£. Desor on the Drift of Lake Superior. 107 
have been the cause of the groovings, it must be admitted that 
_ an agency which was capable of shaving off and wearing down 
such an extent of surface must. also have been able to remove 
the detritus and to transport it from one place to another. Iam 
inclined to ascribe this agency, and to consider as contemporane- 
ous with it, that portion of drift materials which I have described 
a8 Coarse drift, and which, wherever it exists, is regularly found 
at the base of the stratified deposits, having been left undisturbed 
by the waters of the subsequent period. 
We have shown that almost everywhere along the southern 
shore of Lake Superior there is a stratum of red clay resting on 
the coarse drift, or, where this is removed, on the polished rocks. 
From its thickness and the comminuted state of the materials, 
We infer that during its deposition a long interval of time elapsed, 
characterized by no violent agitations. With this stratum begins 
the second era of the drift. As to the boulders distributed 
through it, we may suppose that they were transported by float- 
ing ice, in the same manner as their transport is at this day” 
pose. A 
ing to Mr. H. D. Rogers’s ingenious theory, this feature should 
be ascribed to the temporary operation of earthquake waves, such 
ho 
this €poch had been characterized. by more violent agitations than 
preceding one. We should be careful, however, not to judge 
the power and violence of an agency mer dar from the size © 
oa 
Size in their progress southware 
