130 ON THE GEOLOGY AND NATURAL HISTORY 
and 4, the sediments show that they were accumulated in comparatively deep and quiet 
waters. No. 2 is a black plastic clay, No. 3 gray marl, and No. 4 a dark indurated some- 
times laminated clay, with many calcareous concretions. In No. 5 we gradually approach 
indications of shallow water, until dry land appears, as already stated. 
It will not be possible at this time to mention in detail all the oscillations of surface 
and other physical changes to which we have reason for supposing the country was sub- 
jected during all these periods. It is sufficient for our present purpose to show that ex- 
cept during the middle Cretaceous epoch no long-continued periods of quiet water prevailed 
in these ancient western seas. 
The evidence appears to me to point to the conclusion that a much milder climate pre- 
vailed throughout the western portions of our continent, during a greater part of the 
Tertiary period than that which exists in the same latitudes at the present time. The 
organic remains appear to indicate a subtropical climate, or one similar to that of our Gulf 
States. Near the close of the Cretaceous epoch the waters of the great Cretaceous sea 
receded toward the present position of the Atlantic on the one side and toward that of the 
Pacific on the other, leaving large areas in the central portions of the West, dry land. 
These areas were of course in close proximity to the sea, and comparatively but slightly ele- 
vated above the ocean waters. In regard to the Mollusca which have been found quite abun- 
dantly entombed in the Lignite-bearing strata, it is an interesting fact that the most nearly 
allied living representatives of many of these species are now found inhabiting the streams 
of Southern Africa, Asia, China and Siam, apparently indicating the existence of a tropi- 
cal climate in these latitudes at as late a period as the Tertiary epoch. 
Again, the luxuriance of the flora, which has been so perfectly preserved in the Lignite 
strata of the West, point to the same conclusion. It is true that until recently no forms 
have been found which belong exclusively to a tropical vegetation, but during our last 
expedition we obtained a species of true fan palm, very closely allied to Sabal lamononis, 
figured by Dr. Heer in his “Flora Tertiaria Helvetie.” “The most northern limit of 
palms is that of Chameerops palmetto, in North America, in lat. 34°-36°, and of Chamerops 
humilis in Europe, near Nice, in 43°-44° N. Jat.* The true palms of our present day 
are considered as having their native land within the tropics. That this or a similar con- 
dition of climate continued throughout the accumulation of the Wind river valley depo- 
sits may be inferred from their Molluscan remains, which are more nearly allied to tropical 
forms. 
Again, we have in this region, as before mentioned, a vast area occupied by the Lignite- 
bearing strata. There are from thirty to fifty beds of Lignite, varying in thickness from 
* Lindley’s Vegetable Kingdom, p. 136. 
