Origin of Mountains. d41 
evidences of shallow water origin, indicates a progressing geo- 
synclinal, although the ocean gained no entrance to it. The 
down bending ended probably just after the Miocene 
period without general displacements; but there were tiltings 
along the more western border of the Tertiary in the vicinity 
of the Wahsatch and other mountains. (See note on page 439.) 
(9.) Since the Miocene era, and on through much of the 
Quaternary, there have been vast fissure-eruptions over the 
western Rocky Mountain slopes. They had great extent espe- 
cially in the Snake River region where the successive outflows 
made a stratum 700 to 1000 feet thick, over an area 800 miles 
in breadth. There are other similar regions but of less area. 
t is thus seen that along the Pacific side of the conti- 
nent the crust, under the action of lateral pressure, first bent 
downward profoundly, and then yielded and suffered fracture 
and plications, directly along a belt, parallel with the coast, 
either side of the Great Basin (and perhaps over this basin to some 
extent), the two great lines 400 miles apart. The plicated re- 
gions, thus made, having become firm by the continued pressure 
and the engendered heat and resultant solidification, the crust 
next bent, and then yielded, in a similar way, along an axis 
outside of the former regions of disturbance, the two axes over 
600 miles apart; and again all was mended in the same way. 
Then it bent a third time, just outside of the last range, on each 
side of the same great area, the lines over 700 miles apart; 
and then, over the western of the two ranges, the beds were 
displaced, solidified, and left in high ridges; but over the east- 
ern the final disturbances were local and slight. 
lifted about this time, that is, in the course of the Tertiary era, 
and many of the great voleanoes were made.) 
