The evidence of the authenticity of these implements from Thenay is, moreover, greatly strengthened by the discovery of other Miocene implements at Puy Courny, which have not been seriously impugned, and by the essay of Professor Prestwich, confirming the discovery of numerous flint implements in the upper level gravels of the North Downs, which could only have been deposited by streams flowing from a mountain ridge along the Anticlinal of the Weald, of which 2000 feet must have disappeared by sub-aërial denudation since these rivers flowed northwards from its flanks. How far back such a denudation may carry us is a matter of speculation. Certainly, as Prestwich admits, into the pre-glacial or very early glacial ages, and possibly into the Tertiaries, but at any rate for a time which, by whatever name we call it, must be enormous according to any standard of centuries or millenniums. And what is specially interesting in these extremely ancient implements is that, in Prestwich's words, "these plateau implements exhibit distinct characters and types such as would denote them to be the work of a more primitive and ruder race than those fabricated by palæolithic men of the valley drift times." [Illustration: COMPARE QUATERNARY IMPLEMENTS. SCRAPER. Yorkshire Wold. (Evans, _Stone Implements_.) QUATERNARY. Mammoth Period. River Drift, Mesvin. Belgium. (Congrès Préhistorique, Bruxelles, 1872.) QUATERNARY. Chaleux, Belgium. Reindeer Period. (Congrès Préhistorique, Bruxelles, 1872.)] In fact we have only to look at the figures which accompany Prestwich's essay,[12] to see that their types resemble those of Puy Courny and Thenay, rather than those of St. Acheul and Moustier. [12] _Journal of Anthropological Institute_, Feb. 1892, p. 262. The following remarks of the Professor would apply almost as well to the Miocene implements as to those of the plateau-- "Unlike the valley implements, the plateau implements are, as a rule, made of the _fragments_ of natural drift flints, that are found scattered over the surface of the ground, or picked up in gravel-beds and merely roughly trimmed. Sometimes the work is so slight as to be scarcely apparent; at others, it is sufficient to show a distinct design and object. It indicates the very infancy of the art, and probably the earliest efforts of man to fabricate his tools and weapons from other substances than wood or bone. That there was an object and design is manifest from the fact that they admit of being grouped according to certain patterns. These are very simple, but they answered to the wants of a primitive people.