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or wide belts of low-lying land which within a recent period have been gained from the sea, are all made up of various kinds of sediment arranged in gently inclined or approximately horizontal layers. Now, over considerable areas of the earth's surface the derivative rocks show the same horizontal arrangement, a structure which is obviously original. And this is frequently the case with younger and older sedimentary strata alike. Here, for example (Fig. 1), is a section across a country, the superficial rock-masses of which are horizontally arranged.
The upper line of the section (A-B) represents, of course, the surface of the ground, while the lower we shall take to be the level of the sea. The section thus shows the geological structure or arrangement of the rocks from the surface down to the level of the sea. The strata represented consist of a great series of sandstones and shales with one prominent bed of limestone (/) at the top. In this case we cannot doubt that the horizontal bedding is original—that the strata were accumulated one above the other in the same order as we see them.
Although such horizontal arrangements are of common enough occurrence, and now and again characterise the sedimentary systems over wide areas, yet, as a general rule, strata tend to be inclined. In many regions the inclination, or dip, as it is termed, is sometimes very high—not seldom indeed the beds are seen standing on end, like rows of books in a library. This last appearance of extreme disturbance is not confined to the strata of any system: nevertheless, it is more characteristic of the older than the younger systems. In the sequel we shall have to study these and other rock-structures more particularly, but for the present we need not do more than make some general acquaintance with them.
A very common arrangement is shown in the next diagram (Fig. 2). Here the strata are arranged in the form of a truncated arch, or anticline. At A- the
Fig. a. Section Across An Anticline.
The upper continuous line, A-B, = surface of ground ; the lower continuous line, C-D, = tealevel; X-Y, = vertical axis.
beds are approximately horizontal, but from this point they dip on the right towards B, and on the left in the direction of A. Note further that the angle of inclination is the same on each side of the anticline; in other words, the anticlinal axis (X-V) is vertical. From A to B the distance we shall suppose is six miles.
The succeeding section (Fig. 3) we shall take to be of equal length. Here we have a succession of anticlines, or saddle-backs, separated one from another by troughs, or synclines, as they are termed. In other words, the strata are undulating. From these sections we learn that folds or undulations vary considerably in width. In the region represented by Fig. 2 we have an area six miles in breadth, consisting of a thick series of strata disposed in the form of one single arch or anticline; while in Fig. 3, representing
F1g. 3. Sect1on Across Symmetr1cal Ant1cl1nes And Syncl1nes.
Upper continuous line, -4-/7, = surface of ground; lower continuous line, C-D, — sea-level; a a, anticlines; s s, synclines ; a jr, j jr, axes of folds.
an equal area, the strata are folded into a series of several anticlines and synclines. In both regions the anticlines are symmetrical; that is to say, their axes (a x, s x) are vertical.
But folds or undulations may follow each other much more rapidly than is shown in the preceding section. In countries built up of steeply inclined
F1g. 4. Sf.ct1on Across Unsymmetk1ca1. Ant1cl1nes And Syncl1nes.
Upper continuous line, A -5, = surface of ground; lower continuous line, C-D, = sea-level; a x, s x, axes of folds.
rocks, the undulations of the strata are more abrupt, and the axes of the folds are frequently inclined. In Fig. 4, for example, most of the anticlines and synclines lean over to one side, and this to such a degree, that here and there upper beds are doubled under older beds of the same series of strata; in other words, the order of succession appears to be inverted. From the fact that strata are generally inclined from the horizontal, and frequently curved and folded, it is obvious that they have been subjected to the action of some great disturbing force, for folding and
contortion may affect masses of strata many thousands of feet in thickness. Another evident mark of disturbance is furnished by the presence of dislocations, or faults, as they are technically termed, along the line of which the rocks have been shifted for, it may be, hundreds and sometimes even for thousands of feet. One of the simplest kind of faults is shown in the preceding illustration (Fig. 5). Here, as in preceding figures, the upper line (A-B) represents the surface of the ground. At f the strata are traversed by a fault, which has caused a vertical displacement of the beds to the extent of, say, 500 feet, for it is obvious that the coal and fireclay (8, 9), and the strata amongst which they lie on the left-hand side, were formerly continuous with the corresponding beds on the other side of the fault.
From the facts now briefly set forth we may draw certain conclusions. In the first place, the extensive geographical range of the derivative rocks, most of which are of marine origin, must convince us that the greater portion of our continental areas has been under water. It is not to be understood, however, that all the land-surfaces occupied by sedimentary strata have been submerged at one and the same time. On the contrary, the several geological systems have been accumulated at widely different periods. This is a point, however, to which we shall return: for the present, we need only keep in view the prominent fact that the existing land-surfaces of the globe are composed most frequently of marine strata. There are apparently only two ways in which this phenomenon can be accounted for, and these explanations come to much the same thing. Either the general level of the ocean has fallen, or wide areas of the seafloor have been pushed up from below and converted into dry land. Both changes appear to have taken place. The bed of the sea has sunk from time to