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"6 B. N. A. BOUNDARY COMMISSION. 
place prior to the period of elevation above named the second. Dr 
Hector describes a hill of Carboniferous limestone, which appears to have 
been an island at the time of the deposition of the surrounding Cretaceous 
or early Tertiary rocks, and to have been subsequently pushed up to a 
greater elevation.* Dr. Peale believes that the Bridger Range, in Mon- 
tana, was elevated probably at the close of the Cretaceous, while other 
neighbouring mountains were not formed till the close of the Eocene ;+ 
and Dr. Hayden is also of opinion that a gentle elevation had commenced 
before the close of the Cretaceous. 
173. The great period of elevation, however, was that bringing to a 
close the formation of the Lignite Tertiary, and intervening between it 
and the later rocks assigned to the Miocene and Pliocene. Dr. Hayden 
has accumulated in the course of his surveys an immense number of facts 
proving the violence and universality of this dislocation. He believes 
the series, up to the summit of the Lignite Tertiary, to have passed 
completely over the present position of the mountains, and writes: 
“ From the Silurian to the upper Lignite group inclusive, a thickness of 
10,090 to 15,000 feet extended, in an unbroken, horizontal mass, over 
nearly or quite the entire area of Montana, and probably much more 
widely.” § 
174. The evidence concerning the periods in the formation of the 
mountains, however, is as yet by no means complete ; and it is certain 
that elevatory movements antedating all these above mentioned, have 
taken place. Disregarding those which had effected the Eozoie rocks 
previous to the deposit of the next oldest sediments known in the west ; 
we find at least one break in the series at the base of the Carboniferous, 
and in different localities the Potsdam, Carboniferous limestone, Triassic, 
and Cretaceous are found resting on the denuded edges of Kozoic rocks, 
which must at these periods have formed coast lines. 
175. The comparatively small total thickness of the beds represent- 
ing a great part of the Paleozoic series, in the west, is remarkable, 
especially when taken in connection with the relatively great deposits of | 
the Cretaceous and later ages. This, and the absence of coal deposits, 
even in the Carboniferous series, would tend to show that the area of dry — 
land during the Paleozoic must have been quite small. 
* Exploration cf British North America., p. 115. t U.S. Geol. Surv. Territ. 1872, p. 113. 
t On the Period of Elevation of the Rocky Mountains, Am. Journ. Science and Arts., May, 1862. 
$ U.S. Geol. Surv. Territ. 1872.. p. 83. Geological Report Yellowstone and Missouri Expedition., p. 5, 
Also, for the Black Hills and Laramie Mountains, U, 8, Geol Surv. Territ, 1857-59., p. 70. 

