Part 13 (1/2)
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.]