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It shows that when some of these volcanoes were in action the valleys had already been eroded to their present depth. The tufaceous alluvium called trass, which has covered large areas in the Eifel, and choked up some valleys now partially re-excavated, is unstratified.
M. Mortillet in the same year advanced the theory that after the Alpine lake-basins had been filled up with loose fluviatile deposits, they were re-excavated by the great glaciers which passed down the valleys at the time of the greatest cold, a doctrine which would attribute to moving ice almost as great a capacity of erosion as that which assumed that the original basins were scooped out of solid rock by glaciers.
As it is seen in masses fringing both sides of the great plain, and as occasionally remnants of it occur in the centre of the valley, forming hills several hundred feet in height, it seems necessary to suppose, first, a time when it slowly accumulated; and secondly, a later period, when large portions of it were removed, or when the original valley, which had been partially filled up with it, was re-excavated.
M. Gabriel de Mortillet after a careful study of the glacial formations of the Alps agreed with his predecessors that the great lakes had existed before the glacial period, but came to the opinion in 1859 that they had all been first filled up with alluvial matter and then re-excavated by the action of ice, which during the epoch of intense cold had by its weight and force of propulsion scooped out the loose and incoherent alluvial strata, even where they had accumulated to a thickness of 2000 feet.
As soon as Babylonia was cleared of its enemies, Khammurabi set himself to the work of fortifying its cities, of restoring and building its temples and walls, and of clearing and digging canals. The great canal known as that of "the King," in the northern part of the country, was either made or re-excavated by him, and at Kilmad, near the modern Bagdad, a palace was erected.
We know that many large areas of land are rising and others sinking, and unless it could be assumed that both the upward and downward movements are everywhere uniform, many of the existing hydrographical basins ought to have the appearance of having been temporary lakes first filled with fluviatile strata and then partially re-excavated.
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