ICE-PERIOD IN AMERICA. 
83 
it. But such large boulders, polished and 
scratched like the smaller pebbles, are to be 
found everywhere imbedded in American drift, 
while the angular fragments of rock resting 
above these triturated masses are comparative¬ 
ly rare.* It is evident from this that the ice 
* The greater proportion of large, rounded boulders in the 
American drift, as compared with the European, is a singu¬ 
lar 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 pur¬ 
poses 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. 
