166 GEOLOGICAL SURVEY OF THE TERRITORIES. 



the Mississippi was beneath the ocean I will not now attempt to deter- 

 mine. Restricted portions of the western continent may have been 

 above the ocean level, and some of the mountains may have projected, 

 like rocky islands, above the waters. Near the close of the cretaceous 

 period the great water-shed of the continent was marked out, and the 

 marine waters were separated into more or less shallow seas, lakes, es- 

 tuaries, marshes, &c. Among the marshes sported the reptiles, the 

 remains of which are so abundant in the tertiary deposits; and on the 

 areas raised above the waters grew luxuriant forest trees and other 

 vegetation which contributed to the formation of the coal beds. We 

 shall attempt to show from time to time that, although the coal deposits 

 of the West occupy an enormous area, yet the profitable deposits of coal 

 lie in detached basins, some of which are quite restricted in their area. 

 The study of these coal formations in nature shows most plainly that 

 some of the beds of coal extend, uninterruptedly, over enormous areas, 

 as if the vegetable matter had been deposited in a sea, or that the 

 physical conditions attending its occurrence were widespread and uni- 

 form, while in other localities coal strata of great thickness clearly oc- 

 cupy but a limited area. We are aware that beds of coal, but a few 

 miles apart, and evidently synchronous, show no physical evidence of 

 ever having been connected with each other. There is another curious 

 fact, that, while very nearly the same species of plants occur, the coal 

 strata are nearly, or quite all, marine or brackish, while far removed 

 from the mountain ranges the sediments very soon become purely fresh- 

 water. Ou the Upper Missouri, where the coal-bearing group covers so 

 large an area with remarkable uniformity, only the lowest beds contain 

 marine forms, and very soon we pass up into strata with purely fresh- 

 water fossils. We may suppose that at an early period, during the ter- 

 tiary epoch, this portion was cut off from access to the salt water. If 

 our ideas of the physical geography of these epochs are correct, coal 

 strata of contemporaneous origin may be purely marine, purely fresh- 

 water or brackish, depending upon the proximity of the sea, lake, or 

 marsh, to the ocean waters. 



We have already shown many times that there is no real physical 

 break in the deposition of the sediments between the well-marked cre- 

 taceous and tertiary groups. In some localities the continuity is clear 

 and beautiful in the highest degree. On Green River, and in the Bit- 

 ter Creek Valley, one can trace the continuity step by step, so far as the 

 strata are concerned, from the cretaceous through the greatest thick- 

 ness of clays, sands, and sandstones of the lower tertiary to the purely 

 fresh-water beds of Green River shales, Washakie or Bridger Groups. 

 In these localities the influence of the elevation of the mountain ranges 

 has been such as to expose the outcropping edges of all the strata from 

 the cretaceous to the sands of the most recent tertiary, like the 

 leaves of a book. We have already shown thatin the clays interspersed 

 among the coal beds in the Bitter Creek Valley, are seams of oys- 

 ter shells of several species. A few other marine forms have been ob- 

 served. At Hallville, near Point of Bocks, we have seen that in the 

 slaty shales, above one of the coal beds, are proofs that at that period 

 the physical conditions were most favorable for the existence of a pro- 

 fusion of brackish- water life; that in this locality, from 3,000 to 4,000 

 feet of coal strata were deposited before the salt water ceased to have 

 access to these tertiary lakes. At Bear Biver, also, the same history is 

 written upon the rocky layers. We have well-defined cretaceous strata, 

 and from these we ascend through a series of sandstones and clays, with 

 an abundance of shells of the genus Outrca, and a few other inariue forms 



