Читать книгу The Romance of Modern Geology. Describing in simple but exact language the making of the earth with some account of prehistoric animal life онлайн

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With such tokens of their existence as this, glaciers, as will readily be understood, leave visiting-cards in history that cannot easily be mistaken. Even existing glaciers tell strange stories. Nowadays glaciers are carefully measured and examined both in Switzerland and in Canada. During the last decade of the nineteenth century and the first decade of the twentieth the Swiss glaciers were found to show signs of receding farther up their valleys. The same thing has been observed about some of the Canadian glaciers. There are several plausible reasons for this. Professor Schaeberle says that the earth is growing cooler, and that in the temperate regions the winter rainfall (which would turn to falls of snow in the mountains) is less than it has been. It is certain that a shortage of winter rain over a number of years in succession would account for the shrinkage of the glaciers, but it is not by any means certain that a number of dry winters will not be followed by wet ones, in which case the glaciers would increase again. Some of the glaciers show that during their existence they have shrunk and lengthened alternately like gutta-percha in a variable climate. How do we know that they have shrunk and lengthened? The moraines of which we have spoken give us the testimony. As a glacier shrinks either in length or in breadth and depth it leaves the blocks of rock at its edges stranded on the sides of the valley. Such perched blocks or erratics are the best of glacier marks, and their great size, some of them as large as a cheap villa residence, is such that no current of water could have brought them there. They are often poised on the tops of crags, on the very edges of precipices, or on steep slopes, where they could never have been left by any flood, even had the flood been able to move them. The only thing that could have carried them must have been a vehicle that moved very, very slowly and deposited them very, very gently—in fact, glacier ice. We can see blocks like this on the glaciers now, and others stranded at the sides. In the Swiss valleys the scattered ice-borne boulders may be seen by hundreds far above the glaciers and far beyond the places where the glaciers end. We know they must have been left by glaciers, and by inference we surmise that when we find a valley filled with them, then, though the valley may have no glacier now, it must have once been occupied by one.