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‘CRETACEOUS AND TERTIARY—CONDITIONS OF DEPOSIT. 161 
case, the deposits of this division resemble, in their fineness and homo 
geniety both in time and extension, sediment borne far by marine 
currents and deposited in a sea of some depth, rather than that formed 
from the denudation of neighbouring land. In the Rocky Mountain re- 
gion, however, these deposits still show the existence of somewhat sbal- 
lower water conditions, and though generally maintaining somewhat the 
aspect of these eastern representatives, contain much coarser matter. 
380. In the closing period of the Cretaceous, represented by its fifth 
division, we have continualiy shoaling waters, and the introduction for a 
comparatively limited interval, of deposits, for the most part, coarse and 
arenaceous, and aimost destitute of organic remains. This shoaling of the 
area, may have been produced not so much by any general elevation of 
the continent, as by the increasing thickness of the Cretaceous deposits 
themselves, which may now have amounted in some places to over 2,000 
feet. If this was the case the banks and shoals of the region now marked 
by the ranges of the Rocky Mountains, need not have been raised up into 
extensive land areas, but would retain the character so long maintained 
by that area, and now again assumed by the interior continental region 
generally. The distribution of this arenaceous material was probably 
due to arctic currents, no doubt, then as now forming a part of the 
general oceanic circulation. These, like those still flowing down the 
eastern coast of America, would be urged westward by the rotation of the 
earth, and passing over the extensive shallow water area of the contin- 
ental plateau, would find—if not a continous sea—wide straits and open- 
ings through the Rocky Mountain region, and fall thence into the deeper 
basin of the Pacific. Professor Newberry, from the indications furnished 
by a comparison of the floras, concludes that relative differences of tem- 
perature, similar to those now obtaining, existed during the Cretaceous 
and Tertiary periods, between the interior continental region, and the 
west coast.* If the former region was influenced in the manner above 
supposed, by the arctic currents, while the shores of the latter were 
bathed in warmer waters flowing from the south, as at the present 
day, this may easily be accounted for. 
381. It would thus appear, that the Cretaceous opened with a period 
of considerable land surface, shallow waters, and current-driven sand- 
banks, which also extended to the Mississippi region, described by Prof. 
Hilgard; and probably to the Cretaceous coast-deposits of New Jersey, 
* Geological Report Yellowstone and Missouri Expedition, 
ll Db 
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