REVIEWS. 639 
deposits of fine sand, with bones of mammals and shells of exist- 
ing species, on Loup Fork and its tributaries. The same may be 
said of the bluff deposit, or loess, which i s so well displayed along 
the Missouri from Fort Pierre down w St. sc and pro 
bly, to the Gulf.of Mexico. Ata ait eriod it is probable, 
ready been marked out, consequently we find the yellow marl or 
loess fifty to one hundred and fifty feet thick in the immediate val- 
ley of the Missouri, but thinning out as we recede ig it, or the 
valleys of any of its branches. The existence of so many fresh- 
water mollusca and the entire absence of any marine forms indicate 
that the waters of the Mississippi and Missouri were either cut off 
m the direct access to the sea, or that the influx of such a vast 
quantity of fresh water as must have flowed down from om moun- 
tain inte rendered completely fresh the inland portio 
may suppose the temperature just prior to the eases pe- 
riod és have been extremely low, and that the elevated portions of 
the West were covered with vast masses of snow and ice; that as 
the temperature became warmer this snow and ice melted, produc- 
ing such an accession to the already existing waters that they cov- 
ered -all the country excepting, perhaps, the summits of t 
highest peaks ; that masses of ice filled with fragments of sale: 
worn and unworn, floated off into this great sea, and melting, scat- 
tered the contents over the hills and plains below ; that as the wa- 
ters diminished these masses ‘of ice would accumulate on the 
summits of the foot-hills of the mountains, or at certain localities 
in the plains ; aad thus account for the great local accumulations 
land lake, and then carried beyond the reach of currents, w 
settle quietly to the peak aa almost We gg lines of stratification, 
as we observe in the loess. The last act was the recession of 
these waters to their present positi vey and the formation of the 
terraces. We believe the terraces constitute the last change of 
any importance in the surface of the western continent. We su 
pose that the channels of all the streams on the meen fa of 
the Rocky Mountains were at one time occupied with water from 
hill to Ap and that the drainage was toward the sea. kal in the 
Great Basin, which so far as we know has no outlet, the drainage 
must oe been by evaporation, for the evidence points to the con- 
clusion that it was entirely filled with water high up on the sides 
of the mountains. There is greater uniformity in the terraces in 
the Great Basin than in the valley of the Missouri, which indicates 
a far more equable drainage. Still, those along the flanks of the 
Wasatch Mountains number two or three principal ones, but these , 
