126 W. Cross — Igneous Rooks in Wyoming. 



The optical properties are unlike those of any mineral with 

 which I am acquainted. Extinction seems to be always par- 

 allel to the length axis. The optical scheme is as follows : 



a=a, pale yellow; b=b, red; c~c, bright yellow. The red- 

 dish tones are very similar to those of hypersthene and increase 

 rapidly in intensity with increasing thickness. Absorption : 

 b>c>a. t • 



Needles scraped from the cavities have the same character 

 as the embedded mineral. In the powder obtained from one 

 cavity there was found a flake apparently representing a section 

 normal to the prism as indicated by cleavage, prism angle, and 

 pleochroism. Examination of this plate in convergent polar- 

 ized light showed the figure of an almost uniaxial mineral, the 

 arms barely separating in the 45° position. 



Other constituents. — Bhlogopite and leucite are so identical 

 in development with the forms described for the wyomingite 

 that no further comment is necessary. Apatite appears in a 

 number of clear, more or less irregular prismatic grains, with 

 axial inclusions ; but the analysis indicates a much larger 

 amount of this mineral in the rock than one would infer from 

 the sections. The mineral mentioned above as rutile (?) occurs 

 in minute yellow needles of round prism outline, with strong 

 single and double refraction and extinction apparently parallel 

 to prism, though absorption in this direction is so strong as to 

 obscure extinction. 



The Rock of Pilot Butte. 



Occurrence. — In the words of Mr. Emmons, * " Pilot Butte 

 is a curious little conical castle-like mound, rising about 400 

 feet above the surface of the plateau country, in the angle 

 between Bitter Creek and Green Biver to the north of the 

 [Union Pacific] railroad. It is a rudely circular mass, scarcely 

 1000 yards in diameter, having abrupt faces on all sides, and 

 composed of a rather singular volcanic rock unlike any other 

 found within the limits of the survey. It is evident that the 

 soft Green Biver [Eocene] Tertiaries, which once surrounded 

 and covered it, must have been eroded away. . . ." 



The plateau above which the butte rises is separated from, 

 the principal mesa of the Leucite Hills by the broad shallow 

 valley of Killpacker Creek, and no other mass of volcanic rock 

 is known nearer than the Leucite Hills, some 15 miles to the 

 eastward. Xo observations are reported by either Emmons or 

 Kemp indicating beyond question the character of this mass. 

 Xo contacts seem to have been found. At the eastern base, 

 where I gathered my material, talus effectually concealed the 

 contact, and such may be the condition on all sides. 



* Reports of the Fortieth Parallel Survey, vol. ii. Descriptive G-eologv, 18*77, 

 p. 238. 



