618 ON THE GEOLOGY OF THE ISLAND OF AQUIDNECK. 
fragment of rock would be subjected to great wear, it might, by 
the subposition of fragments more recently sundered, be gradually 
lifted out of the level of greatest attrition into a level where it 
would be carried without much wear. When the rock over which 
the ice flowed yielded fragments more rapidly than they were 
ground up, the conditions would be the most favorable for distant 
transportation; when, on the other hand, if the grinding of the 
pebbles went on more rapidly than they were supplied, the chance 
of distant transportation would be small. This is well illustrated 
over the southern part of Aquidneck island. When the glacier 
swept over the friable slates, the drift drawn from the northern 
end of the island kept in security in the upper part of the débris; 
when it passed on to the southern part of the section where the 
hard felsites and argillites were encountered, the packing of soft 
shale was quickly ground up, and then the syenite pebbles were let 
down upon the bed rock and soon reduced to fine material. The 
result is that the region known as the neck, south of Newport, 
has a very thin coating of drift matter. 
All the boulders, of whatever size they may be and at all posi- 
tions in the débris, show evidences of abrasion which cannot be 
explained without supposing that they were derived from the fog 
of the ice mass, and passed through the inevitable jostling which 
must occur while they were at the bottom of the pack. 
There can be no question but that the retreat of the ice across 
the section given in this island must have been, in a geological oa 
sense, very rapid. The evidence is clear that at one um 4 
extended far to the seaward of the southern end of the island, 
so far indeed, that we can find no distinct evidence at this point of 
the great masses of débris, which must have been de wis 
the outer border in the shape of a great terminal moraine. A w 
ice went back, if its retreat had been by successive steps, ol 
which brought the edge of the ice across this island, ee go 
would have been the formation of a distinct terminal morain® 
cannot be said that such a mark would have been obliterated by 
subsequent changes, for the scratches which cover the up 
pebbles of the drift would surely have been worn away sis 
erosion which could have had great effect on the contour of E so 
face. We are therefore justified in supposing that the pee 
retreated rapidly and steadily, when it passed from the sure 
this part of our shore. It does not seem as if the term ™ 
