376 GEOLOGY OP THE COMSTOCK LODE. 



and primitive liquid inclusions, or not, according to the rock from which 

 they were derived. Base is rare in propylites; where it originally formed 

 a constituent of the rock, it has for the most part undergone devitrification. 



A reexamination has been made of all the slides of propylites from 

 other localities as well as from the Washoe District, descriptions of which 

 have been published in different government reports. These, too, can be 

 referred to other rock species with great probability, in spite of advanced 

 decomposition, and I do not hesitate to affirm that there is no proof yet known 

 of the existence of a pre-andesitic Tertiary eruption in the United States. 



The term "propylite" should not be I'etained in the nomenclature of 

 American geology even to express certain results of decomposition, for the 

 equally loose term "greenstone" seems to cover the same ground and has 

 priority. 



A few minor questions of interest were raised by the microscopic 

 examinations, in addition to those bearing directly upon the identification 

 of the rocks. Such are the occurrence of zonal plagioclases and their bear- 

 ing on Tschermak's feldspar theory; hornblendes with concentric belts of 

 magnetite, and the indications they furnish as to the conditions under which 

 "black borders '' form, and some other small points. 



STEUCTUEAL RESULTS OF FAULTING. 



Evidences of faulting. — Tlic evideucc of faulting on the Comstock is mani- 

 fold, and has been recognized by all observers. The irregular openings in 

 the vein, the presence of horses, the crushed condition of the quartz in man}^ 

 parts, the presence of slickensides and of rolled pebbles in the clays, are 

 all conclusive on this point. Both to the east and west of the vein, too, the 

 country rock shows a rude division into sheets, and along the partings 

 between the plates evidences of movement are perceptible, decreasing in 

 amount as the distance from the vein increases, according to some law not 

 directly inferable. All the evidence points to a relative upward move- 

 ment of the foot wall. 



