Page:Geological Evidences of the Antiquity of Man.djvu/172

154 The alluvium of the Seine and its tributaries, like that of the Somme, contains no fragments of rocks brought from any other hydrographical basin; yet the shape of the land, or fall of the river, or the climate, or all these conditions, must have been very different when the grey alluvium in which the flint tools occur at Paris was formed. The great size of some of the blocks of granite, and the distance which they have travelled, imply a power in the river which it no longer possesses. We can scarcely doubt that river-ice once played a much more active part than now in the transportation of such blocks, one of which may be seen in the Museum of the École des Mines at Paris, three or four feet in diameter.

In the ancient alluvium of the basin of the Thames, at moderate heights above the main river, and its tributaries, we find fossil bones of the same species of extinct and living mammalia, accompanied by recent species, of land and fresh water shells, as we have shown to be characteristic of the basins of the Somme and the Seine. We can scarcely therefore doubt that these quadrupeds, during some part of the post-pliocene period, ranged freely from the continent of Europe to England, at a time when there was an uninterrupted communication by land between the two countries. The reader will not therefore be surprised to learn that flint implements of the same antique type as those of the valley of the Somme have been detected in British alluvium.

The most marked feature of this alluvium in the Thames valley is that great bed of ochreous gravel, composed chiefly of broken and slightly worn chalk flints, on which a great part of London is built. It extends from above Maidenhead through the metropolis to the sea, a distance from west to east