eussell.] PEE-TERTIARY FORMATION. 49 
becoming partially filled with the lacustral and stream sediments and 
lava flows which now occur in it and give it an essentially level floor. 
In traveling through that region nothing in reference to the present 
relief of the surface is more prominent than the fact that the leading 
features in the topography, excepting the broad plains, were in exist- 
ence previous to the deposition of the sediments and the outpouring of 
the lava which had upgraded the great central depression. 
The mountains of central and southeastern Idaho, so far as known, 
are composed mainly of Paleozoic or older rocks. The exceptions are 
certain Mesozoic formations described by the geologists of the Hayden 
survey, which occur in the extreme southeastern portion of the State, 
and tilted Tertiary beds mapped by Lindgren, in the mountains near 
Boise. The Tertiary beds referred to, however, and perhaps the 
Mesozoic (Juratrias) beds to the east of Blackfoot are of fresh-water 
origin. The general interpretation to be put on these facts seems to be 
that southern Idaho has been a land surface, in part occupied by lakes, 
since the close of the Paleozoic. Although up and down movements 
have occurred since the Mesozoic and are, no doubt, still in progress, 
the mountains were formed mainty previous to the beginning of that 
era, and were deeply eroded previous to the beginning of the Tertiary. 
The great valley now occupied by the Snake River Plains, with its 
gnany lateral branches, was in existence previous to the Tertiary, and 
owes its dominant features to the upheaval of its bordering mountains, 
and the accompanying depression of the larger valley areas, together 
with long-continued erosion. During Mesozoic time a river no doubt 
flowed through the Snake River Valley, which received tributaries 
from the secondary valleys leading to it, and the land was deeply 
dissected and was of mature and even old topography previous to 
the Tertiary. The direction of flow of the stream which preceded the 
present Snake River is unknown, but was most probably westward. 
This river continued its work of removing the debris produced by the 
disintegration and decay of the bordering mountains for a very long 
period. No attempt will be made at this time to determine the details 
of the earlier portion of the geographic history of southern Idaho, but 
the general conclusion that we have there a land surface which has 
ibeen in existence since near the beginning of the Mesozoic era is one 
of much interest, on which must be based many conclusions of both 
[ purely scientific and economic importance. 
As has been stated several times, the valle}^s of southern Idaho have 
| become deeply filled in many instances with alluvium deposited by 
! streams, the sediments of lakes, and the lava and other products of 
volcanoes. These deposits demand special consideration, as they are 
e intimately associated with the problem of obtaining artesian water, 
,. the development of springs, the desirability of excavating horizontal 
d wells, etc. 
a Bull. 191)— 02 ± 
