PART III THE STRUCTURE OF THE CRUST OF THE EARTH CHAPTER XII SEDIMENTARY ROCKS-THEIR ORIGINAL STRUCTURES HAVING in the two foregoing chapters considered the more important elementary substances of which the earth's crust is composed and their combinations in minerals and rocks, we have to inquire how these minerals and rocks have been put together so as to build up the crust. A very little examination will suffice to show us that the upper or outer parts of the solid globe consist chiefly of sedimentary rocks. All over the plains and low grounds of the earth's surface, which cover so large a proportion of the whole area of the land, some kind of sediment underlies the soil-clay, sand, gravel, limestone. It is for the most part only in hilly or mountainous regions that anything has been pushed up from below, so as to indicate the nature of the materials underneath. But everywhere we encounter proofs that the sedimentary rocks do not remain as they were deposited. In the first place, most of them were laid down on the sea-floor, and they have been upraised into land. In the next place, not only have they been upheaved, they have not infrequently been bent, broken, and crushed, until sometimes their original condition can no longer be determined. Moreover they have been invaded by masses of lava and other eruptive rocks, which have been thrust in among them and have often burst through them to form volcanoes at the surface. We must now endeavour to form as clear a conception as possible of what, after all these changes, the present structure of the crust actually is. In this chapter, therefore, we may examine some of the leading characters of sedimentary rocks in the architecture of the crust, more particularly those which have been determined by the conditions under which the rocks were formed. In the next chapter we shall consider some of the more important characters which have been superinduced upon the rocks since their formation. Stratification.-It has been shown (p. 148) that one of the most distinctive features in sedimentary rocks is that they are h g e C b α FIG. 79.-Section of stratified rocks. stratified that is, are arranged in layers one above another. As those at the bottom must have been deposited before those at the top, a succession of layers of stratified rocks forms a record of deposition, in which the early stages are chronicled by the lower, and the later stages by the upper layers. An illustration of this kind of record has already been given in the introductory chapter. As a further example, the accompanying section (Fig. 79) may be taken. At the bottom lies a bed (a) of dark shale or clay with fragments of crinoids, corals, shells, and other marine organisms. Such a bed unmistakably points to a former muddy sea-floor, on which the creatures lived whose remains have been preserved in the hardened mud or shale. The next bed (b) is one of limestone full of similar organic remains; it shows that the supply of mud, which had previously made the water turbid and had been slowly gathering in successive layers on the bottom, now ceased. The water became clear and much better fitted for the life of the crinoids, corals, and shells. These creatures accordingly flourished abundantly, living and dying on the spot generation after generation, until their accumulated remains had built up a solid sheet of limestone several feet thick. But once more muddy currents spread over the place, and from the cloud of suspended mud there slowly settled down the layer of blue clay (c) which overlies the limestone. As hardly any remains of organisms are to be seen in it, we may infer that the inroad of mud killed them Next, owing to some new shifting of the currents, a quantity off. But the of sand was brought in and spread out over the mud, forming the sandstone beds (d). The sea in which these various strata were deposited was probably shallow; or its floor may have been gradually rising. At all events, the last layers of sand could have been only slightly below the surface of the water, for they are immediately covered by a hardened silt or fire-clay (e) which, from the abundant roots and rootlets that run through it in all directions, was clearly once a soil whereon plants grew. It was probably part of a mud-flat, on which vegetation spread seaward from the land where the water shallowed, as happens at the present day among the tropical mangrove-swamps (p. 83). The plants that grew on this soil have formed the coal-seam (ƒ), no doubt representing the growth of a long period of time. existence of the coal-jungle came to an end probably by a sinking of the ground beneath the water. Mud, once more carried hither from the neighbouring land, settled down upon the submerged vegetation and formed the clay (g). But that land plants still abounded in the immediate neighbourhood, is shown by their numerous remains in this clay. We notice too that the salts of iron dissolved in the water were eliminated by the decaying plants and animals and were precipitated in the form of carbonate, so as to form concretions round occasional dead shells, fishes, fernfronds, and seed-cones. What were the immediately succeeding events in this ancient history we cannot tell; the layer next in order is a coarse conglomerate (h), originally gravel, which must have been swept along by a swift current that tore away the upper part of the clay-beds (g) and any strata which may once have overlain them. The whole stratified part of the earth's crust is composed of materials which in this way may be made to tell their story. In forcing them to yield up their records of the ancient changes of which they are memorials, scope is afforded for the most accurate and laborious investigation and for the closest reasoning from the facts collected. At the same time, it is obvious that the pursuit is one which constantly exercises the imagination, and that, indeed, it cannot be adequately followed unless, by the proper use of the imagination, the former conditions of the earth's surface are vividly realised. The thinnest layers of a stratified rock form lamina, such as the thin paper-like leaves into which shale can be split. A number of laminæ may be united in a stratum or bed which may vary from less than an inch to several feet or yards in thickness. It is only 15 14 13 the finer kinds of sedimentary rock that, as a rule, are laminated. In other cases a stratum or bed is the thinnest subdivision; it can usually be separated easily from those above and below it, and it may generally be regarded as marking one continued phase of deposit, while the break between it and the next bed above or below probably denotes an interruption of the deposit. The study of the relations of strata to each other is called Stratigraphy. 12 10 9 8 7 6 3 FIG. 80.-Section showing alternation of beds. 15. Shale. 14. Seam of sandstone. 13. Shale with septarian nodules. 12. Sandstone. 11. Mud- 9. Clay. 8. Sandstones. 7. Sandy clays. 6. Limestone with parting of shale. 5. Shale. 4. Limestone. 3. Shale with cement stone passing down into sandstone (2), Layers of deposit usually lie parallel with each other, their flat surfaces marking the general floor of the water at the time of their formation (Figs. 79, 80). But sometimes a series of layers may be found inclined at various angles to what was obviously the original general plane of deposition. In Fig. 81, for example, a series of strata is presented, which are distinguished by a diagonal lamination. This is known as False bedding or Current-bedding. As explained in Chapter III (p. 37), it has been caused by the pushing of layers of sediment over the advancing front of a stratum, and may be compared to the oblique bedding often to be seen in an earthwork, such as a railway embankment, the upper surface of which may be in a general sense parallel with the flat bottom of the valley, while the successive layers of which the mound is made are inclined at angles of 30° or more. False bedding is interesting as affording some indication of the nature and direction of the currents by which sediment has been transported. Proofs of former Shores.-Along the margin of the sea, of lakes, and of rivers, several interesting kinds of markings may be seen impressed on surfaces of sand or mud from which the water has retired. Every one who has walked on a tidal sea- beach is familiar which graduates into fine conglomerate (1). with the Ripple-marks left by the retreating tide upon the bare sands. They are produced by the oscillation of the water driven into movement by wind playing over its surface. They are usually effaced by the next advancing tide; hence, out of the same sand new sets of ripple-marks are made by each tide. But we can understand that now and then, under peculiarly favourable conditions, the markings may not be destroyed. If, for instance, they were made in a kind of muddy sand, which, in the interval between two tides and under a strong sun, could become hard and coherent on the surface, and if the next tide advanced so quietly as not to disturb them, but to lay down upon them a fresh layer They of sand or mud, they might be covered up and preserved. would then remain as a memorial of the shallow rippling water and bare sandy shore where they had been formed. Now evidence of this kind regarding the conditions of deposition occur abundantly among sedimentary rocks (Fig. 82). Ripple-marked surfaces may be traced one over another for many hundred feet in a thick series of sandstones. They bring clearly to the mind that the strata on which they lie were accumulated in shallow water, or along beaches that were often laid dry. Land-Surfaces. Other traces of exposure to the air may be noticed, where ripple-mark is abundant, in what are termed Suncracks, Foot-prints, and Rain-prints. Those who have observed what takes place in muddy places during dry weather will |