WHITE.] MESOZOIC AGE. 27 



with the other groups that are treated of in this report. Until lately, 

 also, these strata have been generally regarded as nufossiliferous, with 

 the exception of the existence here and there of considerable quantities 

 of silicifled exogenous wood ; and they were referred to the Triassic 

 period mainly in consequence of their position between the recognized 

 Carboniferous and Jurassic strata. Serious doubts of the real Triassic 

 age of the strata in question have lately been raised because of the dis- 

 covery of fossils in the lower portion of the group that seem to be not 

 merely similar to some that are found in the group above the one in 

 question, and which ail persons agree in referring to the Jurassic period, 

 but they are apparently specifically identical. 



Dr. Hayden, on page 11 of his Eeport on the Geology of Captain Eay- 

 nolds's Exploration of the Yellowstone and Mississippi Elvers, made in 

 1860 and published in 18G9, states that he found unmistakable Jurassic 

 fossils near the base of this group. 



In 1874, Mr. Edwin E. Howell collected for Professor Powel several 

 species of invertebrate fossils from the lower portion of the division 

 of this group in Southern Utah, some of which, at least, I regard 

 as specifically identical with well known-species, until then found only 

 in unquestionably Jurassic strata.* These facts seem to indicate, 

 with comparatively little doubt, that all the Mesozoic strata of that 

 portion of Western North America, below the Cretaceous, belong to 

 one period only, and that the Jurassic. This opinion is, of course, 

 based upon the invertebrate fossils before referred to alone, no ver- 

 tebrates or plants having been found or examined by myself. If the 

 specific identification of the invertebrate fossils before referred to should 

 be fully verified, there is no sufficient evidence, so far as I am aware, 

 of the existence of any Triassic strata, distinguishable as such by its 

 invertebrate fossils, in any part of the ^reat region east of the great 

 Salt Lake Basin and south of the Union Pacific Eailroad. This ques- 

 tion, however, is of so important a character that notwithstanding the 

 evidence in favor of adopting the view just indicated, I prefer to use the 

 provisional classification already given in this chapter, until material 

 shall have been collected for a more complete discussion of the question. 

 It is especially desirable to collect as much material as possible for this 

 discussion, because' these strata and their equivalents are distributed 

 over so large a portion of the Korth American continent, where their 

 lithological characters are generally so uniform as to render the group 

 recognizable at once from them alone. However, in view of the fact 

 that the relative position of this great series of strata seems to justify 

 its reference to the Triassic period, while all its yet known invertebrate 

 fossils show an intimate relation with those of the accepted Jurassic 

 strata, I shall adopt for present use the general or collective term, 

 Jura-Trias, for these two earlier Mesozoic groups. 



As shown in the description accompanying the figure of the general 

 section of the rocks of this district, the divisions of this group are all 

 sandstones, the middle one being very conspicuous in consequence of its 

 bright-red color. The lower division is sometimes so very much like the 

 sandstone of the Upper Carboniferous group that it is difficult to deter- 

 mine where the one ends and the other begins, but in this district the 

 strata of the former are considerably softer than those of the latter. 

 The exposures of the strata of the middle division are often gorgeous in 

 appearance as seen in the distance, for they are often exposed in exten- 

 sive escarpment faces. Eemarkable and extensive exposures of these 

 red strata are seen in the southern and eastern faces of the great Mid- 



*See Geology of the Uiiita Mountains, pages 80 and 87. 



