of the Rocky Mountains. 79 
fossils, Dr. Shumard gives them as only about 500 feet. In the 
Rocky Mountain district they are seldom more than 80 feet 
and never over 200 feet. Indeed all the Primary fossiliferous 
rocks are but thinly represented there, while the lower Secondary 
formations begin gradually to increase in force until all along 
e eastern slope we have an enormous development of the 
upper Secondary and Tertiary with an aggregate thickness of 
from 8000 to 10,000 feet. 
6. as we yet know there is no unconformability in any 
of the fossiliferous sedimentary rocks of the northwest from the 
Potsdam sandstone to the summits of the true Lignite Tertiary. 
There are proofs of two great periods of disturbance which 
had a marked influence upon the physical geography of the 
West. The one occurred prior to the deposition of the Potsdam 
sandstone when the Azoic or granitic rocks were elevated into a 
more or less inclined position and the other and most important 
riod took place at the close of the accumulation of the great 
ignite Tertiary deposits when the great lines of fracture were 
ry. 
7. What changes took place in the physical geography of 
riod which must have elapsed after 
sandstone until the commence- 
Engelmann have detected proofs of Devonian rocks but they are 
Rot known to be largely develo and on the western declivity 
of the El Paso mountains Dr. G. Shumard found “ well marked 
Strata of the inferior Silurian system corresponding in age to the 
Blue Limestone of Cincinnati and the Hudson River group of 
the New York series.”* But so far as our present knowledge 
extends, rocks of intermediate ages do not form a prominent 
feature in the geology of the west. 
Smithsonian Institution, Washington, Nov., 1961. 
* Transactions of the Academy of Sciences, St. Louis, vol. i, No. 2, page 288, 
