SUMMARY. 375 



and indicates a repetition of absolutely identical physical and chemical con- 

 ditions at distant points, which is far from having received an adequate ex- 

 planation. 



propyiite. — The prcscnt investigation of the geology of the Washoe Dis- 

 trict has failed to establish the existence of propylite. Full proof of this 

 responsible statement cannot of course be given in this summary of results. 

 It consists in a process of exhaustive elimination. A study of each of the 

 rocks of the District, in all stages of decomposition,, has led to the identi- 

 fication of all of them with other and previously recognized species. The 

 reduction of rocks of originally different aspect to an apparently uniform char- 

 acter by chloritic decomposition is strikingly evinced by a mere list of the 

 species in the District, which have been grouped under the terms "propy- 

 lite" and " quartz-propylite." These are granular diorite, porphyritic diorite, 

 diabase, quartz-porphyry, hornblende-andesite, and augite-andesite. The 

 peculiar habitus which is always referred to in descriptions of propylite 

 appears to consist in the impellucidity of the feldspars, the green and fibrous 

 character of the hornblende, the greenish color which often tinges feld.spars 

 and groundmass, and a certain blending of the mineral ingredients. The 

 impellucidity of the feldspars (which surprisingly alters the appearance of 

 rocks originally containing transparent unisilicates) is due to incipient 

 decomposition, especially, as it seems, to the extraction of calcite. The 

 "green hornblendes" are simply pseudomorphs of chlorite after hornblende 

 or augite, as the case may be. Excepting the granular diorite, not one of 

 the rocks from which propylite forms has ever been found in the Washoe 

 District containing primitive green hornblende, though uralite is common. 

 The other characteristics are due to the diffusion of chlorite and the foi'ma- 

 tion of epidote from it. The description of propylite as a species arose from 

 the erroneous determination of chlorite as green hornblende — a very natural 

 mistake before the microscope was brought to bear on the subject, since even 

 with that instrument the same error may be committed if color and dichroism 

 are exclusively relied upon as diagnostic tests. The microscopical charac- 

 teristics of propylite are illusory. Finely disseminated hornblende in the 

 groundmass of a Washoe rock is very rare, and far rarer is the presence of 

 particles of hornblende in feldspars. The propylites contain glass inclusions 



