in the Geological History of North America. 347 
having been in those eras under water. Such beds may here- 
after be detected; but the great fact will still remain, that they 
are there of limited extent, if not wholly absent. As far as 
known, there is no Tertiary on the coasts north of Cape Cod. 
All development or growth there seems to have ceased, or.nearly 
so, with the Paleozoic era or the close of the Carboniferous age. 
But there are Post-tertiary deposits in the Arctic regions in 
many places, situated ee A of feet above the sea, containin, 
shells of existing Arctic species. This alone, independent of 
other evidence, would prove a change in the conditions of geo- 
logical progress after the Tertiary period. The necessa 
ence is, then, that as long as the southwest and southeast forces 
were in active play, and the extremities of the continent were 
thereby in process of growth, there was little change going on 
in the far north. But when the continent was nearly finished, 
its extremities grown, and the stability consequent upon adult 
age acquired, then, through a series of oscillations, a course of 
development was carried on in the more northern regions, giving 
a final completion to the continent—an action, which, as I have 
elsewhere explained, involved the higher latitudes about the 
T- 
east, and the southwest, according to the principle explained, was 
proportioned approximately to the sizes of the oceans, the Arctic, 
the i the Pacific; that the greater forces from the 
plains were spread out in terraces over the 1 
along the ceded that thus a large part of the brighter fea- 
* Address, etc., this volume, p- 327. 
