J, LeConte—Structure and Origin of Mountains. 103 
Finally, the already asymmetric mountain is pushed over 
margin of the Sierra region. The Sierra region, as I have else- 
where shown,* was then a marginal sea-bottom receiving sedi- 
ments from the Basin-region continent, until an enormous 
thickness had accumulated. When these thick sediments 
began to yield from the aqueo-igneous softening of their floor, 
they would first swell up asymmetrically, and then be pushed 
over against the stiffened Basin region land-crust, forming a 
steep slope or even a fault and escarpment on that side. 
c. I have said that the Basin region was land during Meso- 
zoic times ; moreover that the Sierra region was then marginal 
Pacific sea-bottom. Now the Wahsatch region was at the same 
time a marginal sea-bottom of the great interior sea which then 
covered all the Plateau and Plains region. -At the end of 
the Jurassic, as already said, the marginal sea-bottom on the 
Pacific side yielded, and the Sierra was born. Probably at 
the same time the bottom of the Jurassic sea of the Plateau 
region went down and the more open Cretaceous sea of that 
on the east or seaward side and the steep slope on the west or 
landward side. Such according to Emmons is, indeed, the fact. 
The Wahsatch range rises on the east by a gentle slope twenty 
miles long, until it attains a crest 12,000 feet high, and then 
plunges down by a slope so steep that it reaches the plains 
* This Journal, III, vol. iv, p. 460 and seq., 1872. 
