WHITE ON THE LARAMIE GROUP. 871 



surface again beneath the level of the sea ; although jiortious of the area 

 which the intercontinental Mesozoic sea had covered "were afterward 

 occupied b^' great bodies of both brackish and fresh waters. The east- 

 ern border of the hiter Cretaceous deposits was thus carried westw^ard, 

 where its place is now covered like that of the border of the earlier Jura- 

 Trias deposits, but not so deeply. 



The eastern border of the Laramie Group is hidden in the same man- 

 ner, but there is yet no evidence that it is anywhere overlapped by any 

 subsequeut marine deposit j although it is known to have received upon 

 it in several places different groups of fresh-water strata. Perhaps no 

 fact in the physical history of North America is better established than 

 that the elevation of the Eocky Mountains as such are of later date than 

 the Laramie Group, but the foregoing facts show that both oscillatory 

 movements and general continental elevation took place before the 

 beginning of the movements which resulted in the elevation of those 

 mountains. Besides the oscillations of surface which have already been 

 mentioned, there are indications that other similar movements occurred 

 elsewhere within the same limits of time ; such, for example, as the 

 unconformity of the Laramie strata upon those of the Fox Hills Group 

 in Middle Park, reported by Mr. Marvine ; the unconformity in some 

 places of the Jura Trias upon rocks older than the Carboniferous, &c. 



But leaving now the subject of the elevation and subsidence of land- 

 surlace to be briefly resumed further on, a few facts concerning the 

 former physical conditions of what is now the western part of North 

 America may now be considered. No fresh-water deposits of any kind 

 or extent have yet been discovered in any of the Paleozoic rocks of 

 North America, unless the coal of Carboniferous age may be regarded 

 as such ; but even in that case the elevation of the land upon which it 

 was formed could have been only barely above the sea-level; for the 

 conformity of the coal-beds with the strata immediately above and 

 below them is never broken, and the latter strata contain marine fossils. 

 Therefore, for our present i)urpose, all the Paleozoic strata may be 

 regarded as of marine origin. As a rule, also, all the Mesozoic strata, 

 from the Jura Trias to the Fox Hills Group inclusive, are, by the char- 

 acter of their fossils, known to be of marine origin, although at a few 

 localities in some of the strata of each period fresh-water Mollusca have 

 been discovered. These exceptions no doubt indicate the proximity of 

 then existing shores rather than the i)revalence of any such bodies of 

 either brackish or fresh water as afterward covered wide areas in the 

 same region. 



Eesting directly upon the strata of the Fox Hills Group are those of 

 the Laramie, sedimentation having evidently been continuous from the 

 former, notwithstanding the fact that there was such a radical change 

 in the fauna upon the ushering-in of the Laramie period. The geo- 

 graphical extent of the great Laramie Group has already been referred 

 to, as well as its great thickness, the maximum being about 4^000 feet. 



