114 GEOLOGY OF THE COMSTOCK LODE. 



to their direction, and polarize for the most part in dark bluish tints. In 

 many cases the uralite seems to be attacked from innumerable points, and 

 the chlorite then shows a spherolitic structure. There are a few grains of 

 epidote in this slide, associated in a somewhat indefinite manner with the 

 uralite and the chlorite. 



The iron ore seems to be ilmenite. It occurs in the characteristic forms 

 of that mineral, and is accompanied by a very little leucoxene. There are 

 also a very few apatites, a little quartz, which is probably secondary, and 

 one or two particles of sphene. 



Slide 18. Sutro Tunnel. Hanging wall of Lode at Savage connection. 



Fresh diabase used for experiments. — This in hand specimeus is R veiy black rock, 

 with less waxy luster than most diabases show, but with the usual lath-like 

 feldspars. Tlie feldspar does not differ from that in slide 349, and measure- 

 ments of the angles of extinction show it to be labradorite. It contains 

 some fluid inclusions. Most of the augite is fresh, and some crystals show 

 zonal structure : a few are converted into uralite and chlorite. The ground- 

 mass of the rock contains many microlites of augite. There are a few 

 flakes of a brown, highly dichroitic mineral in this slide, which show none 

 of the structure of hornblende, and seem to be biotite. Its quantity is 

 insignificant. The iron ore is at least in part ilmenite. 



This is the freshest diabase known to exist in the District, and as such 

 was selected for the experiments on kaolinization. Assays and a chemical 

 analysis of it will be given at the end of the chapter. Its appearance under 

 the microscope is illustrated in Plate IV., Fig. 28. 



Slide 53. Sutro Tunnel, 19,200 feet from entrance. 



Quartzose diabase. — Macroscopically tWs rock entirely resembles that repre- 

 sented by slide 18. The slide is one of the few containing quartzes which 

 are unquestionably piimitive. In this case the arrangement of the microlites 

 of iron ore round their edges, and the inclusions of groundmass, put their 

 character beyond question. These quartzes are remarkably full of fluid 

 inclusions; the smaller ones with spontaneous bubbles, which do not decrease 

 in size when the slide is heated to above 40° C. The rock contains com- 

 paratively little fresh augite. 



