ON THE GEOLOGY OF THE ISLAND OF AQUIDNECK. 617 
men ; there can be no doubt, however, that it was quite uniformly 
covered with boulders of varied size. The thousand miles or 
thereabouts of stone walls which are found upon the island attest 
the original abundance of these surface boulders. Most of the 
larger masses have been broken up in order to be used for building _ 
purposes, but there still exist several thousand erratics having a 
_ diameter of more than three feet which have been deported from the 
cultivated fields, or lie in the regions which are too barren for cul- 
tivation. If we examine the sections in the drift, visible at various 
points, and notice the comparative variety of large boulders, it 
seems no easy matter to account for these numerous surface erratics 
without resorting to some such agency as floating ice operating 
at the close of the principal glacial period. This method of ex- 
plaining the abundance of surface erratics is much in favor with 
many European geologists. There are many arguments against it, 
however, which make it in the highest degree unlikely that it was 
the main agent. On the highest hills these boulders abound the 
most ; now there is every reason to believe that there has been at 
the most not over forty feet of subsidence on this shore since the 
beginning of the glacial period; a depression so considerable as 
to have lowered the level of the highest hills of Aquidneck island 
beneath the level of the sea, must have left some recognizable 
evidence of its existence. There is, however, no evidence sufti- 
cient of such a change of level. We can more satisfactorily 
account for the facts by assuming that these boulders of large 
size owe their abundance to the following causes. The rock 
masses in the glacier which were furthest from the rock bed would 
receive the least attrition and therefore remain the largest in size ; 
moreover, the continual wear to which the upper part of the drift 
bed has been subjected has served to wash away a large quantity 
of the mud, which forms a great part of the mass, leaving the 
Pebbles of various sizes accumulated on the surface. These great 
boulders should, on account of their superior position, have been 
tiven from their parent rock at a greater distance from their 
Present position than the other which lies below them. It is a 
tolerably safe conclusion that the lowermost pebbles in the mass 
which were riven by the ice from the bed rock, would be the 
last additions to the accumulation. Each successive accumulation 
Would lift the débris which was already in. existence, so that while 
in the time immediately after its separation from its bed the 
