866 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



be about 1,000 miles, and east and west a maximum of not less than 500 

 miles. The full length of the area once occupied by the group is prob- 

 ably considerably greater than here indicated, and we may safely esti- 

 mate that it originally com j)rised not less than 50,000 square miles. The 

 present range of the Eocky Mountains traverses this great area, against 

 both flanks of which, as well as those of the Black Hills, the Laramie 

 strata are upturned. These mountains, therefore, did not exist during 

 the Laramie period, and the continuity of the waters of the Laramie Sea 

 over tbeir present site is also shown by the specific identity of aqueous 

 molluscan fossils in its strata on both sides of those mountains. 



The prevailing material of the strata, especially those of Mesozoic and 

 Cenozoicage, in all the Western Territories, whether of marine, brackish-, 

 or fresh-water origin, is sand 5 and consequently those of most of the 

 groups have certain characteristics in common. 



Not only in this general way, but in other respects also, the litholog- 

 ical characteristics of the Laramie Group are similar to those of the Fox 

 Hills Group of the Cretaceous Series, upon which the former group rests, 

 and with which, so far as is now known, iu is everywhere apparently con- 

 formable ;* that is, it has the appearance of a widespread marine for- 

 mation, consisting mainly of sandstones and sandy shales; but that it 

 was not, like the Fox Hills Group, an open-sea deposit, is shown by its 

 fossils. Its resemblance to the Fox Hills Group is still further increased 

 by the presence in the latter, as well as the former, of many important 

 beds of coal. It is true that no coal has been found in the Fox Hills 

 Group in the Upper Missouri Eiver region, nor in Eastern Colorado, 

 but it is not uncommon among the strata of that group in Wyoming, 

 Utah, and Western Colorado. 



Although there is sufiQcient evidence that the Fox Hills Group, which 

 immediately preceded the Laramie, was deposited in a comparatively 

 shallow sea, the bottom of which was slowly but constantly subsiding, 

 its waters seem to have been everywhere truly marine except in a few 

 estuaries jt and the whole area occupied by the group where it has been 

 studied seems also to have been always and entirely submerged, except, 

 perhaps, those surfaces upon which the coal-plants grew, and these 

 could have been above the water-level only during the growth of that 

 vegetation and the accumulation of its carbonized remains. The 

 Laramie Group seems also to have been deposited in waters that were 

 constantly shallow, and as the group has a maximum thickness of not 

 less than 4,000 feet, the bottom must have been constantly subsiding.| 



*There must necessarily be some unconformity between tbese two groupsin tbe periph- 

 eral portions of tbe Laramie, because, as will be sbown further on, the area upon which 

 its waters rested was cut off from the great open sea by the elevation of portions of 

 the bottom upon which the Fox Hills deposits were made. 



tAn interesting essemblage of fossils from a deposit of one of these estuaries has 

 been obtained near Coalville, Utah. 



t Similar remarks may be made concerning all the other groups of the "Western for- 

 mations from the Jura Trias to the Bridger Group inclusive, as will appear further on. 



