124 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



tions, an examination of nine specimens proving that, with one exception, they 

 are non-magnesian limestones with a variable but generally small percentage of 

 insoluble material, and less than i per cent of (FeMg)COs. In the limestone 

 from Tucker Mountain, which forms the exception, the dirty brown weathered 

 surface and granular texture already noted suggest the dolomitic character and 

 the presence of iron and manganese carbonates. Furthermore, in the absence of 

 definite faunal distinctions the limestone beds have been used to define the limits 

 of the formation, the base being taken at the limestone belt known as the Robinson 

 limestone, and the top of the formation at the Jacque Mountain limestone. 

 Both these limestones contain an invertebrate fauna of upper Coal Measure 

 type. The Robinson limestone is somewhat dolomitic at the base, containing 

 nearly 7 per cent of magnesium carbonate. The Jacque Mountain limestone is 

 characterized in certain layers by an oolitic structure. The rock is light bluish 

 gray in color, and the oolitic grains are embedded in a finely granular matrix 

 of similar color. They are about the size of mustard shot, and have a normal 

 concentric structure and sometimes a radiate appearance; grains of sand or 

 crystal particles serve as nuclei. This structure disappears with recrystallization, 

 and is entirely wanting in certain layers. 



"Wyoming formation. The beds above the Jacque Mountain limestone, 

 which have a maximum thickness of about 1,500 feet, have been given this name, 

 not because of any fossil evidence of their age that could be found, but because 

 by their position and petrological character they most nearly correspond to the 

 beds of this formation which elsewhere in the Rocky Mountain region have, on 

 fossil evidence, been determined to be Triassic. If the Permian is represented 

 in Colorado, the evidence of which appears to the writer as yet very uncertain, 

 it would be included in these beds, which have evidently been deposited in direct 

 and unbroken succession over the upper Carboniferous. 



"They consist principally of sandstones, of intensely brick-red color where not 

 metamorphosed, with a moderate development of thin shales between their more 

 massive beds. Limestones are practically absent, having been found only at a 

 few isolated points, generally at about the same horizon, showing that the condi- 

 tions favoring calcareous sedimentation, which had hitherto prevailed so irregu- 

 larly throughout the region, had at this time almost entirely ceased. The sand- 

 stones are often coarse, sometimes conglomeratic, and are composed mainly of 

 distinctly recognizable Archean debris. Feldspar and mica are the most abundant 

 constituents next to quartz. Near the top of Jacque Mountain there is a con- 

 glomerate bed containing Archean bowlders as large as 2 feet in diameter; finer 

 conglomerates are abundant, in beds usually not over I or 2 feet in thickness. 

 In one place, on the Tenmile slope of Mayflower Hill, a conglomerate bed was 

 observed where the pebbles were entirely of white quartzite in a matrix of nearly 

 pure quartz sand. On the slope of the hills bordering Tenmile Valley on the 

 west, especially of Jacque, Tucker, and Copper mountains, where metamorphic 

 action has been most pronounced, the red color has disappeared from the Wyoming 

 sandstones, and the rock has become dark and quartzitic and contains much 

 bright-green epidote." 



Similar conditions occur in the Anthracite-Crested Butte and the Aspen 

 quadrangles. 



It is obvious that the red sandstone here mentioned above the Weber 

 grits is not to be directly correlated with the Wyoming formation or its 



