white.] GENERAL DISCUSSION. 261 



zoic rocks of !N"orth America, unless the coal of the 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, because the conformity of the coal-beds with the strata above and 

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

 Therefore, for our present purpose, all the Paleozoic strata may be re- 

 garded 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 character 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 dis- 

 covered. These exceptions, no doubt, indicate the proximity of then ex- 

 isting shores rather than the prevalence of any such bodies of either 

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

 region. 



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

 the Laramie Group, the latter, as already shown, having been at least in 

 part deposited continuously with the former. The geographical bounda- 

 ries of the great Laramie formation are not known, but its area embraces 

 many thousand square miles, for it is known to extend from Southern 

 Colorado and Utah northward beyond the northern boundary of the 

 United States ; and from the Wasatch Mountains, eastward far out on 

 to the great plains. It reaches a maximum thickness of about 4,000 teet, 

 and its general lithological characteristics are similar to those of the 

 Fox Hdls Group, a known marine formation. Its fauna, however, has 

 been shown to be largely of brackish and partly of fresh water origin, 

 and not marine. Furthermore, the brackish-water species are distrib- 

 uted throughout its entire thickness and its whole geographical extent. 

 These facts, together with the absence from all the strata yet examined 

 of any true estuary characters, show that the Laramie Group was depos- 

 ited in a great brackish -water sea. This being the case, it must have 

 received its peculiar character, as well as its boundaries, by having been 

 separated from the great open sea by an encircling elevation of land ; 

 the continuity of shore-line having been comj>leted by elevations con- 

 necting the two great continental factors at the northern and southern 

 portions of the inter-contmental Mesozoic sea. Whether the brackish salt- 

 ness of the Laramie sea was sustained throughout the period by limited 

 communication of its waters with those of the great open sea, or whether 

 such communication was entirely cut off, and the supply of salt above 

 that which was originally retained of its marine saltness came by adja- 

 cent continental drainage in amount sufficient to balance the waste by 

 overflow, can probably never be known, but the latter seems probable.* 

 If the former condition existed, one of the places of communication was 

 no doubt at the southeastern border of the Laramie sea, and some for- 

 tunate exposure of strata in the region between Western Kansas and 

 the Gulf of Mexico may yet reveal the true relations of the Laramie 

 Group -with the Cretaceous and Eocene deposits of the Gulf border. If 

 tide-level communication between the Laramie sea and the great open 

 sea was entirely cut off, as there is much reason to believe it was, the 

 question of such relationship or contemporaneousness of deposition must 

 ever remain an open one. 



It is evident that the movements which caused the inclosure of the 



* The frequent presence of fresh-water forms in the strata of this group, from its base 

 to top, such as Unio, Alelania, Viciparus, Campeloma, Goniobasis, &c, are suggestive of 

 the non-existence of tides in its waters, such as would have existed if they had com- 

 municated freely with the open sea, for the living representatives of these mollusks do 

 not hnd a congenial habitat in tide-water, even if it be fresh. 



