J. D. Dana on American Geological History. 325 



whose influence on the general features we cannot yet make full 

 allowance. 



Through all this time, central British America appears to have 

 taken little part in the operations ; and what changes there were, 

 except it may be, in the Arctic regions, conformed to the system 

 prevailing farther south, for the rocks of the Jurassic Age, like 

 the Connecticut Kiver sandstone, are found as far north as Prince 

 Edward's Island, in the Gulf of St. Lawrence. 



But the Tertiary Period does not close the history of the con- 

 tinent. There is another long Period the Post-tertiary, — the 

 period of the Drift, of the Mastodon and Elephant, of the lake 

 and river terraces, of the marine beds on Lake Champlam and 

 the St. Lawrence, — all anterior to the Human Era. 



From this time there is a fundamental change in the course of 

 operations. The oscillations are from the north, and no longer 

 from the southeast. 



The drift is the first great event, as it underlies the other loose 

 material of the surface ; and all recognize it as a northern phe- 

 nomenon, connected with northern oscillations. 



The upper terrace of the lakes and rivers, and also the marine 

 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 northern depression, as no 

 one denies. 



The subsequent elevation to the present level again, by stages 

 marked in the lower river terraces, was also northern, affecting 

 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 

 uneasy 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 were lifted and borne along for miles in the 

 Drift epoch. 



