Thus we learn that when one agency fails, Nature makes use of another to take up the important work of rock-building. How the other rocks which we mentioned in our list were formed,—such as granite, basalt, and the metamorphic rocks,—we must explain in a future chapter dealing with volcanoes and their work.
CHAPTER VI.
HOW THE MOUNTAINS WERE UPHEAVED.
The notion that the ground is naturally steadfast is an error,—an error which arises from the incapacity of our senses to appreciate any but the most palpable, and at the same time most exceptional, of its movements. The idea of terra firma belongs with the ancient belief that the earth was the centre of the universe. It is, indeed, by their mobility that the continents survive the increasing assaults of the ocean waves, and the continuous down-wearing which the rivers and glaciers bring about.—Professor Shaler.
We have found out the quarries which supplied the rocky framework of mountains, and have learned how the work of transporting these vast quantities of stone was accomplished by the agency of ever-flowing glaciers, rivers, and streams.
We must now consider the second stage of the work, and inquire how the mountains were raised up. Referring back to our illustration of the cathedral (see pages [143]-[147]), it will be remembered that this work was included under the head of Elevation. But perhaps some one might ask: "How do you know that the mountains have been elevated or upheaved? Is it not enough to suppose that they owe their height entirely to the fact that they are composed of harder rock, and so have been more successful in resisting the universal decay and destruction?" Now, such an objection contains a good deal of truth, for mountains are formed of hard rocks; but at the same time we know that the agents of denudation are more active among them than on the plains below, so that, in the higher mountain regions at least, the work of demolition may actually proceed faster than it does on low ground.
Mountains are higher than the rest of the world, not merely because they are built of more lasting material, but also because they have been uplifted for thousands of feet above the level of the sea; and the evidence of their upheaval is so plain as to be entirely beyond doubt.
Let us inquire into the nature of this evidence. We have seen that the rocks of which mountains are composed were for the most part formed at the bottom of the sea. When the geologist finds, as he frequently does, buried in mountain rocks the fossil remains of creatures that must have lived in the sea (and often very similar to those living there now), he is compelled to think of the gigantic upheavals that must have taken place before those remains could arrive at their present elevated position.
Numerous examples might be given; but we will only mention three. In the Alps marine fossils have been detected at a height of 10,000 feet above sea-level, in the Himalayas at a height of 16,500 feet, and in the Rocky Mountains at a height of 11,000 feet.
Again we must take it for granted that all the stratified or sedimentary rocks (see pages [148]-[149]) with some trivial exceptions, such as beds of shingle and conglomerates, have been formed in horizontal layers. This is one of the simple axioms of geology to which every one must assent.