Vanadiferous Minerals in Western Colorado. 125 



as a cement or matrix for the sand grains. At a distance of a 

 foot below the limestone, the sandstone shows, on close inspec- 

 tion, numerous specks of carnotite and gives a qualitative reac- 

 tion for vanadium (Hillebrand). Examined in thin section 

 under the microscope, this portion of the bed shows a fine- 

 grained homogeneous sandstone, in which well rounded grains 

 of quartz are held together by a rather abundant matrix of 

 calcite. The latter is crystallized as a fine granular aggregate 

 and includes small indeterminable particles of various kinds. 

 Many of the quartz grains as seen in section are entirely sur- 

 rounded by calcite, and so isolated from adjacent grains. A 

 pale lemon-yellow substance occurs sporadically in the section 

 as an interstitial material between the quartz grains. This is 

 probably the carnotite which was seen as small yellow specks 

 in the hand specimen. It is very minutely crystalline, recall- 

 ing the habit of some obscure chloritic aggregates in decom- 

 posed igneous rocks, and is too indistinct for successful optical 

 study. Small crystal grains of zircon are scattered through 

 the sandstone, and are readily concentrated in the re'sidue after 

 treating with hydrofluoric acid. 



In the next two feet below where it exhibits the foregoing 

 facies, the sandstone becomes pinkish in color, due to the 

 presence of ferric oxide, and the lens fails to reveal any ros- 

 coelite or carnotite. But just below the pink coloration, the 

 sandstone begins to show yellow and green specks. The latter 

 especially become more numerous and larger, until at from 3 

 to 4 feet below the limestone the sandstone has a decided 

 green tint. This deepens, going downward, until near the 

 floor of the tunnel the sandstone is a deep uniform olive-green, 

 rich in roscoelite (over 20 per cent) and showing many small 

 yellow specks of carnotite. This is regarded as the first-class 

 ore, and the chief value of the deposit is supposed to be in its 

 vanadium, the uranium being very subordinate. In this 

 respect the Placerville deposit differs from those on La Sal and 

 Roc Creeks, presently to be described, where the mineral sought 

 for and occurring most abundantly is carnotite. 



Thin sections of the green-spotted sandstone, when exam- 

 ined microscopically, resemble those of the light-buff sandstone 

 just above it, as far as character of quartz grains and relative 

 abundance of matrix are concerned. But the character of the 

 matrix or cement is different. Calcite is much less abundant 

 and its place is largely taken by roscoelite. This is grass-green 

 in thin section, and might readily be mistaken for indistinct 

 wisps and areas of chlorite. It sometimes forms a distinct 

 envelope around the quartz grains, showing an indistinct foli- 

 ated or fibrous structure normal to the surface of the latter. 



