« PreviousContinue »
time to greater and greater depths, and has thus tended to draw the water away from the surface of what are now continental areas. But if the earth■s crust under the ocean has subsided, it has also been elevated within what are now dry lands again and again. The folds and corrugations of the strata, and the numerous dislocations by which rocks of all kinds are traversed, clearly demonstrate that movements of the solid crust have taken place. Such crustal disturbances are probably in chief measure due to the fact that the earth is a cooling body. As the solid crust sinks down upon the cooling and contracting nucleus, it must occupy less superficial space. Hence its rocky framework becomes subjected to enormous tangential squeezing and compression to which it yields by bending and folding, by fracture and displacement. Obviously, then, the mysterious subterranean forces must have played an important part in the formation of earth-features. Disturbed rocks are of more frequent occurrence than strata which have retained their original horizontality. It is no wonder, therefore, that for a long time the general configuration of the land was believed to have been impressed upon it by plutonic agency. Indeed, in the case of certain mountain chains, we cannot fail to see that the larger features of such regions often correspond to a considerable extent with the main flexures and displacements of the underlying rocks. In many elevated tracts, however, composed of highly disturbed and contorted strata, no such coincidence of surface-feature and underground structure can be traced. The mountain ridges do not correspond to great swellings of the crust; the valleys neither lie in trough-shaped strata, nor do they coincide with gaping fractures. Again, many considerable mountains are built up of rocks which are not convoluted at all, but arranged in horizontal beds. More than this, many plateaux and even lowlands are composed of as highly flexed and contorted strata as are to be met with in any mountainous country. Evidently, therefore, crustal movement is not the only factor in the production of surface-features.
The sections already given will serve to illustrate the general fact that underground structure and superficial configuration do not necessarily correspond. Thus in Fig. i we have a series of pyramidal mountains developed in horizontal strata. The slope of the surface, therefore, frequently bears no relation to the "lie" of the beds below. This is further illustrated in the succeeding figures, where we find depressions at the surface, while the rocks immediately underneath show an anticlinal arrangement; and, conversely, where the strata are trough-shaped the surface-feature is not a depression but an elevation.
In the case of the horizontal strata shown in Fig. i we have■no difficulty in perceiving that the present surface is not that of original deposition. It is impossible that sedimentary deposits could have been piled up in the shape of great pyramids : obviously the beds were formerly continuous, as shown by the dotted lines. Clearly some "monstrous cantles" have been cut out and removed. And the same is necessarily true of the folded strata. In each case (Figs. 2, 3, 4) masses of strata have disappeared; the tops or backs of the anticlinal arches have been more or less deeply incised, and the material carried away. In subsequent pages it will be shown that the thickness of rocks thus removed can be proved to amount in many cases to thousands of feet.
Not less striking is the evidence of rock-removal furnished by the phenomena of faults. At the surface there may be no inequality of level corresponding to that seen below (see Fig. 5). Obviously, however, a considerable thickness of rock has vanished. Were the missing continuations of the strata to be replaced upon the high side of the fault, they would occupy the space contained within the dotted lines above the present surface A-B. Such dislocations often interrupt the continuity of the strata in our coalfields. In such regions we may traverse level or gently undulating tracts, and be quite unconscious of the fact that geologically we have several times leaped up or jumped down hundreds of feet in a single step. Nay, some rivers flow across dislocations by which the strata have been shifted up or down for thousands of yards, and in some places we may sit upon rocks which are geologically more than a thousand fathoms below or above those on which we rest our feet. Faults, then, afford clear evidence of the wholesale removal of rocks from the surface of the land.
Such proofs of rock-removal can be appreciated by anyone, and will come frequently before us in the discussion that follows. There is another kind of evidence, however, leading to the same general conclusion, which may be briefly touched upon at this stage of our inquiry. In this and other countries there are enormous masses of rock, often widely extended, which have cooled and consolidated from a state of igneous fusion. Some of these, it is well known, have flowed out as lavas at the surface, while others were never so erupted, but have solidified at greater or less depths below ground. Among the latter is granite, a rock believed to be of deep-seated origin. Its plutonic character is evinced not less by its composition and structure than by its relation to the rock-masses that surround it. Every mass of granite, then, has cooled and consolidated, probably very slowly, and certainly at a less or greater depth in the earth's crust. When this rock is met with over a wide area at the actual surface, therefore,—forming, it may be, great mountains or rolling and broken lowlands,—we know that in such regions thick masses of formerly overlying rocks have been removed. The granite appears at the surface simply because the covering of rocks underneath which it cooled and solidified has been subsequently carried away.
The occurrence at the surface of crystalline schists and other metamorphic rocks has a similar significance. Although the processes by which rocks become so highly altered are still more or less obscure, yet there can be no doubt that the metamorphism had taken place when the rocks affected were more or less deeply buried in the crust.
While we may safely infer, from the general phenomena of geological structure, that earth-movements have shared in the production of surface-features, we must be convinced, at the same time, that some other factor has aided in the work of shaping out our lands. Earth-movements quite account for the folding and fracturing of strata, for the uplifting of great mountain masses, but they cannot have caused the general loss which these masses have sustained. We may conceive it possible that subterranean action may now and again have resulted in wide-spread shattering of rocks at the surface, but such action could not have caused the broken material to disappear. Further, when we bear in mind that the thickness of rock removed from the surface of the land is sometimes to be measured by many thousands of feet, or even yards, we see at once that subterranean action cannot have been directly implicated in the spoliation of the land. How, then, have anticlines been truncated? What power has removed the strata from the high side of a fault? What, in a word, has produced that truncation and discontinuity of beds which is so common a feature of derivative rocks all the world over? And how shall we account for the presence at the surface of deep-seated plutonic rocks and metamorphic masses? When we have satisfactorily answered such questions we shall have solved the problem of the origin of surface-features.