Illustrations of the Huttonian Theory of the Earth

by John Playfair

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.