ON THE GEOLOGY OF THE ISLAND OF AQUIDNECK. 755 
tions which have given the character to the topography of New 
England were already in a position * to be exposed to erosion. 
We have already noticed the close similarity observable between 
the conglomerate of the primordial time and that which has been 
produced during the last geological period. It is not easy to find 
any sharp line of demarcation between the characters of these two 
detrital rocks. The older conglomerate is always more compact 
and has usually a trace of bedding, though this feature is often 
wanting. The pebbles are rather more regularly arranged even 
‘where the distortion of the pebbles (a point to be treated here- 
after) is least or entirely wanting. Traces of an arrangement of 
the pebbles as if they had been under flowing water are evident. 
The fact that at certain points the pebbles have had most, if not 
all, of the sand and clay washed away between them is a strong 
proof of their having been exposed to a degree of washing which 
has been unfelt by more modern deposits. The great similarity 
in the characters of these two conglomerates, the recent glacial 
and the carboniferous’ beds, is strong evidence of relation of origin. 
The conditions under which conglomerates can be formed are few 
and peculiar ; mountain torrents or the sea rolling upon a shore of 
tain torrents can only make band-like beds or heap up their débris 
in delta accumulations where their rapid streams merge in water 
of less carrying force. The sea cannot move pebbles except 
within the narrow range of its breakers ; it rarely has tidal cur- 
rents which can sweep coarse sand along. The only agent we 
know to have been generally in action on the earth which is capa- 
ble of moving pebbles in such a manner as to produce broad 
deposits of boulders is ice. That it is able to do such work is 
fully shown by the great accumulations of the drift period. The 
-Tesearches of Mr. James Croll have shown that the conditions 
_ which were probably instrumental in producing the last glacial 
period have operated again and again in the past to effect the same 
result, There are many points in the great geological section which 
ee *Should it be proven that these Rhode Island conglomerates are of the same atten 
_ the materials of similar character in the neighborhood of Boston, we may rs = - 
to extend much further this system of reasoning. But it is more than likely, as I sha 
_ try to show in the discussion of the geology of Massachusetts Bay, that the conglome- 
s found there are of still earlier age, having probably been 
S of the primordial time. Should this conjecture prove true the ee a 
> > materials composing these beds will prove an even more interesting key 
: ancien t history of New England. 
rock may form very local accumulations of this nature, but moun-. 
formed during the later 
of 
d 
