36 Geological Extinction and (January, 
to 700 feet, while farther north, in Colorado and Wyoming, the 
Rocky mountain plateau rose from 4000 to points 10,000 feet 
above the ocean. . 
The plains east of the Rocky mountains are underlaid by beds 
deposited by vast inland, fresh-water lakes. In Texas these beds 
dip under the Gulf of Mexico, but at the base of the Rocky moun- 
tains in Colorado they are 7000 feet above the sea. They have 
been tilted up. Gen. Warren and Mr. King have shown that after 
the Pliocene epoch such a tilting took place. These lakes dried 
up, and the marvelously abundant mammalian life which thronged 
about their shores became extinct as the Quaternary period opened. 
May not the extinction of life so widespread throughout the West, 
particularly at the end of the Eocene, the Miocene and the Plio- 
cene, have been mainly due to the great changes in the physical 
geography of that vast region? We see also whya semi-tropical 
climate and flora and fauna continued to exist around the Gulf of 
Mexico, but ceased to live on the elevated Rocky Mountain pla- 
teau, as well as the Sierra Nevada and Cascade plateaus. The 
whole western portion of the continent was carried up bodily, the 
lakes drained off by the Missouri, Columbia and Colorado rivers 
and the air at such an elevation becoming rarified, dry and cooler, 
the tropical life became either extinct or migrated southward to 
warmer and lower regions. Towards the end of the Pliocene mul- _ 
titudes of llamas, droves of horses, mylodons, elephants and mas- — 
todons, with lions, cats and dogs, flourished in Oregon, Montana, 
Utah, Wyoming, Colorado and New Mexico; changes of level 2 
and consequently of climate were perhaps the main factors con- 
cerned in their demise. 
There were throughout the Tertiary most widespread and all : 
pervading geological changes, culminating in the upheaval of the 
two great mountain chains of the West. Horizontal Cretaceous 
strata lie on the Rocky mountains at an elevation of 10,coo feet, 
the sign and proof of an extensive upheaval. We know that the T 
movement in South America, while gradual for the continent, was 
more or less locally paroxysmal. Was it not the case also in _ 
North America ? 
_ By the end of the Pliocene, North America assumed its present — 
continen.al proportions. The Rocky mountains and Sierras shot — 
their peaks into the sky to elevations of 10,000 and 1 5,000 feet 
above the Pacific. These great walls shut off the moist trade winds © 
“es tars aie ye es 
Sree eee 
