a 
J. D. Dana on American Geological History. 325 
whose influence on the general features we cannot yet make full 
owance. 
Through all this time, central British America appears to have 
ken 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 River 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 Champlain 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 
ons. 
_ 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 
uaurentian 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. want 
_ The south felt but slightly these oscillations. 
There are thus the following epochs in the Post-tertiary :—the 
Drift Epoch ; the Laurentian Hpoch, an epoch of depression; the 
Lerrace Epoch, an epoch of elevation; three in number, unless the 
Drift and Laurentian Epochs are one and the same. - 
As this particular pein is one of much interest in American 
fe gy, 1 will briefly review some of the facts connected with 
The drift was one of the most stupendous events in geological 
In some way, by a cause as wide as the continent,— 
Ro rock strata, from the first to the last, do we find imbedded 
