A Manual of Elementary Geology Part 13

On following one of the anticlinal ridges of the Jura, before mentioned, A, B, C, fig. 71., we often discover longitudinal cracks and sometimes large fissures along the line where the flexure was greatest. Some of these, as above stated, have been enlarged by denudation into valleys of considerable width, as at C, fig. 71., which follow the line of strike, and which we may suppose to have been hollowed out at the time when these rocks were still beneath the level of the sea, or perhaps at the period of their gradual emergence from beneath the waters. The existence of such cracks at the point of the sharpest bending of solid strata of limestone is precisely what we should have expected; but the occasional want of all similar signs of fracture, even where the strain has been greatest, as at _a_, fig. 71., is not always easy to explain. We must imagine that many strata of limestone, chert, and other rocks which are now brittle, were pliant when bent into their present position. They may have owed their flexibility in part to the fluid matter which they contained in their minute pores, as before described (p. 35.), and in part to the permeation of sea-water while they were yet submerged.

[Ill.u.s.tration: Fig. 78. Strata of chert, grit, and marl, near St.

Jean de Luz.]

At the western extremity of the Pyrenees, great curvatures of the strata are seen in the sea cliffs, where the rocks consist of marl, grit, and chert. At certain points, as at _a_, fig. 78., some of the bendings of the flinty chert are so sharp, that specimens might be broken off, well fitted to serve as ridge-tiles on the roof of a house. Although this chert could not have been brittle as now, when first folded into this shape, it presents, nevertheless, here and there at the points of greatest flexure small cracks, which show that it was solid, and not wholly incapable of breaking at the period of its displacement. The numerous rents alluded to are not empty, but filled with calcedony and quartz.

[Ill.u.s.tration: Fig. 79. Cross section.

_g._ gypsum.

_m._ marl.]

Between San Caterina and Castrogiovanni, in Sicily, bent and undulating gypseous marls occur, with here and there thin beds of solid gypsum interstratified. Sometimes these solid layers have been broken into detached fragments, still preserving their sharp edges (_g g_, fig.

79.), while the continuity of the more pliable and ductile marls, _m m_, has not been interrupted.

[Ill.u.s.tration: Fig. 80. Cross section.]

I shall conclude my remarks on bent strata by stating, that, in mountainous regions like the Alps, it is often difficult for an experienced geologist to determine correctly the relative age of beds by superposition, so often have the strata been folded back upon themselves, the upper parts of the curve having been removed by denudation. Thus, if we met with the strata seen in the section fig. 80., we should naturally suppose that there were twelve distinct beds, or sets of beds, No. 1. being the newest, and No. 12.

the oldest of the series. But this section may, perhaps, exhibit merely six beds, which have been folded in the manner seen in fig. 81., so that each of them is twice repeated, the position of one half being reversed, and part of No. 1., originally the uppermost, having now become the lowest of the series. These phenomena are often observable on a magnificent scale in certain regions in Switzerland in precipices from 2000 to 3000 feet in perpendicular height. In the Iselten Alp, in the valley of the Lutschine, between Unterseen and Grindelwald, curves of calcareous shale are seen from 1000 to 1500 feet in height, in which the beds sometimes plunge down vertically for a depth of 1000 feet and more, before they bend round again.

There are many flexures not inferior in dimensions in the Pyrenees, as those near Gavarnie, at the base of Mont Perdu.

[Ill.u.s.tration: Fig. 81. Cross section.]

[Ill.u.s.tration: Fig. 82. Curved strata of the Iselten Alp.]

[Ill.u.s.tration: Fig. 83. Unconformable junction of old red sandstone and Silurian schist at the Siccar Point, near St. Abb's Head, Berwicks.h.i.+re. See also Frontispiece.]

_Unconformable stratification._--Strata are said to be unconformable, when one series is so placed over another, that the planes of the superior repose on the edges of the inferior (see fig. 83.). In this case it is evident that a period had elapsed between the production of the two sets of strata, and that, during this interval, the older series had been tilted and disturbed. Afterwards the upper series was thrown down in horizontal strata upon it. If these superior beds, as _d_, _d_, fig. 83., are also inclined, it is plain that the lower strata, _a_, _a_, have been twice displaced; first, before the deposition of the newer beds, _d_, _d_, and a second time when these same strata were thrown out of the horizontal position.

Playfair has remarked[60-A] that this kind of junction which we now call unconformable had been described before the time of Hutton, but that he was the first geologist who appreciated its importance, as ill.u.s.trating the high antiquity and great revolutions of the globe. He had observed that where such contacts occur, the lowest beds of the newer series very generally consist of a breccia or conglomerate consisting of angular and rounded fragments, derived from the breaking up of the more ancient rocks.

On one occasion the Scotch geologist took his two distinguished pupils, Playfair and Sir James Hall, to the cliffs on the east coast of Scotland, near the village of Eyemouth, not far from St. Abb's Head, where the schists of the Lammermuir range are undermined and dissected by the sea.

Here the curved and vertical strata, now known to be of Silurian age, and which often exhibit a ripple-marked surface[60-B], are well exposed at the headland called the Siccar Point, penetrating with their edges into the inc.u.mbent beds of slightly inclined sandstone, in which large pieces of the schist, some round and others angular, are united by an arenaceous cement.

"What clearer evidence," exclaims Playfair, "could we have had of the different formation of these rocks, and of the long interval which separated their formation, had we actually seen them emerging from the bosom of the deep? We felt ourselves necessarily carried back to the time when the schistus on which we stood was yet at the bottom of the sea, and when the sandstone before us was only beginning to be deposited in the shape of sand or mud, from the waters of a superinc.u.mbent ocean. An epoch still more remote presented itself, when even the most ancient of these rocks, instead of standing upright in vertical beds, lay in horizontal planes at the bottom of the sea, and was not yet disturbed by that immeasurable force which has burst asunder the solid pavement of the globe.

Revolutions still more remote appeared in the distance of this extraordinary perspective. The mind seemed to grow giddy by looking so far into the abyss of time; and while we listened with earnestness and admiration to the philosopher who was now unfolding to us the order and series of these wonderful events, we became sensible how much farther reason may sometimes go than imagination can venture to follow."[60-C]

In the frontispiece of this volume the reader will see a view of this cla.s.sical spot, reduced from a large picture, faithfully sketched and coloured from nature by the youngest son of the late Sir James Hall. It was impossible, however, to do justice to the original sketch, in an engraving, as the contrast of the red sandstone and the light fawn-coloured vertical schists could not be expressed. From the point of view here selected, the underlying beds of the perpendicular schist, _a_, are visible at _b_ through a small opening in the fractured beds of the covering of red sandstone, _d d_, while on the vertical face of the old schist at _a' a"_ a conspicuous ripple-mark is displayed.

[Ill.u.s.tration: Fig. 84. Junction of unconformable strata near Mons, in Belgium.]

It often happens that in the interval between the deposition of two sets of unconformable strata, the inferior rock has not only been denuded, but drilled by perforating sh.e.l.ls. Thus, for example, at Autreppe and Gusigny, near Mons, beds of an ancient (paleozoic) limestone, highly inclined, and often bent, are covered with horizontal strata of greenish and whitish marls of the Cretaceous formation. The lowest and therefore the oldest bed of the horizontal series is usually the sand and conglomerate, _a_, in which are rounded fragments of stone, from an inch to two feet in diameter.

These fragments have often adhering sh.e.l.ls attached to them, and have been bored by perforating mollusca. The solid surface of the inferior limestone has also been bored, so as to exhibit cylindrical and pear-shaped cavities, as at _c_, the work of saxicavous mollusca; and many rents, as at _b_, which descend several feet or yards into the limestone, have been filled with sand and sh.e.l.ls, similar to those in the stratum _a_.

_Fractures of the strata and faults._--Numerous rents may often be seen in rocks which appear to have been simply broken, the separated parts remaining in the same places; but we often find a fissure, several inches or yards wide, intervening between the disunited portions. These fissures are usually filled with fine earth and sand, or with angular fragments of stone, evidently derived from the fracture of the contiguous rocks.

The face of each wall of the fissure is often beautifully polished, as if glazed, and not unfrequently striated or scored with parallel furrows and ridges, such as would be produced by the continued rubbing together of surfaces of unequal hardness. These polished surfaces are called by miners "slickensides." It is supposed that the lines of the striae indicate the direction in which the rocks were moved. During one of the minor earthquakes in Chili, which happened about the year 1840, and was described to me by an eye-witness, the brick walls of a building were rent vertically in several places, and made to vibrate for several minutes during each shock, after which they remained uninjured, and without any opening, although the line of each crack was still visible. When all movement had ceased, there were seen on the floor of the house, at the bottom of each rent, small heaps of fine brickdust, evidently produced by trituration.

[Ill.u.s.tration: Fig. 85. Faults. A B perpendicular, C D oblique to the horizon.]

It is not uncommon to find the ma.s.s of rock, on one side of a fissure, thrown up above or down below the ma.s.s with which it was once in contact on the other side. This mode of displacement is called a s.h.i.+ft, slip, or fault. "The miner," says Playfair, describing a fault, "is often perplexed, in his subterraneous journey, by a derangement in the strata, which changes at once all those lines and bearings which had hitherto directed his course. When his mine reaches a certain plane, which is sometimes perpendicular, as in A B, fig. 85., sometimes oblique to the horizon (as in C D, ibid.), he finds the beds of rock broken asunder, those on the one side of the plane having changed their place, by sliding in a particular direction along the face of the others. In this motion they have sometimes preserved their parallelism, as in fig. 85., so that the strata on each side of the faults A B, C D, continue parallel to one another; in other cases, the strata on each side are inclined, as in _a_, _b_, _c_, _d_ (fig.

86.), though their ident.i.ty is still to be recognized by their possessing the same thickness, and the same internal characters."[62-A]

[Ill.u.s.tration: Fig. 86. E F, fault or fissure filled with rubbish, on each side of which the s.h.i.+fted strata are not parallel.]

In Coalbrook Dale, says Mr. Prestwich[62-B], deposits of sandstone, shale, and coal, several thousand feet thick, and occupying an area of many miles, have been s.h.i.+vered into fragments, and the broken remnants have been placed in very discordant positions, often at levels differing several hundred feet from each other. The sides of the faults, when perpendicular, are commonly separated several yards, but are sometimes as much as 50 yards asunder, the interval being filled with broken _debris_ of the strata. In following the course of the same fault it is sometimes found to produce in different places very unequal changes of level, the amount of s.h.i.+ft being in one place 300, and in another 700 feet, which arises, in some cases, from the union of two or more faults. In other words, the disjointed strata have in certain districts been subjected to renewed movements, which they have not suffered elsewhere.

We may occasionally see exact counterparts of these slips, on a small scale, in pits of fine loose sand and gravel, many of which have doubtless been caused by the drying and shrinking of argillaceous and other beds, slight subsidences having taken place from failure of support. Sometimes, however, even these small slips may have been produced during earthquakes; for land has been moved, and its level, relatively to the sea, considerably altered, within the period when much of the alluvial sand and gravel now covering the surface of continents was deposited.

I have already stated that a geologist must be on his guard, in a region of disturbed strata, against inferring repeated alternations of rocks, when, in fact, the same strata, once continuous, have been bent round so as to recur in the same section, and with the same dip. A similar mistake has often been occasioned by a series of faults.

[Ill.u.s.tration: Fig. 87. Apparent alternations of strata caused by vertical faults.]

If, for example, the dark line A H (fig. 87.) represent the surface of a country on which the strata _a b c_ frequently crop out, an observer, who is proceeding from H to A, might at first imagine that at every step he was approaching new strata, whereas the repet.i.tion of the same beds has been caused by vertical faults, or downthrows. Thus, suppose the original ma.s.s, A, B, C, D, to have been a set of uniformly inclined strata, and that the different ma.s.ses under E F, F G, and G D, sank down successively, so as to leave vacant the s.p.a.ces marked in the diagram by dotted lines, and to occupy those marked by the continuous lines, then let denudation take place along the line A H, so that the protruding ma.s.ses indicated by the fainter lines are swept away,--a miner, who has not discovered the faults, finding the ma.s.s _a_, which we will suppose to be a bed of coal four times repeated, might hope to find four beds, workable to an indefinite depth, but first on arriving at the fault G he is stopped suddenly in his workings, upon reaching the strata of sandstone _c_, or on arriving at the line of fault F he comes partly upon the shale _b_, and partly on the sandstone _c_, and on reaching E he is again stopped by a wall composed of the rock _d_.

[Ill.u.s.tration: Fig. 88. Cross section.]

The very different levels at which the separated parts of the same strata are found on the different sides of the fissure, in some faults, is truly astonis.h.i.+ng. One of the most celebrated in England is that called the "ninety-fathom dike," in the coal-field of Newcastle. This name has been given to it, because the same beds are ninety fathoms lower on the northern than they are on the southern side. The fissure has been filled by a body of sand, which is now in the state of sandstone, and is called the dike, which is sometimes very narrow, but in other places more than twenty yards wide.[64-A] The walls of the fissure are scored by grooves, such as would have been produced if the broken ends of the rock had been rubbed along the plane of the fault.[64-B] In the Tynedale and Craven faults, in the north of England, the vertical displacement is still greater, and has extended in a horizontal direction for a distance of thirty miles or more. Some geologists consider it necessary to imagine that the upward or downward movement in these cases was accomplished at a single stroke, and not by a series of sudden but interrupted movements. This idea appears to have been derived from a notion that the grooved walls have merely been rubbed in one direction. But this is so far from being a constant phenomenon in faults, that it has often been objected to the received theory respecting those polished surfaces called "slickensides" (see above, p. 61.), that the striae are not always parallel, but often curved and irregular. It has, moreover, been remarked, that not only the walls of the fissure or fault, but its earthy contents, sometimes present the same polished and striated faces.

Now these facts seem to indicate partial changes in the direction of the movement, and some slidings subsequent to the first filling up of the fissure. Suppose the ma.s.s of rock A, B, C, to overlie an extensive chasm _d e_, formed at the depth of several miles, whether by the gradual contraction in bulk of a melted ma.s.s pa.s.sing into a solid or crystalline state, or the shrinking of argillaceous strata, baked by a moderate heat, or by the subtraction of matter by volcanic action, or any other cause.

Now, if this region be convulsed by earthquakes, the fissures _f g_, and others at right angles to them, may sever the ma.s.s B from A and from C, so that it may move freely, and begin to sink into the chasm. A fracture may be conceived so clean and perfect as to allow it to subside at once to the bottom of the subterranean cavity; but it is far more probable that the sinking will be effected at successive periods during different earthquakes, the ma.s.s always continuing to slide in the same direction along the planes of the fissures _f g_, and the edges of the falling ma.s.s being continually more broken and triturated at each convulsion. If, as is not improbable, the circ.u.mstances which have caused the failure of support continue in operation, it may happen that when the ma.s.s B has filled the cavity first formed, its foundations will again give way under it, so that it will fall again in the same direction. But, if the direction should change, the fact could not be discovered by observing the slickensides, because the last scoring would efface the lines of previous friction. In the present state of our ignorance of the causes of subsidence, an hypothesis which can explain the great amount of displacement in some faults, on sound mechanical principles, by a succession of movements, is far preferable to any theory which a.s.sumes each fault to have been accomplished by a single upcast or downthrow of several thousand feet. For we know that there are operations now in progress, at great depths in the interior of the earth, by which both large and small tracts of ground are made to rise above and sink below their former level, some slowly and insensibly, others suddenly and by starts, a few feet or yards at a time; whereas there are no grounds for believing that, during the last 3000 years at least, any regions have been either upheaved or depressed, at a single stroke, to the amount of several hundred, much less several thousand feet.

When some of the ancient marine formations are described in the sequel, it will appear that their structure and organic contents point to the conclusion, that the floor of the ocean was slowly sinking at the time of their origin. The downward movement was very gradual, and in Wales and the contiguous parts of England a maximum thickness of 32,000 feet (more than six miles) of Carboniferous, Devonian, and Silurian rock was formed, whilst the bed of the sea was all the time continuously and tranquilly subsiding.[65-A] Whatever may have been the changes which the solid foundation underwent, whether accompanied by the melting, consolidation, crystallization, or desiccation of subjacent mineral matter, it is clear from the fact of the sea having remained shallow all the while that the bottom never sank down suddenly to the depth of many hundred feet at once.

It is by a.s.suming such reiterated variations of level, each separately of small vertical amount, but multiplied by time till they acquire importance in the aggregate, that we are able to explain the phenomena of denudation, which will be treated of in the next chapter. By such movements every portion of the surface of the land becomes in its turn a line of coast, and is exposed to the action of the waves and tides. A country which is undergoing such movement is never allowed to settle into a state of equilibrium, therefore the force of rivers and torrents to remove or excavate soil and rocky ma.s.ses is sustained in undiminished energy.

FOOTNOTES:

[46-A] In the first three editions of my Principles of Geology, I expressed many doubts as to the validity of the alleged proofs of a gradual rise of land in Sweden; but after visiting that country, in 1834, I retracted these objections, and published a detailed statement of the observations which led me to alter my opinion in the Phil. Trans. 1835, Part I. See also the Principles, 4th and subsequent editions.

[46-B] See his Journal of a Naturalist in Voyage of the Beagle, and his work on Coral Reefs.

[46-C] See chapters xxviii. to x.x.xi. inclusive.

[48-A] Edin. Trans. vol. vii. pl. 3.

[50-A] Proceedings of Geol. Soc. vol. iii. p. 148.

[53-A] See plan by M. Chevalier, Burat's D'Aubuisson, tom. ii. p. 334.

[55-A] See M. Thurmann's work, "Essai sur les Soulevemens Jura.s.siques du Porrentruy, Paris, 1832," with whom I examined part of these mountains in 1835.

[57-A] I am indebted to the kindness of T. Sopwith, Esq., for three models which I have copied in the above diagrams; but the beginner may find it by no means easy to understand such copies, although, if he were to examine and handle the originals, turning them about in different ways, he would at once comprehend their meaning as well as the import of others far more complicated, which the same engineer has constructed to ill.u.s.trate _faults_.

[60-A] Biographical account of Dr. Hutton.

[60-B] See above, p. 49. and section.

[60-C] Playfair, ibid.; see his Works, Edin. 1822, vol. iv. p. 81.

[62-A] Playfair, Ill.u.s.t. of Hutt. Theory, -- 42.