PRESIDENT’S ADDRESS—SECTION C. da 
were formed ; extrusions of granite and gneiss took place, the 
sedimentary rocks on the eastern side were placed vertically or 
thrust into folds on the Jura type, while on the western side the 
disturbance was small. In California, to the west of the Sierra 
Nevada, where the sediments had been heavier, numerous foldings 
took place, and the coast range was formed. The plateau region 
of Colorado between the Wahsatch and Rocky Mountains, became 
a vast fresh-water lake, the bottom of which appears to have 
subsided while 5000 feet of fresh-water sediments were placed 
upon it ; then the lake shrank and disappeared at the close of the 
Eocene. General elevation re-commenced in the Miocene, the 
floor of the old lake was thrown into long swellings, and volcanic 
eruptions of a basic character took place on the margins of the 
plateau, and on a still larger scale in California. This upward 
movement ceased in the middle of the Pliocene, but commenced 
again in the Pleistocene, and appears to be still going on. The 
total elevation of the plateau region from the close of the Eocene 
was no less than 20,000 feet, most of which occurred in the 
Miocene. It was accompanied by a number of north and south 
monoclinal folds and faults, which continued into the Pleistocene. 
The erosion that accompanied this elevation is as much as 10,000 
feet in places, and averages 5500 to 6000 feet. In the Uinta 
Mountains, which were raised 4000 feet above the rest of the 
plateau at the close of the Cretaceous, the erosion has been in 
places over 18,000 feet. 
The geological history of the western United States shows 
clearly that the forces which contorted the Sierra Nevada and 
coast ranges were not distinct from those which produced the 
regional uplift, and it is in this very region that we find in the 
Uinta and Rocky Mountains those intermediate types which I 
have already mentioned. Fifty years ago C. Darwin came to a 
similar conclusion from a study of the geology of South America, 
and stated his belief that mountain chains were only subsidiary 
and attendant operations on continental elevation; a view which 
has since been lost sight of, but is now re-established by the 
labours of American geologists. 
The conclusions which can, I think, be fairly drawn from the 
facts I have just narrated are :— 
1. The plicated bands of the earth’s crust have been formed 
along areas of previous heavy sedimentation, that is in geosyn- 
clinals. 
2. The formation of a geosynclinal is a very slow process, 
extending through at least two geological periods. 
3. The subsidence that accompanies the sedimentation is not 
continuous, but is often broken by periods of elevation without 
contortion, followed by depression. 
4. The subsequent elevation with plication, and the formation 
of what Dana calls a synclinorium, is comparatively rapid, com- 
