Imagini ale paginilor
PDF
ePub

CHAPTER IV.

UPHEAVAL, SUBSIDENCE, AND DENUDATION.

Upheaval, subsidence, and denudation considered in their effects on all classes of rocks-Alterations of level, and contemporary geological action-The cañons of Colorado-Denudation on river basinsRecent volcanic action; appearance and disappearance of islands within historical times-Earthquake waves, geysers, etc.-Raised beaches; terraces of Norway; sinking shores-Conclusions from animal remains.

FROM occasional former references to the great and frequent displacements which the strata have suffered, the reader will have concluded that there has never been a truly stable and permanent condition throughout the whole history of the earth. The results of oscillations above and below the mean level, ranging from a few inches to several miles, are everywhere apparent. The great difference observable between the summit of a mountain chain and the general level of the ocean-which is always our standard of reference-cannot be accounted for by the agency of water, but must be referred to another cause. Mountain systems may be formed of unstratified or stratified rocks-of granites, limestones, or sandstones—or partially of all; and it is now generally

[ocr errors]

understood that they owe their elevation to the gradual contraction of the earth in cooling, perhaps supplemented by active volcanic movements. Experiments undertaken to illustrate the effect of lateral pressure in forming these great inequalities, have confirmed this view. Professor Favre of Geneva found that, by subjecting a horizontal layer of plastic clay to pressure from two or more directions laterally, contortions and foldings of the material, strongly resembling mountains, valleys, and other natural appearances of the strata were obtained.

A

20

FIG. 1.-Diagram of positions of rocks affected by subterranean action. A. Horizontal strata conformable with respect to each other; and resting unconformably on the inclined strata 1, 2, 3, 4, which are dislocated by fault, f.

Vaults, dykes, anticlines, synclines, faults, and fissures were represented, often with minute accuracy, by the clay as it yielded to the pressure; and the material was in some instances folded back upon itself, as actually occurs in nature. The sides of the ridges presented steep escarpments and gentle inclines; and, while in some places the lower strata remained horizontal, those

above them were much dislocated. We know the earth to be a gradually cooling body, and in the course of ages it must necessarily contract. The axis thus becomes shorter, and the mass of matter composing the globe is compelled to occupy a smaller space. In accommodating itself to the urgent force of contraction, some portions are pushed upwards, and others, by comparison, appear to have been depressed, and really have in many instances fallen below the normal level. The unequal resistance of contiguous strata contributes to this, and the depressions and elevations are therefore not uniform over the whole surface; for the matter of the globe varies very greatly in density, in both the horizontal and perpendicular directions. Thus, by measuring the undulations of contorted strata between two given points, it has been found that they occupy a space only twothirds of that covered by them before their compression.

The casual observer, in looking at the sections of Professor Favre's layer of clay after compression, would be struck by the close resemblances they afford to every peculiarity in the structure of a mountain system; and, when tested by measurements, they show how much exaggerated our ideas of mountains are. A ridge 10,000 feet high is really a very moderate and almost inappreciable object on the surface of the globe, when we compare it with the immense diameter and vast superficial extent of the earth. Relatively to ourselves, and to the objects of comparison within the immediate range of our vision, such a ridge appears of stupendous magnitude. But our view of the subject undergoes a complete

change when we measure the original length and thickness of M. Favre's experimental layer of clay, and note the effect of crushing. The ridges raised upon this are frequently one-tenth of the original length of the material operated upon, and the depressions proportionate to them. It is, then, possible that mountains five miles high might be produced on the surface of the earth in an area of crushing or compression no more than fifty miles in extent, with corresponding valleys of great depth, and those extensive "faults," so frequently-indeed almost invariably-observable in mountain systems.

Thus the chains of America, Asia, and Europe have been squeezed up into irregular ridges high above the general level, by the shrinking of the crust. When the mountains are of stratified character (as, for instance,

[blocks in formation]

FIG. 2.-1. Horizontal strata; 2. Inclined; 3. Almost perpendicular; 4. Synclinal; 5. Anticlinal.

portions of the Pyrenees and Alps), this "upheaval" must have taken place after their deposition in water, and they are consequently comparatively new. Besides these grand examples of elevating force, there are many of minor extent, but no less significance, stretching along the shores of continents, where the coast is gradually raised, and new islands are lifted from

the bed of the sea; or sea-beaten beaches are left high above the water-line, to testify to their late immersion, by water-worn cliffs and stones, and remains of marine animals. Strata become thus necessarily inclined at very various angles, from an almost imperceptible slope to the vertical; and in the latter case their edges are exposed for thousands of feet, and their thickness is easily measured. Elevation must be compensated for by a corresponding amount of depression or subsidence of some other part of the surface; and the alternation of these two forces has constituted a more potent distributor of land and sea than even water itself-fashioning the rough outlines of continents and oceans, and placing them successively within reach of atmospheric and aqueous influences.

A simple illustration of one form of subsidence will be seen in the following diagram, where D B is the

[blocks in formation]

sea level, and A B C the line of dip of a stratum whose lower part, A B, is now immersed up to the point of contact of land and water at в; B C representing the

F

« ÎnapoiContinuă »