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] [32] Geikie. 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? Questions such as these are deeply interesting, and present to the geologist some of the most fascinating problems to be met with in the whole range of this science. And though at first sight they might seem hopelessly beyond our reach, yet even here the prospect is by no means unpromising; and it is quite possible to show that they can be answered to some extent. Here we shall find our illustration of the cathedral (see chapter v., pages 143-147) holds good once more. It is perhaps hardly necessary to explain that by looking at a Gothic cathedral one can say at what period or periods it was built. Perhaps it has a Norman nave, with great pillars and rounded arches. Then the chancel might be Early English, with pointed windows and deep mouldings, and other features that serve to mark the style of the building, and therefore its date,--because different styles prevailed at different periods. Other parts might contain work easily recognised as belonging to the "Perpendicular" period. Now, as there have been periods in the history of architecture and art, so there have been periods in the history of our earth. What these periods were, and how we have learned to recognise them, we must first very briefly describe.[33] [33] For a fuller account see the writer's "Autobiography of the Earth."