Note H — ICE-PERIOD IN AMERICA.

The greater proportion of large, rounded boulders in the American drift, as compared with the European, is a singular fact not fully met by the above explanation; since, while the number of mountain-peaks rising above the ice in Europe would account for the frequency of large angular fragments transported upon its surface, there would seem to be no reason why the drift, carried along by a mass of ice having the same thickness in both continents, should not contain as many rounded masses in one as in the other. The facts, however, are as I have stated them, and the difference may be due partly to the broken character of the ground over which the drift must have passed in Europe, subjecting it to a more violent process of friction and grinding than in America, and partly to the use that has been made of the drift-boulders during so many centuries for building-purposes in the Old World, the drift-boulders being naturally taken first, because they are more easily reached, while the angular ones are frequently perched on almost inaccessible spots. Indeed, the stone fences in both countries tell us the use to which many of the rounded boulders have been put, and the ground in many parts of the United States has already been cleared to a great extent of its rocky fragments for this and like purposes. In the course of time they will, no doubt, disappear from the surface of this country, as they have done from that of Europe.