There are many other deeply interesting questions with regard to the upheaval of mountains which at present cannot be answered.
We have already learned to alter our preconceived ideas about the stability and immovable nature of the earth's crust, and have seen that it is in reality most unstable, and is undergoing continual movements, both great and small. But here we have an equally startling discovery, which quite upsets all our former ideas of the hard and unyielding nature of the rocks composing the earth's crust; for we find that not only can they be bent into innumerable folds and little puckerings, but that in some cases they have been drawn out and squeezed as if they were so much soft putty. The imagination almost fails to grasp such facts as these.
Of late years geologists in Switzerland and in Great Britain have discovered that in some parts of mountains rocks have been enormously distorted and crushed, so that they have assumed very different states from those in which they were made, and curious mineral changes have taken place under the influence of this crushing.
In the very complicated region of the Northwest Highlands of Sutherland and Ross, the structure of which has only lately been explained, some wonderful discoveries of this nature have been made. Certain of the crystalline schists found there have been formed by the crushing down and rearrangement of older rocks that once presented a very different appearance. In this district, where the rocks have been squeezed by enormous lateral pressure, the dislocations sometimes have assumed the form of inclined or undulating planes, the rocks above which have been actually pushed over those below, and in some cases the horizontal displacement amounts to many miles.
Not only have the rocks been ruptured, and older, deep-seated masses been torn up and driven bodily over younger strata (that once were above them), but there has been at the same time such an amount of internal shearing as to crush the rocks into a finely divided material, and to give rise to a streaky arrangement of the broken particles, closely resembling the flow-structure of a lava. In the crushed material new minerals have been sometimes so developed as to produce a true schist.[32]
CHAPTER X.
THE AGES OF MOUNTAINS, AND OTHER QUESTIONS.
O Earth, what changes hast thou seen!
Tennyson.
It might naturally be asked at what period in the world's primeval or geological history some particular mountain-range was upheaved; whether it is younger or older than another one perhaps not very far away; and again, whether the mountain-chains of the world have been uplifted all at once, or whether the process of elevation was prolonged and gradual?