SOME CONSTITUENTS OF SANADINE-BEAEING ROCK. 179 



for microchemical tests of it with hydrochloric acid yield hut few cubes 

 of sodium chloride. 



In specimens from the south mesa, the augite shows occasionally a ten- 

 dency to develop larger crystals than the minute rods and needles in the 

 groundmass. One was noted of perhaps 0.5 of a millimeter in diameter, 

 but in Black Rock butte the large augites are more frequent and may 

 reach 2 millimeters. They are colorless and are surrounded by a rim 

 of biotite crystals as is shown in figure 3. Olivine is also met less com- 

 monly forming the interior core. In one or two slides an opaque de- 

 composition product was noted, apparently limonite in largest part, that 

 in one case had some unaltered strongly pleochroic strips still remain- 

 ing. It corresponded in pleochroism to biotite or hornblende, but the 

 dark color and almost opaque character prevented its sharp determina- 

 tion. If biotite, it must be of a different kind from the common light 

 brown variety, which is perfectly unaltered. The light brown biotite, 

 which is the most widespread and uniformly present mineral, has been 

 quite fully described by Zirkel. Hexagonal plates do, however, appear 

 in sections taken parallel to the flow, and exhibit a marked biaxial char- 

 acter. 



The amygdaloidal cavities are almost invariably empty. In only two 

 cases were fillings detected, and in each they were 'chalcedonic silica. 

 Although often rusty and more or less weathered the rocks show sur- 

 prisingly few secondary minerals of this character. 



INCL USIONS. 



As stated above, inclusions of foreign rock, especially sandstone, are 

 common. In thin section the biotite flakes eddy around them and 

 produce very pretty flow-lines and similar phenomena. An inclusion of 

 orthoclase was also noted, showing strongly undulatory polarization ap- 

 parently from strains. In the specimens studied practically no contact 

 effects were observable. 



PETROGRAPHY OF PILOT BUTTE. 



Fresh specimens from Pilot butte are not readily obtainable, and are 

 in most cases seamed with veinlets of secondary minerals. The rock has 

 a dense, massive texture, gray in the least altered specimens, and giving 

 to the unaided eye but slight clue to its composition. It is a difficult 

 one to make transparent in thin sections, and of these only the outer 

 edges are, as a rule, available for fine determinations. The light brown 

 mica of the Leucite hills is present in considerable amount, but augite 

 is much more in evidence and is the predominating mineral in the rock. 



