118 GEOLOGY OF THE GOMSTOCK LODE. 



Slide 229. From the same locality as 228. 



Specimen containing ilmenite. TlllS COntainS RH RUglte wllich shoWS excellent 



pinacoidal as well as prismatic cleavage. It also contains a few patches 

 of a finely granular mineral, which shows very feeble tints between crossed 

 Nicols, and might be taken for sphene. It is in reality epidote, which often 

 behaves in this way when finely divided. 



The hornblendes in this slide are, as a rule, less decomposed than the 

 augites. Indeed, the hornblendes in the Washoe andesites frequently, 

 though by no means always, resist decomposition better than the augites, 

 perhaps on account of the heavy black border. Much of the chlorite formed 

 from the augite has further decomposed into calcite and quartz. One 

 pseudomorph of chlorite after augite has been attacked from within by epi- 

 dote, and from without by calcite. 



To test the nature of the iron ore in this rock the cover of the slide 

 was removed, and the balsam well washed off with alcohol. Careful draw- 

 ings were made with the camera of certain portions of the slide, which were 

 then treated with strong chlorhydric acid. A drop of acid was placed upon 

 the area to be tested and the slide warmed over a lamp for several minutes. 

 The acid was then washed off" with water, and the operation repeated five 

 times. After each treatment the slide was inspected, and the result showed 

 that while the black border and certain grains of iron ore were completely 

 soluble, others were only coated with a white film, and remained undis- 

 solved. The etching also brought out faint straight lines on the undissolved 

 grains, at an angle of approximately 60°, which seems to complete the 

 proof that the mineral is ilmenite. I find myself unable to distinguish 

 under the microscope the diff"erence in tint between magnetic and titanic 

 iron, which is so perceptible in the streak. 



Slide 208. North Twin Peak. 



Partially decomposed hornblende-andesite. A dark, bluisll, fresll-lookiugandesite, but 



in reality much more decomposed than those just described. The feldspars 

 contain glass inclusions, but no fluid ones were observed. They are but 

 slightly decomposed, showing a little calcite and a few porous streaks and 

 spots. They contain many yellow, rounded microlites, some of which extin- 



