50 GEOLOGY OF THE COMSTOCK LODE. 



iiiuler discussion shows in all cases a thoroughly ciTstalliue structure, and 

 the grouiidmass is always composed of" granitoid grains. The feldspars -of, 

 I believe, every slide of the augite-andesite show glass inclusions; and I 

 have not met one fluid inclusion in that rock which a[)peared to me of pri- 

 marv origin.' In the diabase the occurrence of fluid inclusions and the 

 absence of those of glass is equally universal. The augite of the augite- 

 ■ andesites shows no pinacoidal cleavages, and only one locality has been 

 detected at Washoe in which it has passed into uralite. The change even 

 there is so exceedingly local that although a dozen slides have been ground 

 from the same cropping, but one shows the alteration of augite into born- 

 blende. In the diabase the passage of augite into uralite is the u.sual pre- 

 liminary to chloritic decomposition. Fiualh', if there is one point of struct- 

 ure incapable of two interpretations, it is that the l)lack dike is of later 

 origin than the east and west country rocks. As will be shown, the black 

 dike is an ordinary diabase, and the hanging wall is consequently a pre- 

 Tertiary rock, and would necessarily be classed as a diabase were its resem- 

 blance to the volcanic series much more thorough than it really is. 



Decomposition. — lu decomposiug, tlic diabase shows few peculiarities. As 

 has already been mentioned, the augite is apt to be converted into uralite 

 and then into chlorite. Epidote almost always forms to some extent from 

 the chlorite, but the latter does not generalh- seem to pass so readih^ and 

 comjiletelv into epidote as does that which results from the degeneration of 

 hornblende. Instances occur, however, where the conversion is complete. 

 The decomposition of the feldspars presents no peculiarity. They change 

 slowly to quartz and calcite, and become porous and suffused with chlorite, 

 just as in the diorites. The final result is a mass showing aggregate polar- 

 ization with a few determinable grains of silica and carbonates, and par- 

 ticles of a whitish opaque substance, but nothing determinable as kaolin 



' It lias bceu shown of late .years (hat the evidence afforded hj fluid inclusions needs to be treated 

 •with caution, for they are reported as present in all the ;s ounger rocks. No one, however, has claimed, so 

 far as I am aware, that such inclusions are frequent in or characteristic of the Tertiary eruptives. Pro- 

 fessor Rosenlinsch, in his "Physiog. der Gesteine," does not mention a single observation of his own on 

 fluid inclusions in augite andc-^ites, and cites only one instance of such an occurrence noted by others. 



If my inferences as to tlic secondary nature of certain fluid inclusions (p. 79) are correct, a deduc- 

 tion may need to be made from the number of fluid inclusions, to which a genetic significance can prop- 

 erly be attributed. 



