126 Hillebrand and Ransome — Carnotite and Associated 



The uniformly dark green sandstone richest in roscoelite 

 does not effervesce with acids and shows no calcite cement in 

 thin section. The usual quartz grains are held together by 

 roscoelite. This is crystalline, but the highest available powers 

 show only an indistinct and minute foliation, snch as may be 

 observed in some very finely crystallized chlorites. 



Close to the floor of the tunnel is a fairly regular, nearly 

 horizontal streak of carnotite, varying in width, but usually 

 less than an inch, and showing noticeable diminution in thick- 

 ness in the face of the tunnel. This small seam is not solid 

 carnotite, but is merely a zone in the sandstone impregnated 

 with the bright-yellow uranium mineral. It is not nearly so 

 continuous as the main vanadiferous belt, and was seen only at 

 three or four places along the 2000 feet or so of outcrop of the 

 latter. When this seam is closely examined, it is seen that 

 narrow bands rich in carnotite alternate with green bands 

 carrying mostly roscoelite. There is also usually present a 

 seam generally about an eighth of an inch in thickness (though 

 often thicker) which is almost wholly quartz. The microscope 

 shows it to be a true quartzite, in which the original rounded 

 detrital grains of quartz have been cemented by fresh quartz in 

 optical continuity with the older granules. A similar quartzite 

 occurs in the green vanadiferous sandstone above the carnotite, 

 where it forms concretionary knots and nodules. It was not 

 noted in the buff sandstone however, where the cementing 

 material is calcite. 



Immediately below the carnotite seam there is a parting or 

 " floor " in the sandstone, probably originally a very thin layer 

 of shale, which forms the working floor of the tunnel. The 

 sandstone below this floor is plainly impregnated for a short 

 distance with roscoelite, but the thin shale seam is regarded as 

 the practical bottom of the deposit. 



The questions of the origin and actual extent of this deposit 

 are closely related, and of much interest. Their discussion will 

 be deferred until the other deposits visited have been described. 



Some distance below the Placerville vanadium deposit, sand- 

 stone, presumably belonging to the Dolores formation, was 

 observed to be colored green, of a somewhat brighter hue than 

 the vanadiferous sandstone higher up the slope. As there are 

 some copper prospects near by, from which ore has been taken, 

 this was supposed to be a copper stain. Qualitative tests by 

 Dr. Hillebrand show however that the color is due to a com- 

 pound of chromium. 



Similar green sandstones occur on the western side of Sinbad 

 Yalley in what is apparently the La Plata formation, and were 

 originally supposed to be impregnated with roscoelite. Dr. 

 Hillebrand's investigations, however, show that they too owe 



