We have no means of measuring the time which may have elapsed since these stone weapons and tools were made. And we need not speak here of the geological changes which must have passed over the surface of the earth since they were deposited upon it. All we know is that, after the great streams flowing through wide valleys have dug these implements from under the earth which time had heaped over them, carried them along and deposited them once more amid sand and pebbles in a bed upon some point of its course, the river must through long subsequent years have cut so much deeper into the valley through which it flowed, and at the same time probably so shrunk in its bed, that these river drifts, as they are called, stand in many cases fifty, eighty, a hundred feet above the level of the present stream. It is because they are found in the beds made by the ancient rivers, that the implements of this period are called _drift implements_. The river Ouse, of which we spoke just now, which, though to-day a small river, drains a large and level country as it runs through the counties of Bedford, Huntingdon, and Cambridge, has been one of the most prolific in this class of pre-historic remains. Another river which still better deserves to be remembered in this respect is the Somme in the north of France. For it was in the beds of this stream, by Abbeville and Amiens, that the drift implements were first discovered, or first recognized for what they really are, the earliest traces of human labour; and it was here that the foundation was laid for this branch of pre-historic study by M. Boucher de Perthes. This was forty-one years ago, in 1847. These _drift implements_, then, form a class apart--apart even from all other stone implements made by man, and probably earlier than any other class. Very simple and rude are these drift implements. It would require a skilled eye to detect any difference between most of them and a flint which had only been chipped by natural means. But the first thing to remember is, that the makers of these implements had nothing but other still ruder materials to help them in this manufacture of theirs. Metals of all kinds were as yet utterly unknown to man.