Before the close of the tertiary period, a change occurred, and probably a rapid one, to a more rigorous climate than now exists. The destruction of the elephant in Siberia was evidently sudden, and was followed by extreme cold; for the animals are in some cases entirely preserved in ice, and in so perfect a state that, when the ice which surrounds them becomes melted, the flesh is devoured by carnivorous animals. There are occasionally found, in the drift of the boulder period, shells similar to those of the Arctic regions, and in a condition to show that they have not been transported. The clay beds of the northern portion of the United States and of Canada were deposited during the last depression of that portion of the continent, and they contain the remains of marine animals identical in several instances with species now living, but confined to more northern regions. It must therefore be admitted that the interval between the middle tertiary and the modern era was one of great cold. It is generally referred to as the _Glacial period_. Very considerable local changes of climate have also occurred within the historical period. Thus the mean temperature of the Alps has been so reduced that the ancient passes have in modern times become choked up with snow, and other passes have been sought,--a result, perhaps, of additional upheaval. It would seem that Siberia is now receiving a milder climate. The ice in which elephants have for centuries been imbedded has been slowly melting for at least thirty years. SECTION VI.--ADVANTAGES RESULTING FROM GEOLOGICAL CHANGES. 1. The division of the general surface into land and water, as well as the diversified form of the land, the existence of mountains and low lands, and the consequent modifications of climate, the waterfalls, and the river-systems, constituting the drainage of continents, are all results of the process of upheaval. 2. A large part of the mineral substances employed for architectural and economical purposes are oceanic deposits, such as the marbles, slates, sandstones and mineral salt, and would have been inaccessible if they had not been elevated from the position in which they were formed. And the elevation of them above the bed of the sea would have exposed only the superficial layer, if they had not been either irregularly uplifted, as at _e c_ (Fig. 69), or unequally worn down, as at _b_. [Illustration: Fig. 69.] The granitic rocks, as they were formed below the aqueous rocks, must have remained unknown and useless, if they had not been brought to the surface, as at _c_, by the most convulsive efforts of nature of which we have any knowledge. Thus, natural mechanical forces have effected for man what the mechanical forces under his control would be entirely insufficient to accomplish.