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ON THE GEOLOGY OF THE ISLAND OF AQUIDNECK. 613 
Anthony’s, become rare when we get as far south as the glen, 
and are almost entirely wanting at Sachuest Neck. The conglom- 
erate, with very much compressed pebbles, which does not occur 
north of the east and west line drawn from Black Point to Law- 
ton’s valley, is not found in the drift to the north of that line, 
though not infrequent in the beds of that age at all points to the 
southward of it. The north end of the island has a great quan- 
tity of conglomerate erratics but they belong to the type of that 
rock found to the north of Somerset on the Taunton River, and 
probably at other points in the direction whence came the glacial 
stream. 
It will be shown further on that at various points the under- 
lying rock of the island is traversed by veins of white quartz 
often having a thickness of many feet; the position of these veins, 
even when the rock in place is hidden, is often made evident by 
the trains of quartz blocks which lie mingled with the boulder 
mass, to the south of the outcrop of the vein. The actual 
thickness of the drift diminishes pretty steadily.as we go south- 
ward; this is doubtless to be attributed to the soft character of 
the rock which underlies the island. That which the glacial 
stream rent from the bottom, in that part of its course which lies 
just north of the island, was of a much more resisting character 
than that which it encountered in its course over the island itself; 
naturally the endurance of the rock masses ground beneath the 
glacier was in proportion to the hardness of the material of which 
they were composed, so that where the rock was very soft the 
rapid disappearance of its fragments would prevent a great accu- 
mulation of débris. 
Professor Agassiz has already called attention to the essential 
difference in the character of the débris of the wearing in the two 
modes in which glaciers may act. 1st, where the ice is in the form 
isolated streams bounded by distinct rock walls, as in the existing 
glaciers of Switzerland. 2d, where the ice is spread broadly over 
the surface of the continent, attaining to a depth which quite 
deprives it of all bounding walls, giving us a sea of glacial matter 
in place of the rivers of ice in the other condition. In the first 
Condition of the ice, the rock boundaries of the streams furnish 
Comparatively the larger part of the material transported by the 
acier; something is torn away from the bed by the moving 
Stream, but its comparatively thin mass gives it no such abrading 
