CORRELATION OF FORMATIONS 493 



Searching for probable equivalents of the Dolores Triassic to the east 

 of the mountains, we are at once confronted with the condition, long 

 known, that the Morrison formation — whether Jurassic or Lower Cre- 

 taceous matters not to this discussion — rests unconformably on the older ■ 

 formations, the transgression being very gradual. 



It is plain that the Morrison beds rest on the Fountain grits in the 

 Pikes Peak quadrangle, the " Creamy sandstone" and the entire Upper 

 Wyoming section of the Denver area being missing. The thickening of 

 the Chugwater group northward, recorded by Darton, is probably due, 

 in part at least, to this same transgression. 



The Spearfish formation of the Black hills, embracing some 700 feet of 

 gypsiferous sandy Red beds, is referred by Darton (7) to the Trias, with- 

 out fossil evidence, from its stratigraphic position between the supposed 

 Permian and the marine Jura (Sundance). The Spearfish is conform- 

 able with the beds below, but the Sundance is unconformably on it. 

 In view of the apparent absence of the widely distributed Triassic verte- 

 brate fauna and the conformity with the Permian (?) below, the Triassic 

 age of the Spearfish seems to me open to much doubt. I think it may 

 more plausibly be viewed as embracing apart of the uppermost Palezoic 

 section not now preserved at any point to the south, as far as known. 

 Whether the Spearfish beds of the more southern area were removed by 

 pre-Triassic erosion, or, together with Triassic sediments, by the pre* 

 Morrison erosion, is an interesting question upon which there is little 

 evidence. 



Of important bearing on the problem of Triassic sediments to the 

 east of the mountains is the discovery recorded by Darton (7) of a Tri- 

 assic vertebrate, determined by Lucas as belonging to a belodon, in Red 

 beds of the canyon of the Purgatoire, a southern branch of the Arkansas 

 river, in Colorado, some 70 miles below Pueblo. Since this locality is 

 on the plains, many miles from any well-known section of the foothills, 

 and without exposure of underlying formations, the correlation of the 

 fossiliferous horizon with any part of the Front Range Red bed section 

 is hazardous, as Darton points out (p. 441). But since the belodonts 

 are the most common forms in the Triassic beds of a great area, as 

 brought out by this discussion, the general correlation of the Purgatoire 

 horizon with that elsewhere known to contain the belodont fauna is 

 directly suggested. 



According to Darton (7) the fossiliferous Red beds of the Purgatoire 

 have been traced by Lee into northern New Mexico, and this at once 

 raises the interesting question as to the relation of the Purgatoire horizon 

 with that of the Gallinas mountains, some 200 miles farther west in New 

 Mexico, where the first Triassic fossils of the West were discovered by 

 Newberry in 1859 ; but before further discussion of this connection, it 



