a large scale, and with great variety. A section of some of them is given by Dr Hutton, in his Theory of the Earth, vol. i. from a drawing made by Sir James Hall. The nature of the curve superficies into which the schistus is bent, is the better understood from this, that, besides transverse sections from north to south, the deep indentures which the sea has made, and the projecting points of rock, exhibit many longitudinal sections, in a direction from east to west.

203. The dock-yards at Plymouth are in several places cut out of a solid rock of primary schistus, singularly incurvated. The inflections are seen there to great advantage, being exhibited in three sections, at right angles to one another, transverse, longitudinal and horizontal.

204. From these instances, to which it were easy to add many more, two conclusions may be drawn. The first of these is very obvious, viz. that the strata must have been pliant and soft when they acquired their present form. The bending of an indurated bed of stone into an arch of great curvature, and without fracture, as in the preceding examples, is a physical impossibility. Saussure has indeed observed a fracture to accompany the bending, in one or two cases; but it is an uncommon phenomenon, and, where it happens, must no doubt be understood to indicate an imperfect flexibility. Now, if it be granted that the strata were at any time soft and flexible, since their complete formation, it will be found impossible to deny their having been softened by the application of heat.

205. The second conclusion, alluded to above, results from a property, which belongs very generally, if not universally, to the inflections of the strata. This consists in their curvature being simple, or in one dimension only, like a cylindric superficies, not double, or in two dimensions, like the superficies of a sphere or spheroid. This may be otherwise expressed by saying, that the sections of the bent strata, by a horizontal plane, are straight lines, parallel to one another. On this account, every such stratum seems as if it were bent over all axis and the axes of all these different bendings, for a great extent of country, are nearly parallel.

The truth of this is evident, where the strata are seen both transversely and longitudinally. It holds remarkably of the primary schistus on the coast of Berwickshire; where the beds of rock, if cut transversely, by a vertical plane, exhibit the figures of very complicated curves, with various maxima and minima, and points of contrary flexure; but, if they are cut by a horizontal plane, the section will produce nothing but straight lines, nearly parallel.

206. The constancy of the direction of the primary strata, when estimated by their intersection with the horizontal plane, is often very remarkable. Their elevation and flexure are subject to great and sudden changes, so as to pass not only from greater to less, but from one side to the opposite, within a small distance; but the horizontal line in which they stretch, usually preserves the same bearing to a great extent. The general direction of the primary strata, in the south part of Scotland, is from E.N.E. to W.S.W.; and the same is nearly true of those which compose the ridge of the Grampians on the north, and the hills of Cumberland and Westmoreland toward the south, though between the schistus of these three tracts, there is no communication at the surface, each being entirely separated from the one next it, by the interposition of secondary strata. I have already mentioned the observations of Lord Webb Seymour and myself, at the foot of Ingleborough; and it appears from them, that the vertical schistus on which that mountain rests, though it still preserves an eastern and western direction, varies several points from that of the more northern strata. The strata of Wales return more to the first mentioned direction, and those of Devonshire and Cornwall agree with it very nearly. In all this, it will be easily conceived, that I do not mean to speak with absolute precision, or to deny the existence of great local irregularities. The result given is only a kind of average, deduced from observations hardly susceptible of great exactness, and not yet sufficiently multiplied to give to the conclusion all the accuracy it may attain.

207. This tendency of the primary strata to take a uniform direction, has also been observed in other countries. Saussure remarked in the Alps, that the beds of schistus are generally parallel to the chains of mountains composed of them;[104] and this remark is probably applicable to all mountains consisting of primary strata. The general direction, therefore, of the schistus of the Alps, must be confined between W. 10° S. and W. 40° S. In the Pyrenees, the direction of the strata is about W.N.W.[105] If Saussure's rule may be depended on, the schistus of the Altaic, and most of the other great chains in the old continent, are in directions that run considerably to the south of west. The Urals, and perhaps some other of the northern chains, are however entirely different. In the Urals, as we learn not only from the general direction of the chain, but from a section of it in the 10th volume of the Nova Acta of Petersburgh (Tab. 12,) the direction of the strata is nearly from N. to S. This last is probably the direction in the great chains of South America; so that the uniformity of direction in the primary strata, which some mineralogists would extend to those of the whole earth, is certainly imaginary, though there can be no doubt that it extends over very large portions of the earth's surface.[106]

[104] Voyage aux Alpes, tom. i. § 577.

[105] Essai sur la Mineralogie des Pyrenées.

[106] It is perhaps unnecessary to observe, that the two propositions, that the intersections of the strata with the horizon are parallel lines; and that they are lines which reserve the same bearing with respect to the points of the compass; are nearly the same thing for tracts of moderate extent, but for large portions of the earth's surface are extremely different. If, for instance, the belt of primary vertical schistus, which traverses the south of Scotland, were to be produced eastward in the same plane, from its northern extremity, where its direction is E.N.E. and its latitude 55° 57', it would cut the meridian always less obliquely as it advanced, till, having increased it longitude about 26° 28', it would be at right angles to the meridian, and its direction of consequence due east and west. This would happen in the parallel of 58° 51', (on the shore of the Gulf of Finland, near Revel,) the strata being now extended about 880 G. miles from the Siccar Point. Conversely, vertical strata, having the same bearing with respect to the meridian, may be in planes very much inclined to one another. A stratum which bears east and west in Cornwall, and one that does the same at the east end of the Altaic, will be in planes, which, if produced, would cut one another at right angles. All this is sufficiently plain from the doctrine of the sphere, and is mentioned here merely as a caution to prevent too hasty conclusions from being drawn from any correspondence of bearing among the strata of remote countries.

For the sake of those who would deduce the medium bearing of any body of strata from a number of observations, it may be proper to take notice, that the true average is not to be found by simply taking an arithimetical mean among all the observations. A more exact way is to work by the traverse table, as in keeping a ship's reckoning, (supposing the distance run to be always unity,) and to compute from the observed bearings the amount of all the southing or northing, and also all the easting and westing. The sum of all the latter, divided by the sum of all the former, is the tangent of the angle which the general direction of the strata makes with the meridian.

208. The tendency of the primary strata to remain straight in the horizontal direction, and to be bent in the vertical, is a phenomenon which points very directly to the causes from whence it has arisen. A surface of simple curvature, or a surface straight in one direction, is what the application of forces to different points of a plane, which is flexible, though with a certain degree of rigidity, will naturally produce. The supposition, therefore, that these strata were once flat and horizontal, and were impelled upward from that situation before they had become rigid or hard, will explain their having the kind of curvature which removes them as little as possible from their original condition. But no other hypothesis affords any reason why they should have that curvature more than any other. From the falling in of roofs of caverns, we might expect fracture and dislocation, without any order or regularity; but certainly no bending or sinuosity, nor any symmetrical arrangement. If, as some mineralogists allege, the curvature, as well as inclination of the strata, arose from the irregularities of the bottom on which they were deposited, why is the former in one dimension only, and why is it not in every direction, like that of hills and valleys, or the actual surface of the earth? Or, lastly, if the whole structure of the primitive mountains is an effect of crystallization, and if these mountains are now such as they have ever been from the time of their consolidation, whence is it, that, in their bendings the law just mentioned is so constantly observed? Indeed, the idea of ascribing the inflections of the strata to crystallization, though suggested by Saussure,[107] and since become a favourite system with several mineralogists, appears to me in the highest degree unsatisfactory and illusive. The purpose for which crystallization is here introduced, is not to give a specific figure to a particular substance, but to arrange the substances which it has formed and figured, according to certain rules; a work which we know not how it is to perform, and in which we have no experience of its power. Accordingly, this principle does not account, in any way whatever, for the circumstances which attend the inflection of the strata, for the simple curvature which they affect, nor for that parallelism of their layers, which, in all their bendings, is so accurately preserved. It does, indeed, so little serve to explain these facts, that, were the appearances completely reversed; did the strata assume the most complex, instead of the most simple curvature; instead of equidistant, were they converging, or alternately receding and approaching to one another; the theory of crystallization might be equally applied to them. The state of the phenomena is a matter of perfect indifference to such a theory as this; all things are explained by it with the same facility; the straight and the crooked, the square and the round, the moveable and the immoveable. Is it not evident that such an explanation is a mere word; or, if any thing more than a word, an expression of our ignorance, so awkward and indirect, as to deprive us of whatever credit might have been gained by a plain and candid avowal of it?

[107] Voyages aux Alpes, tom. i. § 475.

It should never be forgotten, that a theory which accounts for any thing, and a theory which accounts for nothing, stand precisely on the same footing, and ought to be banished from all parts of philosophy, as they have been from those sciences which are justly honoured with the name of accurate. The animated orbs of Aristotle, and the vortices of Des Cartes, have long ceased to be mentioned in physical astronomy; the first, because, they accounted for every thing alike; the second, because, when they accounted for one thing, they never could be made to account for another. Both theories, therefore, have very properly been rejected; and, when geology shall undergo a similar purification, the principle we have been considering will not be the only sacrifice required of the Neptunian system.

209. An appearance observed in some kinds of primary schistus, which clearly indicates their deposition by water, and in planes very different from those in which we now see them, though it might have been introduced before, is also much connected with the present argument. This appearance consists of small wavings or undulæ on the surface of the plates of schistus, precisely similar to these marks which are left by the sea on a gently inclining beach of sand, at the ebbing of the tide. All the species of schistus do not seem to afford instances of these wavings. The rocks which do so, are, I think, chiefly of the argillaceous kind, but often highly indurated; so that the laminæ containing the impressions are not to be torn asunder but with great difficulty. Instances of it abound in the schistus of Berwickshire, and are also not unfrequent in that of Galloway. All must agree about the agent which produced these marks; it could be no other than the sea; but it must have been the sea acting on loose, small and round particles, lying on a surface which was nearly horizontal.

210. Dr Hutton's theory is no where stronger, than in what relates to the elevation and inflection of the strata; points in which all others are so egregiously defective. The phenomena to be connected are here extremely various, and even in appearance contradictory: the horizontally of one part of the strata; the inclined or vertical position of another; the perfect planes in which one set are extended; the breaking and dislocation found in a second; the inflection and sinuosity of a third; and almost every where the utmost rigidity and induration, combined with appearances of the greatest softness and flexibility; the preservation of a parallelism of superficies in the midst of so much irregularity, and the assumption of a determinate species of curvature, under circumstances the most dissimilar; all these appearances were to be connected with one another, and with the consolidation of the strata, and this is done by the twofold hypothesis, of aqueous deposition, and the action of subterraneous heat. When these circumstances are fairly considered, and when the shifts which other systems are put to on this occasion are remembered, I think it will be granted, that few attempts at generalization have been more successful, than that which has been made by the Huttonian Theory.

211. To the fact of the elevation of the strata, the study of geology is much indebted. The stratified form of a great proportion of the earth's surface, gives to minerals that organization and regularity, which makes their disposition an object of science, and their inclined position serves to bring that organization into view, from far greater depths than we can ever reach by artificial excavations. If, for instance, the termination of strata, that make with the horizon an angle of 30°, lying one over another, is seen for a horizontal distance of two miles; then it is certain, that if these strata have that extent under ground, which may be reasonably supposed, the thickness of the whole mass, measured by a line perpendicular to its stratification, is half the horizontal distance, or amounts to one mile. It would also require a pit to be sunk from the uppermost of these strata, to the depth of (2 miles × tan 30°, =) 6093 feet before it could intersect the undermost; and therefore, if we suppose the same stratum to preserve the same character for the extent of some miles, we obtain the same information from inspecting the edge-seams, and see in reality as far into the bowels of the earth, as if we had sunk a perpendicular shaft to the depth of 6000 feet.

In general, the length of the horizontal line drawn across the strata, from the lowest in position to the highest, multiplied into the sine of the inclination of the strata to the horizon, gives the thickness of the whole, measured perpendicularly to the plane of the stratification: and the same horizontal distance, multiplied into the tangent of the inclination, gives the actual depth at which the lowest stratum would meet a perpendicular to the horizon, drawn from the highest extremity of the upper stratum.

In many cases, the extent of stratified materials admitting of such an examination as this, is much greater than has now been supposed. M. Pallas describes a range of hills on the south-east side of the peninsula of the Tauride, which is cut down perpendicularly toward the sea, and offers a complete section of the parallel beds of a primary, or, as he calls it, an ancient limestone, inclined at an angle of 45° to the horizon; and this section continues for the length of 130 versts, or about 86 English miles. The beds are so regular, that M. Pallas compares them to the leaves of a book.[108] The height of these hills does not exceed 1200 feet, but the real height of the uppermost stratum above the undermost, is 86 × √½ = 86 × 5/7 = 61 miles nearly.

[108] See Nova Acta Acad. Petropol. tom. x. (1792,) p. 257.

If therefore we conceive that there is no shift in all this great system of strata, we in reality are enabled, by means of it, to see no less than 61 miles into the interior of the earth, nearly a 65th part of the radius of the globe. It is true, that we can hardly suppose so great a body of strata to have been raised without shifting, so that we must diminish this depth considerably; but were it reduced even to one-half it will appear, that men see much farther into the interior of the globe than they are aware of, and that geologists are reproached without reason for forming theories of the earth, when all that they can do is but to make a few scratches on its surface. Art indeed can do little more; but nature supplies the deficiency, and makes discoveries to the attentive observer, on the same great scale with her other operations.

The simplest account that can be given of the vast body of parallel and highly inclined strata just mentioned, is, that it consists of the ends of horizontal strata, or of strata not greatly inclined, that have been forced up when they were all soft and flexible. This is a much more conceivable supposition than Pallas's, viz. that the greater part of this mass has sunk down into some vast cavern in the interior of the earth.