in the Geological History of Nor-th 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 Palaeozoic era or the close of the Carboniferous age. 

 But there are Post-tertiary deposits in the Arctic regions in 

 many places, situated hundreds of feet above the sea, containing 

 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 necessary infer- 

 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 

 whole sphere, north and south of the equator.* 



We shall understand more definitely the relations of the 

 later to the older oscillations, if we consider that all were due to 

 one grand cause, influencing the whole extent of the continent 

 even to the Arctic ocean ; that the force from the north, the south- 

 east, and the southwest, according to the principle explained, was 

 proportioned approximately to the sizes of the oceans, the Arctic, 

 the Atlantic and the Pacific ; that the greater forces from the 

 southeast and southwest acted against that from the north, 

 and through their superior strength or the concurrent greater 

 flexibility of the crust, kept up those vibrations in the progress 

 of which the border mountains were made ; but at last, the south- 

 east and southwest action almost ceasing through the stiffening 

 and 'uplifting of the crust, then the northern force, having a 

 stable fulcrum, made itself felt in the long and slow oscillations 

 of the Post-tertiary. Under this mode of view it will be seen 

 that all was part of one system of development. 



If we rightly apprehend the results of the Post-tertiary period, 

 we shall perceive that there was vast importance in these finish- 

 ing operations over the sphere : — that during its progressing cen- 

 turies, the great phenomena of the drift took place, covering 

 hills and plains with earth ; that the valleys for our rivers were 

 then either made or vastly enlarged; that immense alluvial 

 plains were spread out in terraces over the interior and in flats 

 along the shores ; that thus a large part of the brighter fea- 



* Address, etc., this volume, p. Z21. 



