68 GEOLOGY OF THE COMSTOCK LODE. 



tlie species of feldspar, the mass of g-raiiis was first placed in a solution of the 

 double iodide of potassium and mercury, which possessed a specific frnivity 

 of 2.95. A portion of the substance imniediateh' fell to the bottom. When 

 examined with the microscope this was found to consist of hornblende, 

 augite, nnca, and iron oxide. The sjjecific gravity of the fluid was then 

 diminished to 2.85, when a small portion settled out. The ])recii)itate was 

 found under the microscope to consist of composite grains including por- 

 tions of one of the previously mentioned minerals. At the specific gravity 

 2.75 only a few grains of the same character fell down, and these were 

 more largely feldspathic. 



On reducing the fluid to 2.70, a large amount of clear white grains 

 fell from the fluid. At 2.G8 another large portion was precipitated, and 

 these precipitates when examined under the microscope proved to be com- 

 posed entirely of grains of feldspar. On reducing the specific gravity to 

 2.fi7 very little fell down, and this was of a red color, and consisted mostly 

 of grains containing clear feldspar, together with portions of the ground- 

 mass. Subsequent reductions of the specific gravity caused the remaining 

 substances to fall to the bottom in successive portions, and when the fluid 

 had reached the specific gravity of 2.61, only a very small amount of ma- 

 terial floated. This examined under the microscope was found to consist 

 entirely of groundmass. There appeared to be no portion of the glassy 

 fekhspar crystals in any of the substances which had a specific gravity 

 below 2.65. As the amount of rock which will float at any specific gravity 

 which approaches that of orthoclase is very small, it would seem that 

 under no circumstances could this feldspar be considered as a preponder- 

 ating species, and that, if present at all, it must be in very small amount. 



Mr. F. P. Dewey, at Dr Hawes' request, analyzed the feldspar which 

 fell Avhen the specific gravity of the solution was 2.70 and found its oxygen 

 ratio 1:2.89:7.95. This I find would correspond to a mixture of 39 per 

 cent, labradorite and 61 per cent, oligoclase, supposing these the only feld- 

 spars present. He also analyzed the portion which fell at a specific gravity 

 of 2.68 and found the oxygen ratio 1:2.96:8.69, corresponding to 12 per 

 cent, labradorite and 88 per cent, of oligoclase. The entire feldspar analyzed 

 was 31 per cent, of the rock, or 8 per cent, labradorite and 23 per cent, oligo- 



