DETAILED DESCRIPTION OF SLIDES. 1 19 



guish light at an angle of above 30°, and are probably angite. The rest 

 of the augite is decomposed to chlorite, of which there are excellent pseu- 

 domorphs. The hornblende, too, is decomposed. With a low power it 

 seems as if the space within the heavy black border were filled with calcite, 

 quartz, and magnetite ; but a No. 7 objective shows that the apparently 

 opaque particles are in reality minute grains of a strongly refracting min- 

 eral, no doubt epidote. Epidote in deterniinable grains also occurs in the 

 chlorite masses. There is considerable magnetite in this slide, as well as 

 many colorless apatites and one or two zircons. The groundmass shows 

 well marked fluidal structure, but no glass base. 



This is the rock described by Professor Zirkel as from the first hill 

 north of Gold Hill Peak, and analyzed by Dr. Kormann. 



Slide 209. Quarry 1,000 feet west of Yelloic Jacket east shaft. 



Considerably decomposed hornbiende-andes.te. — A gray-gTCcn porphyritlc rock, imme- 

 diately overlying and passing into ordinary bluish hornblende-andesite. 

 Under the microscope it appears that the bisilicates are wholly decomposed, 

 the hornblendes being traceable only by the black borders now filled with 

 quartz, calcite, and oxides. A few pseudomorphs of chlorite after augite 

 remain. The feldspars also are considerably attacked, and contain second- 

 ary fluid inclusions. 



Slide 210. .')00 feet nortli of North Twin Peak. 



Much decomposed specimen. — This spcclmcn is from thc same mass of rock 

 as that represented by slide 209, and resembles it, except in the fact that the 

 feldspars have lost their transparency. Under the microscope it is also 

 plain that it is the same rock in a more advanced stage of decomposition. 

 The feldspars are in part filled with specks of calcite ; in part the calcite 

 appears to have been removed by solution, and in some instances the cavi- 

 ties thus formed seem to have been filled with liquid, accompanied by a 

 bubble; or in other words the feldspars contain secondary liquid inclusions. 



Secondary fluid inclusions. — I base tlic opiuion that these inclusions are second- 

 ary on the following grounds: They do not occur in the fresh hornblende- 

 andesite from the same locality, or in unattacked feldspars in decomposed 



