XXXVIU PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



and waterworn by their transit. Thus we should have the phse- 

 nomena of fine detrital mater below and blocks above, apparently- 

 referable to separate successive periods of time, during the first of 

 which one kind of agency should have transported the finer sediment, 

 and during the second another and much more powerful agency 

 should have transported the blocks and coarser detritus, while, in 

 fact, the whole phsenomena would be really referable to a repetition 

 of precisely the same agency during the whole period of transport. 

 That period, therefore, except in a limited sense, and not with refer- 

 ence to the whole area of transport, could not, in the case now 

 supposed, be divided into two, but must be regarded as one single 

 period. 



I do not mean here to assert the opinion that the actual glacial 

 period recognized by geologists was characterized by a uniform suc- 

 cession of exactly similar events producing erratic dispersion. There 

 might be particular portions of that period in which accidental cir- 

 cumstances produced a greater or less prevalence of each particular 

 mode of transport ; but I am satisfied that some of the attempts 

 which have been made to subdivide the glacial period have been made 

 without due regard to such considerations as those which I have 

 given above. 



Let us now turn again to the drift of North America. The Ameri- 

 can geologists appear for the most part to recognize three distinct 

 peiiods into which the whole period of the drift may be divided. 

 The first period was one of the transport of blocks and coarse ma- 

 terials ; the second, one of tranquil deposition ; and the third was 

 again a period of transport of large blocks and coarser matter. This 

 generalization appears to have been principally founded on the cha- 

 racters of the drift of Lake Champlain and that of the general valley 

 of the St. Lawrence, where the beds of the second period not only 

 consist, in great part, of finer matter, but are also, in many instances, 

 distinctly stratified, and filled with organic remains. But before we 

 can adopt these subdivisions of the general period with reference to 

 so many distinct modes of action of the transporting agencies, or of 

 the different degrees of intensity with which they acted, it will be 

 necessary to prove the above-mentioned succession of beds to be 

 general and not merely local. If local, I should be disposed to refer 

 the tranquil deposition of the fossiliferous and associated beds, partly 

 at least, to the condition of a deeper submergence than at the periods 

 of the transport of the coarser beds and blocks above and below the 

 finer beds. I see no reason in local facts of this kind to infer that 

 there were three distinct periods with reference to the intensity or 

 mode of action of the dispersing forces. I may here observe that 

 Dr. Bigsby detected no evidence of this subdivision of the drift in the 

 region which he examined further to the west. 



Some of the American geologists appear to have entertained the 

 opinion that the Mastodon existed in that region after the latest 

 period of the Drift, and seem to refer its final destruction to some 

 upheaval of the American continent. It may be doubted, however, 

 whether any evidence has been offered of the existence of that animal 



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