2 PROCEEDINGS OF THE NATION" AL MUSEUM vol.72 



Dutton ^ concluded that the erosion of the Grand Canyon began in 

 Eocene time. Situated as it is on a small tributary near the upper 

 end of the Grand Canyon, erosion probably exposed the Bridger 

 Jack area to oxidation and the action of meteoric water at a con- 

 siderably later period in the Tertiary. 



The veins are later than the usual carnotite deposits of the 

 plateau region, for the ordinary deposits are impregnations of sand- 

 stone in connection with leaves or fillings of cavities in old tree 

 trunks {ani/r'acarioxylon) some of which were hollow and all were 

 partly decayed before petrifaction. What the form of the minerals 

 may have been when deposited is uncertain, but carnotite and other 

 minerals now present were not formed until the rocks were eroded 

 and exposed to the percolation of meteoric waters. Generally 

 carnotite and related minerals have moved out far enough to make 

 an aureole around the vegetal masses, but after the soft sandstones 

 were brought near enough to the surface to allow the formation of 

 open cracks, the carnotite at this place was moved from the aureole 

 and deposited in thle cracks. 



PHYSICAL AND OPTICAL PROPERTIES 



GontriT)uted 6y WilUMn F. Foshag 



The carnotite (U.S.N.M. 95332) forms crusts from 1-2 millimeters 

 in thiclmess and with rough botyroidal surface on calcareous sand- 

 stone. The outer surface is colored greenish to brownish yellow 

 and shows only faint suggestions of crystal faces. The inner portion 

 is made up of coarse plates in parallel position or roughly radiated. 

 These plates are of a deep lemon yellow color with a decided tinge 

 of green and have a pearly silky luster. The powdered mineral 

 is of a strontian yellow color. 



Under the microscope the mineral is seen to be made up of clear 

 yellow plates showing a perfect platy cleavage. There appears how- 

 ever, to be no other cleavage. These plates show no pleochroism but 

 grains oriented normal to the cleavage are strongly pleochroic. The 

 scheme is : X grayish yellow with strong absorption, Y lemon yellow, 

 Z lemon yellow. As far as could be determined the plates show 

 parallel extinction. The cleavage flakes show a biaxial interference 

 figure and a medium dispersion of the optic axes. The axial angle 

 (2 V) on material containing 1.32 per cent of water was measured 

 with the aid of a micrometer ocular and found to be 50° ±2°. The 

 indices of refraction of the mineral were found to be somewhat higher 

 than amorphous sulphur. Material dried over concentrated sulphuric 



1 Button, Clarence E. The physical geology of the Canon district. U. S. Geol. Survey, 

 Second Ann. Rep., p. 119, 1882. 



