And here we may point out that the Alps are only a portion of a vast chain of mountains stretching right across Europe and Asia in a general east and west direction, beginning with the Pyrenees and passing through the Alps, the Carpathians, the Caucasus, and the range of Elbruz to the Hindoo-Koosh and the high plateau of Pamir, called "the roof of the world," which stands like a huge fortress, fifteen thousand feet high. Thence it passes to the still higher tracts of Thibet, great plains exceeding in height the highest summits of the Alps, being enclosed between the lofty ramparts of the Himalayas on the south and the Kuen-Lun Mountains on the north; and thence the mountain wall is prolonged in the Yuen-Ling, In-Shan, Khin-Gan, and other ranges till it finally passes to the Pacific Ocean at Behring's Strait. All these ranges are, as it were, the backbone of the great continental plateau of the Old World, and doubtless are chiefly due to those earth-movements by means of which the Alps were upheaved. The last grand movement, which raised the Mont Blanc range, was probably rather later, and seems to have taken place as late as the Pliocene Period. At the present day no great movements are taking place in the Alps; but now and then earthquakes visit this region, and serve to remind us that the process of mountain-making is still slowly going on. Probably there have been times in the history of all these mountain-ranges when movements took place of a more violent and convulsive kind than anything with which we are familiar at the present day; and the age we live in may be one of comparative repose. This is of course somewhat a matter of speculation; and we only allude to it because there has been a tendency on the part of some to carry the theory of uniformity in all geological operations much farther than Hutton or Lyell ever intended. But at the same time there is no need to go back to the old teaching of sudden catastrophes and violent revolutions. We only wish to avoid either of these two extremes and to take a safe middle course.