274 EXPLORATIONS ACROSS THE GREAT BASIN OP UTAH. 



older rocks are several miles distant. Near Dear Creek, and for some distance west 

 from there to the lied l.nttes, they are partly granitic, but mostly composed of 

 upheaved stratified rocks. At the lied Unites we find more Triassic outcrops. 



The mountains south of Sweetwater River, west of the Devil's Gate, are also 

 mostly granitic, with altered Paleozoic rocks on their slopes. On the east side of the 

 Rocky Kidev 1 found the last outcrops of this age. They are then succeeded by meta- 

 morphic schists. 



TRIASSIC AND JURASSIC FORMATIONS. 



To within a short period it has been problematical whether the Jurassic and 

 Triassic formations were represented in the territory of the United States, although 

 Middle Jurassic strata are known in the Russian territory, on the northwest coast of 

 this continent. Their discovery was repeatedly claimed, but every time it was found 

 that a mistake had been made. Although some of the strata which Mr. J. Marcou 

 described as Jurassic and Triassic are, perhaps, of that age, still he based his con- 

 clusions chiefly upon tossils which have since been recognized as Cretaceous forms, 

 and nearly the whole area which he colored upon his map indiscriminately as covered 

 by those formations, is now well known to be Tertiary and Cretaceous. 



On an expedition under command of Lieut. G. K. Warren, Topographical Engi- 

 neers, in the year 1857, Dr. Hayden collected, in the Black Hills, north of Fort 

 Laramie, a series of fossils, in which he and Mr. Meek recognized the Jurassic for- 

 mation. I he lull report of tins exploration, and of the highly interesting geological 

 discoveries connected with it, has not been published yet, but a short account of them 

 has been given in a "Preliminary Report of Lieutenant Warren, Washington, 1869, 

 Doc, Secretary of War," and in a paper read before the Academy of Science of Phila- 

 delphia, March, 1858. 



Our observations fully confirm the conclusion in regard to the Jurassic age of 

 that formation. At various points I observed strata which are evidently coeval with 

 those described by Dr. Hayden, and occupy an analogous position between the Cre- 

 taceous and older beds. A few of the fossils, of our collection, a full description of 

 which is given in the^ subjoined report of Mr. Meek, are identical with those of Dr. 

 Hayden, while we have, also, several new ones which, like his, are closely allied to 

 Kuropeau Jurassic forms. The only disputable point is now, to which horizon of the 

 Jurassic series these strata correspond. Mr. Meek suggests that they are Liassic, bas- 

 ing his opinion chiefly upon the similarity of several of Dr. Hayden's fossils with 

 huropean species of that age, while our fossils, although from strata which apparently 

 form the continuation of those observed by Dr. Hayden, seem to be more closely 

 related to Middle Jurassic types. We have an Ostrea, scarce!}- distinguishable from 

 0. Marskii (0. Enpelmanni, Meek), a leading type of the Middle Jura of Europe- 

 "J™." [>l rtl " » Fej 7 ™ilar t. . P. h ,,s- ( P. h Uastriata, Meek), which does not occupy 

 a distinct horizon and furnishes, therefore, no proof pro or contra. BvlemnUes rhmus, 

 .M.-ek. Ims a slight ventral groove, and is thus allied to the CanaUuilati which are 

 characteristic „f the Middle Jurassic formation, and it is perhaps not distinct from B 

 eccentrtCHS, Blainville, of that period. In order to settle this question of age it will, 



