612 ON THE GEOLOGY OF THE ISLAND OF AQUIDNECK. 
reminds one of the pudding stone in the neighborhood of Boston, 
but it differs from that-rock in the comparative abundance of the 
matrix, and scarcity of pebbles of large size. The pebbles are, 
moreover, composed of different sorts of rock from those in the 
Boston conglomerates. After a careful inspection, I am compelled 
to conclude that none of the conglomerate masses are from that 
set of beds, which extend over the country to the north and east 
of the Blue Hills near Boston. : 
On the northern part of the island, a few of the syenite cliffs, 
which rise at steep angles from the plain, are the only points which 
have no drift matter upon them. Between these cliffs and thè 
north slope of Butts Hill is a. plain much indented by singular 
depressions already described (vide supra p. 523). The drift here 
has an unknown depth; it is known to be more than fifteen feet 
in thickness, and is probably at some points as much as forty 
feet thick. The pebbles here are with very rare exceptions less 
than six inches in diameter and are heaped together with only 
imperfect stratification. The cementing matter differs in no way 
from that which is always found in our boulder deposits, being & 
mixture of all the kinds of materials which go to make up 
pebbly part of the deposit. There is no trace of true “ay : 
heaps, such as fill the valleys in front of existing glaciers, in this : 
part of the island. All the drift has, more or less, the character 
described by Agassiz, and by him referred to the melting a 
great ice sheet, and the deposition, in a great unstratified pe o 
all the pebbles which it had torn from its bed. Followms i - 
drift southwards, we find that without much change of volume Eo 
constituents become greatly altered in character. A large part aie 2 
the materials which are found in it at the extreme northern end . 
í ; 
appear, and are replaced by fragments from the rocks which ee ia , 
: 
3 
in the immediate neighborhood. This is very clearly ia per 
beds which overlie the portion of the island which coat a 
The matrix or cement of the mass is here much darker than a fe 
drift near Blue Bill Cove; a large part of the fragments ae 
slate of a carbonaceous character. This blackish color 
to the drift found to the south of the coal field, though it De" of : 
boulders behind us aś we go from north to south. Fr rife nest | 
less and less marked as we go away from the source aa 
the coal. On the eastern shore we gradually leave the 53 : 
this sort of rock, which made up about one-half of the 
