[H] 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.
[I] Fuller descriptions of these polished hills may be found in my work
on Lake Superior.