Vanadiferous Minerals in Western Colorado. 123 



tion* (Triassic), the La Plata formation (Jurassic), the McElmo 

 formation (Jurassic), the Dakota sandstone (Cretaceous), and 

 the Mancos shales (Cretaceous). Carboniferous rocks occur in 

 Sinbad Valley, and a series of gypsum-bearing shales of 

 unknown age in the bottoms of Sinbad and Paradox Valleys, 

 but as the ore-deposits to be described all occur in the beds of 

 the La Plata and McElmo formations, these older rocks need 

 not be again referred to. The sediments making up these 

 various formations lie usually nearly horizontal, but they are 

 sometimes flexed and frequently faulted. 



In all of the prospects examined, the ore of uranium occurs 

 in the form of the recently described bright yellow carnotite. 

 In one case this is intimately associated with a dull olive-green 

 mineral which according to Dr. Hillebrand is either identical 

 with, or very closely allied to, the vanadium-mica roscoelite. 

 Deposits of one or both of these minerals occur widely scat- 

 tered over San Miguel and Montrose Counties, Colorado, and 

 in the Blue Mountain (Sierra Abajo) district of southeastern 

 Utah ; but a portion only of the known deposits were per- 

 sonally examined. 



The Placerville Deposits. — These are essentially vanadium 

 deposits, and occur 4500 feet nearly northeast of the railway 

 station near Placerville, and about 1000 feet above the San 

 Miguel River. The lower 900 feet of the San Miguel canon is 

 cut in the typical " red beds " of the Dolores formation. Above 

 the Dolores comes the La Plata sandstone in typical develop- 

 ment as described in the Telluride folios, viz., two heavy beds 

 of light-colored sandstone, separated by a much thinner bed of 

 dark limestone. The roscoelite occurs as an impregnation in 

 the lower bed of the La Plata sandstone, about 100 feet above 

 the base, and just beneath the bed of dark limestone. All the 

 beds at this point are practically horizontal. The dark green 

 vanadiferous sandstone occurs in a nearly continuous band, 

 approximately parallel to the bedding planes, and varying in 

 thickness from a few inches up to five or six feet. This band 

 extends along the sandstone cliffs for an estimated distance of 

 about 2000 feet. The roscoelite occurs more or less thoroughly 

 impregnating portions of the fine-grained sandstone which con- 

 stitutes the mass of the bed. It sometimes makes up more 

 than 20 per cent of the vanadiferous facies. Normally the 

 sandstone of this lower bed of the La Plata is light buff in 

 color, with patches, mottlings, and stripes of pale pink, the 

 latter color being apparently due to small amounts of ferric 

 oxide. But when richly impregnated with roscoelite, this light- 



* The formation names used in these notes are those adopted by Cross in the 

 text of the Telluride Folio, of the U. S. Geol. Survey. The reader is referred to 

 this folio for fuller descriptions. 



Am. Jour. Scl — Fourth Series, Vol. X, No. 56. — August, 1900. 

 9 



