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POSTDILUVIAN DETRITUS. 
and can many of them be traced to their original source in that 
direction. Between Lake Erie also and Lake Huron, he states the 
beaches and woods to be strewed with masses of gneiss, porphyries, 
conglomerates, and greenstones; and that similar blocks appear on 
the north coast of Lake Erie, which itself is for the most part com¬ 
posed of a series of clay-cliffs and sand-hills. 
He moreover draws an accurate distinction between these diluvial 
driftings of the great debacle, and the small and usually angular 
debris of strata produced by causes now in existence: this latter 
remains unmoved nearly in the place in which it is formed, at the 
base of the parent-cliffs from which it has fallen. Thus the opposite 
shores of Pelletau, on Lake Huron, being of different formations, the 
one limestone, the other greenstone, each is lined with its own debris, 
and without admixture. 
In the fifth volume, number 2, of Silliman’s American Journal of 
Science, he also gives an excellent example of another species of 
postdiluvian operations, viz. the forming of terraces, like the parallel 
roads described by Dr. Macculloch, at Glenroy, in Scotland, around 
the edges of lakes and on the flanks of great rivers, at considerable 
elevations above the level of their present waters: terraces of this 
kind are not uncommon in North America. The case I now quote is 
that of the valley of St. Etienne, near Malbay, in which Dr. Bigsby 
accompanies his description with a map; by which it appears that 
this valley has been a postdiluvian lake, which has lowered its level 
at successive periods, by the breaking down at distant intervals 
of the gorge through which the Malbay river now flows into the 
