STRUCTURE AND ARRANGEMENT OF ROCK-MASSES.
34. The student can hardly learn much about the mineralogical composition of rocks, without at the same time acquiring some knowledge of the manner of their occurrence in nature. We have already briefly described certain sedimentary rocks, such as conglomerate, sandstone, and shale, and have in some measure touched upon their structure as rock-masses. These rocks, as we have seen, are arranged in more or less thick layers or beds, which are piled one on the top of the other. Rocks which are so arranged are said to be stratified, and are termed strata. We may also use the word stratum as an occasional substitute for bed. The planes of bedding or stratification are sometimes very close together, in other cases they are wide apart. When the separate beds are very thin, as in the case of shale, it is most usual to term them laminæ, and to speak of the lamination of a shale, as distinguished from the bedding of a sandstone. Planes of bedding are generally more strongly marked than planes of lamination. The laminæ frequently cohere, while beds seldom do. In the above figure, which represents a vertical cutting or section through horizontal strata, the planes of lamination are shewn at l, l, l, and those of stratification at s, s, s. There are hardly any limits to the thickness of a bed—it may range from an inch up to many feet or yards, while laminæ vary in thickness from an inch downwards.
| Fig. 1.—st, sandstone, and sh, shale: s, lines or planes of bedding; l, lines or planes of lamination. |
35. Hitherto we have been considering the laminæ and strata as lying in an approximately horizontal plane. Sometimes, however, the layers of deposition in a single stratum are inclined at various angles to themselves, as in the following figure. This structure is called false bedding; the layers or laminæ not coinciding with the planes of stratification. It owes its origin to shifting currents, such as the ebb and flow of the tide, and very often characterises deposits which have been formed in shallow water. (Hillocks of drifting sand frequently shew a similar structure, but their false bedding is, as a rule, much more pronounced.)
| Fig. 2.—False Bedding. |
36. Mud-cracks and Rain-prints.—The surfaces of some beds occasionally exhibit markings closely resembling those seen upon a flat sandy beach after the retreat of the tide—hence they are called ripple-marks or current-marks. They are, of course, due to the gentle current action which pushes along the grains of sand, and hence, such marks may be formed wherever a current sweeps over the bottom of the sea with energy just sufficient for the purpose. But since the necessary conditions for the formation of ripple-mark occur most abundantly in shallow water, its frequent appearance in a series of strata may often be taken as evidence, so far, for the shallow-water origin of the beds. Besides ripple-marks we may also detect occasionally on the surfaces of certain strata mud-cracks and rain-prints. These occur most commonly in fine-grained beds, as in flagstones, argillaceous sandstones, shales, &c. The mud-cracks resemble those upon a mud-flat which are caused by the desiccation and consequent shrinkage of the mud when exposed to the sun. The old cracks have been subsequently filled up again by a deposition of mud or sand, usually of harder consistency than the rock traversed by the cracks. Hence, when the bed that overlies the mud-cracks is removed, we find a cast of these projecting from its under surface, or frequently the cast remains in its mould, and forms a series of curious ridges ramifying over the whole surface of the old mud-flat. Rain-prints are the small pits caused by the impact of large drops. They are usually deeper at one side than the other, from which we can infer the direction of the wind at the time the rain-drops fell. Like the mud-cracks, they are most commonly met with in fine-grained beds, and have been preserved in a similar manner. Some geologists have also been able to detect wave-marks, 'faint outlinings of curved form on a sandstone layer, like the outline left by a wave along the limit where it dies out upon a beach.'
37. Succession of Strata.—The succession of strata is often very diversified. Thus, we may observe in one and the same section numberless alternating beds of sandstone and shale from an inch or so up to several feet each in thickness, with seams of coal, fireclay, ironstone, and limestone interstratified among them. In other cases, again, the succession is simpler, and some deep quarries shew only one bed, as is the case with certain limestones, fine-grained sandstones (liver-rock), and many volcanic rocks. Some limestones, indeed, shew small trace of bedding throughout a vertical thickness of hundreds of feet.
38. Beds, their Extent, &c.—Beds of rock are not only of very different thicknesses, but they are also of very variable extent. Some may thin gradually away, or 'die out' suddenly, in a few feet or yards, while others may extend over many square miles. Beds of limestone, for example, can often be traced for leagues in several directions; and if this be the case with certain single beds, it is still more true of groups of strata. Thus the coal-bearing strata belonging to what is called the Carboniferous period cover large areas in Wales, England, Scotland, and Ireland, not less, probably, than 6000 square miles; and strata belonging to the same great period spread over considerable tracts on the Continent, and a very extensive area in North America. It holds generally true that beds of fine-grained materials are not only of more equal thickness throughout, but have also a wider extension than coarser-grained rocks. Fine sandstones, for example, extend over a wider area, and preserve a more equable thickness throughout than conglomerates, while limestones and coals are more continuous than either.