Now, it is believed that all mountain-ranges have been enormously squeezed by lateral pressure; and the little experiment with the sheet of paper furnishes a good illustration of what has happened. A table-cloth lying on a smooth table will serve equally well. You can easily push it into a series of folds; notice how they come nearer as you continue pushing. You see also that in this way you get long narrow ridges with valleys between. These represent the original anticlines and synclines of mountain-ranges, which in course of time are carved out, as explained above, until the synclines become hills and the anticlines valleys.
Every mountain-chain must originally have had long ridges like these, which in some cases determined the original directions of the streams and valleys; and it is easy to see now why mountain-chains are long and narrow, why their strata have been so greatly folded, and why we get in every mountain-chain long ranges of hills roughly parallel with each other (see chapter vi., pages [177]-[178]).
The reason why granite, gneiss, and crystalline schists are frequently found in the central and highest peaks of mountain-ranges is that we have the oldest and lowest rocks exposed to the surface, on account of the enormous amount of denudation that has taken place. There may be great masses of granite underlying all mountain-chains; but it is only exposed to view when a very great deal of overlying rock has been removed.
It was thought at one time that granite was the oldest of all rocks, and that mountain-chains had been upheaved by masses of granite pushing them up from below; but we know now that both these ideas are mistaken. Some granites are certainly old geologically, but others are of later date; and it is certain that granite was not the upheaving agent, but more likely it followed the overlying rocks as they were heaved up by lateral pressure, because the upward bending of the rocks would tend to relieve the enormous pressure down below, and so the granite would rise up.
MONT BLANC. SNOWFIELDS, GLACIERS. AND STREAMS.
We now pass on to a very different example, where mountains are the result of huge fractures and displacements; namely, the numerous and nearly parallel ranges of the Great Basin, of Western Arizona, and Northern Mexico. The region between the Sierra Nevada and the Wahsatch Mountains, extending from Idaho to Mexico, is composed of very gently folded rocks deeply buried in places by extensive outflows of lava.
Now, in this case the earth-movements caused great cracks, or splits, doubtless attended by fearful earthquakes. We find here a series of nearly parallel fractures, hundreds of miles long, and fifteen to thirty miles apart. These traverse the entire region, dividing the rocks into long narrow blocks. There is evidence to show that the whole region was once much more elevated than it is now, and has subsided thousands of feet. During the subsidence along these lines of fracture, or faults, the blocks were tilted sideways; and the uptilted blocks, carved by denudation, form the isolated ranges of this very interesting region (see illustration, chap. viii., p. [273], Fig. 1). The faults are indicated by arrows pointing downwards; and the dotted lines indicate the erosion of the uptilted blocks.
But this must be regarded as a very exceptional case, for we do not know of any other mountain-range formed quite in the same way. Why the strata, although only slightly bent, should have snapped so violently in this case, while in other mountain-ranges they have suffered much more bending without so much fracture and displacement, we cannot tell, but can only suggest that possibly it was because they were not buried up under an enormous thickness of overlying rocks, which would exert an enormous downward pressure, and so tend to prevent fracturing.