University of California Publications. 



[Geology 



of the quartz. In the ease of those rocks containing' biotite it 

 may he due to decomposition of that mineral, but in the present 

 instance it is not clear from what source, the water was derived, 

 for the bubble inclusions occur along the fractures of the quartz. 



No. 89 is an even grained rock, composed of light green horn- 

 blende and feldspar, showing polysynthetic twinning as viewed 

 in the hand-specimen. The rock occurs at the east base of Sing- 

 ats'-e Ridge, and half way from the top of the map PI. 1. It 

 apparently is not of great extent. Hornblende appears to be 

 about one-third of the rock. The grain of the rock is about 2.5 

 mm. In the slide the last named mineral is pale green and some- 

 what pleochroic. No section normal to the prisms was obtained 

 in which to measure cleavage angles, but in some sections, show- 

 ing the cleavages parallel or very nearly so, the extinction was as 

 great as 15° or over. The cleavage is very fine and lamellar. Ortho- 

 clase and quartz have less prominence. The plagioclase differs 

 not essentially from that seen in previous sections. They show 

 zonal structure, and their decomposition has had zonal progress. 

 The quartz is fractured, and through its rifts are minute liquid 

 inclusions. The cracks are quite parallel throughout the slide. 



No. 141 is the granite of Mason Butte. The light and dark 

 minerals are about of equal proportions. The dark minerals are 

 hornblende, biotite and chlorite. Epidote is noticeable. Quartz 

 and feldspar comprise the light colored minerals. The average 

 grain of the rock is 4 mm. In the slides the feldspar appears to 

 be mostly orthoclase, since it is free from lamellar twinning and 

 possesses low relief and birefringence. Quartz is next in import- 

 ance, and there are a few individuals of microcline. The horn- 

 lende is sparingly present, its cleavage and form normal to prism 

 zone indicating its presence beyond doubt. Brown biotite is 

 believed to have been once present in an abundance, for chlorite, 

 with a few remaining shreds of brown biotite, is very plentiful. 

 The chlorite shows the micaceous cleavage, and the extinction is 

 parallel to the cleavage. Its very low double refraction, "ultra 

 blue,'* between crossed nicols, makes it easily recognizable. Some 

 iron ore and perhaps secondary silica is in evidence. Epidote is 

 easily recognizable by its relief and strong double refraction. It 

 is thought to be secondary because of its intimate association with 



