MESOZOIC TIME — CRETACEOUS, 827 



were, to a large extent, fresh, and only occasionally, or else locally, brackish. 

 Moreover, at many intervals, great areas emerged which were speedily 

 covered with marshes and forests in the warm and moist climate, and thus 

 peat-beds were made, which later became coal-beds. 



The length of the Laramie Interior Sea in this condition was nearly 2000 

 miles, it reaching to the parallel of 57° N. ; and another, the Mackenzie valley 

 area, opening on the Arctic Ocean, was 500 miles long. The southern of 

 these Laramie areas was probably tidal as well as the northern. For the 

 width south of 49° IST. was 600 to 800 miles, — which is too great for fluvial 

 waters. Besides, the strata are generally cross-bedded in stratification, and 

 they include occasionally conglomerates, proving seemingly strong move- 

 ments in opposite directions, and at times in some parts violent currents. 

 Moreover, although the waters were generally fresh, still Sea-Saurians, Sharks, 

 and other marine species occasionally ascended to Dakota and beyond. 

 The bay received the drainage from all the bordering lands for the 2000 

 miles from the Mexican Gulf to the limit of the Laramie beds in British 

 America ; and hence a great amount of fresh water flowed southward toward 

 the outlet. 



Hence the tides from the western part of the gulf generally carried 

 in salt waters for a short distance only, and thence the tidal movement 

 was propagated northward by the fresh waters. But occasionally the 

 Gulf waters were able, through a subsiding in the land, to flow far north- 

 ward, and let in the Sea-Saurians, and Sharks, the Oysters, and other Sea- 

 Mollusks, so as to make the brackish-water fossiliferous beds of the Laramie 

 formation. The spawn of Oysters and other Mollusks would have been 

 rapidly transported. 



If the above explanation of the conditions in the Laramie epoch is 

 correct, the distance to which the salt waters of the Gulf were carried in 

 westward and northward, whether one mile or many, is a subject for investi- 

 gation. The Laramie beds derived their material from the land on the 

 borders of the Interior Sea. The existence of Paleozoic and Mesozoic rocks 

 of various ages about the base of the Black Hills, where there is also the 

 Cretaceous formation, indicate how the other adjoining Archaean lands may 

 have been skirted, where now covered by Tertiary beds and those of the 

 later Cretaceous. 



The Upper Laramie or Denver group was first defined by Cross and 

 Eldridge in 1888. It derives the latter name from its distribution about 

 the city of Denver, east of the Front Eange (Archsean) of the Rocky Moun- 

 tains, where it overlies the Lower Laramie. It is described as resting on 

 the latter unconformably, — the unconformity being, however, not that of 

 bedding in a marked degree, but the unconformity consequent on the previous 

 erosion of the beds on which the formation was deposited. The upper por- 

 tion in that region, 1400 feet thick, consists largely of the debris of eruptive 

 rocks, mostly different kinds of andesytes ; while the lower part, 800 feet 

 thick, distinguished as the Arajjahoe beds, is mostly made up of conglom- 



