420 On American Geological Historf, 



The upper terrace of the lakes and rivers, and also the mariD<? 

 beds four hundred feet above the level of Lake Champlain, and 

 five hundred above the St. Lawrence, which have been called 

 Laurentian deposits, are marks of a Twrtherii depression, as no 

 one denies. : 



The subsequent elevation to the present level again, by stages 

 marked in the lower river terraces, was also northern^ afiecting 

 the region before depressed. 



The south felt but slightly these oscillations. 



There are thus the following epochs in the Post-tertiary : — the 

 Drift Epoch ; the Laurentian Epoch, an epoch of depression ; the 

 Terrace Epoch, an epoch of elevation ; three in number, unless the 

 Drift and Laurentian Epochs are one and the same. 



As this particular point is one of much interest in American 

 Geology, I will briefly review some of the facts connected with 

 the drift. 



The drift was one of the most stupendous events in geological 

 history. In some way, by a cause as wide as the continent,— 

 and, I may say, as wide nearly as the world, — stones of all sizes, 

 to immense boulders of one or two thousand tons weight, were 

 transported, along with gravel and sand, over hills and valleys, 

 deeply scratching the rocks across which they travelled. Al- 

 though the ocean had full play in the many earlier ages, and an 

 uneaay earth at times must have produced great convulsions, in 

 no. rock strata, from the first to the last, do we find imbedded 

 stones or boulders at all comparable in magnitude with the 

 immense blocks that w^ere lifted and borne along for miles in the 

 Drift epoch. 



Much doubt must remain about the origin of the drift, until 

 the courses of the stones and scratches about mountain ridges 

 and valleys shall have been exactly ascertained. The general 

 course from the north is admitted ; but the special facts proving 

 or disproving a degree of dependence on the configuration of the 

 land have not yet been sufiiciently studied. 



One theory, the most prevalent, supposes a deep submergence 

 over New England and the north and west, even to a depth of 

 four or five thousand feet, and conceives of icebergs as floating 

 along the blocks of stone, and at bottom scratching the rocks. 

 Another, that of the Professors Rogers, objects to such a sub- 

 mergence, and attributes the result to an incursion of the oceaa 



