76 QUICKSILVER DEPOSITS (JV THE PACIFIC SLOPE, 



and avigitic groups. No clear indication has been detected that the mode 

 of occurrence differs for the two chxsses. This may nevertheless b3 the 

 case, for the amphibolic and pyroxenic rocks are macroscopically indis- 

 tinguishable, excepting in a few cases, and differences in occurrence would 

 thus readily escape detection. At present it seems more likely that the 

 controlling factor is an unknown and certainh^ very slight difference of 

 chemical composition. Observation has shown me that it is absolutely 

 necessary in some eruptive rocks to resort to a chemical explanation of the 

 replacement of one of those minerals by the other without affecting the 

 probability, in another class of instances, that the same replacement is due 

 to differences of physical condition.^ 



Green hornblende is also very abundant. Much of this is certainly 

 uralitic and some of it appears probably due to the alteration of brown 

 hornblende. There are also cases in Avhich the green hornblende, so far as 

 can be judged, is a direct product of metasomatic action, but none in which 

 every other explanation is excluded. There are further instances which 

 suggest the existence of a brown nralite, but these cases are believed to be 

 better explained by envelopment. 



The authigenetic hornblendes are readily distinguished from allothi- 

 genetic fragments, the latter being commonly of a dark, dirty-green color 

 and much more pleochroitic than the newly formed mineral. The outline 

 of clastic fragments is usually characteristic. In extreme cases there is some 

 difficulty in distinguishing green hornblende from chlorite, but where the 

 particles are not excessively minute the oblique extinction of the former is 

 generally perceptible. 



Giaucophane. — TliIs Is a prouilnent component of the micaceous schists' 

 and occurs also in the more composite granular rocks and in the amphlbo- 

 lite. Cross-sections frequently show the amphibolic outline and cleavage. 

 The pleochroism and absorption are strong. The pleochroic colors are a, 

 brownish yellow to colorless; h, violet; c, ultramarine blue. The absorp- 



' For some curious evidence bearing ou tliis point, .stc iny'Geology of the Comstock Lode I'.nd the 

 Washoe District, Mou. U. S. Geol. Survey No. 3, p. CO. 



-According to llr. H. G. Ilanks, gl.iucophane was detected by Mr. Micbel-Ldvy, iu 1878, iu speci- 

 lueus of micaceous schist from the Wall Street quicksilver mine, Lake County, exhibited at the Paris 

 exposition in l'-'78. (Finirth Aiiunal Keport of the Stale Mineralogist of California, IS-'S-'SJ.p. 1S>,) 



