THEIR DEPOSITS AND DRAINAGE. 653 
Nevada, voleanie materials have accumulated in the lake 
basins to a much greater extent than east of the Rocky 
Mountains ; and we have abundant. evidence that during the 
Tertiary period the western margin of the continent was the 
scene of far greater volcanic activity than we have any record 
of in the Rocky Mountain belt. 
The deposits formed by the lake basins of the Upper Mis- 
souri region are shales, marls and earthy limestones, with 
immense quantities of lignite, but with almost no traces of 
volcanic products. The number of fossil plante and animals 
is much greater there than farther West; and we have, in 
these deposits, proof that during unnumbered ages this por- 
tion of the continent exhibited a diversified and beautiful 
surface, which sustained a luxuriant growth of vegetation 
and an amount of animal life far in excess of what it has 
done in modern times. This condition of things existed 
long enough for hundreds and even thousands of feet of 
sediment to accumulate in the bottoms of extensive fresh- 
water lakes. These lakes were gradually and slowly dimin- 
ished in area by the filling up of their basins and by the 
slow wearing away of the barriers over which passed their 
gently flowing, draining streams.” Since the deposition of 
the fresh-water Tertiaries, which occupy the places of the 
old lakes, great changes have taken place in the topography 
of this region by the upheaval of portions of the Rocky 
Mountain ranges. In some localities these lake deposits are 
found turned up on edge and resting on the flanks of the 
. mountains which border the plains on the west. ]t is cer- 
tain, however, that much of the Rocky Mountain belt existed 
anterior to this date. We have in these, and many other 
facts that might be cited, proofs of the truth of the assertion 
I have elsewhere made that these great mountain chains, 
though existing at least in embryo from the earliest paleo- 
zoic ages, have, since then, been subject to many and varied 
modifieations—that they have been, in fact, hinges upon 
Whieh the great plates of the continent have turned — lines 
