142 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



granites. Of these the first are long prisms of oligoclase (vvitli appropriate 

 extinctions), more or less irregular prisms of hornblende (often with good 

 hexagonal cross-sections), and irregular foils of blotite. The minerals of 

 secondary consolidation are quartz and orthoclase, with perhaps a portion 

 of the oligoclase. This division is only approximate, however, for grains 

 of quartz are embedded in well developed hornblende prisms of first con- 

 solidation. 



The plagioclase in some of these specimens is much more striking 

 under the microscope than the orthoclase, and this fact might lead an ob- 

 server to doubt which mineral predominated. In cases of this kind there 

 is really no means of determining by microscopical examination the rela- 

 tive quantities present, for, since the areas of the grains cut by the slide 

 vaiy with the form and position of the grains as well as with their cubic 

 contents, the most careful study of the areas exposed, or even of the areas 

 of each grain, will lead to no definite result unless the diff"erence in quan- 

 tity is very great. To test tlie matter 42 grams of such a specimen were 

 reduced to a grain of 0.5""", which, in consideration of the coarseness of the 

 rock, was considered sufficiently fine, and separated by the Thoulet method. 

 The separation seemed very successful, there being a very small loss from 

 dust. The following is the result: 



Per cent. 



(1) At specific gravity 2.77, ferromagnesian silicates 17 



(2) At specific gravity 2.67, impure feldspar 5 



(3) At specific gravity 2.64, feldspar 12 



(4) At specific gravity 2.62, quartz 34 



(5) At specific gravity 2.00, feldspar, with some quartz 1) 



(G) At specific gravity 2.58, feldspar 12 



(7) At specific gravity 2.56, feldspar 5 



(8) At specific gravity 2.54, feldspar 6 



The only triclinic feldspar detected under the microscope was oligo- 

 clase, and the feldspar heavier than quartz was undoubtedly of tliis species. 

 As appears from the table, about 1 7 per cent, of this mineral fell before the 

 quartz. At first sight it would appear that orthoclase predominated greatly 

 ill the rock, since the larger part of the feldspar is lighter than quartz. It 

 is a suspicious circumstance, however, that the range of densities is so great, 

 and mixtures are to be suspected. Clieraical tests of (.5), (G), (7), and (8) 



