128 ON THE GEOLOGY AND NATURAL HISTORY 
nite beds dip five to ten degrees. Along the Platte I have seen the former inclining 
five degrees, especially on La Bonte creek, and about fifteen miles east of the mouth of 
that creek. Often the beds seem to have been raised up several hundred feet above their 
original position, without inclination, resting upon the upturned edges of the lignite beds, 
which we have before observed partook equally of the disturbing influences which have 
given so great an inclination to the older fossiliferous rocks. Along the Big Horn moun- 
tains and the North Platte the lignite beds sometimes incline from the foot of the moun- 
tains eighty degrees, and often the influence of the elevatory power has affected them far 
out into the plain country. 
In the above accounts of the Tertiary deposits of the West, we have shown that the 
older members are clearly separable into four divisions, exclusive of the Pliocene deposits 
of the Niobrara. Let us examine the evidence in regard to the age of these deposits. If 
we study the upper portions of Cretaceous formation No. 5, when not removed by the ero- 
sive power of water to any great extent, we then observe, from the time we pass from No. 
4 to No. 5, a gradual change in the sediments, and other indications of a slow approach 
to shallow water: arenaceous sediments begin to take the place of argillaceous, so that we 
have alternate thin layers of sand and clay, the sand continuing to increase until the upper 
part becomes a yellow, ferruginous, coarse sandstone, with most conspicuous examples of 
ripple-mark and oblique lamine. As the waters of the Cretaceous sea were gradually re- 
ceding, toward the Atlantic on the one side and toward the Pacific on the other, remnants 
were left, in the form of lakes, estuaries, &c., which now afford us the last indications of 
marine and brackish water deposits in the central portions of the West. In these deposits 
we have first a mingling of brackish and freshwater forms, gradually passing up to pure 
freshwater and terrestrial species, with no return to the marine condition again. 
In the upper part of the Cretaceous formation No. 5, on the Moreau, we find the Ostrea 
subtrigonalis, and in the Judith deposits a form occurs in the greatest abundance which is 
undistinguishable from it. 
We have also mentioned the fact that the fossils of the upper part of No. 5 seem to 
have existed upon the verge of the Tertiary period, that they sometimes present peculiar 
forms more closely allied to Tertiary types than Cretaceous, and were it not for the presence 
of the genera Baculites, Ammonites, Inoceramus, &c., which are everywhere supposed to 
have become extinct at the close of the Cretaceous epoch, we would be in doubt whether 
to pronounce them Tertiary or Cretaceous. ‘These facts would seem to indicate a fore- 
shadowing of the Tertiary era, and that the transition from one great period to the other | 
was gradual and quiet, the change in the physical conditions being ultimately sufficient to 
destroy the Cretaceous fauna and bring into existence that of the Tertiary. Again, in 
numerous localities where No. 5 is fully developed and a large thickness of Tertiary de- 
