138 GEOLOGY OF THE COMSTOCK LODE. 



brown hornblende rocks; but I failed to find green hornblende, fibrous horn- 

 blende, or hornblende without a black border. There are a few excellent 

 augites and many capital pseudomorphs of chlorite after augite. This chlorite 

 shows the usual structure, dichroism, extinction parallel to the fibers, etc. 

 The feldspars are triclinic, the large ones seemingly labradorite, and they 

 appear to contain devitrified glass inclusions. There are many brown and 

 dusty apatites. The groundmass has the microlitic structure of hornblende- 

 andesites, nor can I see any reason for separating this rock from that species. 



Exploration of the Fortieth Parallel. Slides Nos. 218 and 219, specimen No. 22,694. 

 Ophir Ravine, Washoe. 



These slides I have sufficiently discussed in describing my own thin 

 sections from the same locality. I have there considered the rock as a dio- 

 rite-porphyry. 



Exploration of the Fortieth Parallel. Slide No. 220, specimen No. 22,588. Hill east 

 of Steamboat Valley, Virginia Eange. 



This is a brown rock which looks like an impure limonite. Under the 

 microscojje nothing is visible excepting ferric hydrate and a little secondary 

 quartz. 



Exploration of the Fortieth Parallel. Slide No. 221, specimen No. 22,574. Sheep 

 Corral Canon, Virginia Range. 



This is a light greenish, granular rock, evidently composed of feldspar 

 and hornblende. The slide shows that the hornblende is wholly decom- 

 posed. The crystals of this mineral appear to have had black borders, 

 which are now in part replaced by higher oxides. When fresh it contained 

 great numbers of small augites, which are now converted into the ordinary 

 chlorite. The same product of decomposition is also disseminated through 

 the groundmass, and is accompanied by quartz and calcite. The feldspar 

 is fresh and striated, and the general character under the microscope is that 

 of an andesite. I can see no reason for calling it anything but hornblende- 

 andesite. 



Professor Wiedemann analyzed this rock and found 64.62 per cent, silica. 

 In discussing this analysis the fact should not be overlooked that a relative 

 increase in the quantity of silicic acid commonly accompanies decomposition. 



