88 GEOLOGY OF THE COMSTOCK LODE. 



be deep brown black-bordered crystals. There is no green hornblende and 

 there are glass inchisions. In short, there is no assignable reason for sepa- 

 rating- this rock from the andesites. There are many other localities in the 

 District where the propylitie rocks are quite as puzzling as in the three 

 described, but it is sufficient to state that they were studied with equal care 

 and with similar results. 



Conclusions reached. — Field obscrvatious, aided by microscopical examina- 

 tion, show that the mineralogical composition and the structure of the proj)v- 

 lites of the Washoe District in their original state were identical with those 

 of certain fresh rocks found in the same region, namely, granular diorite, 

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

 great and misleading similarity of the propylites to one another is due not 

 to original constitution, nor to their geological relations, but to the identity 

 of the decomposition processes to which they have all been subjected. The 

 failure to detect the lithological relations of these rocks arose principally 

 from a confusion between green hornblende and the green and tlichroitic, 

 but uniaxial, minerals grouped under the tei'ui chlorite; but a neglect to 

 give due weight to evidences of pseudomorphism, partial devitrification 

 and other phenomena of decomposition, materially aided in obscuring the 

 true nature of the supposed rock-species. 



Causes of error. — It appears to me by no means superfluous to consider how 

 so keen an observer as Baron v. Richthofen came to regard propylite in the 

 Washoe District as an independent rock-specyes, and as a volcanic of Ter- 

 tiary age; and while I have no authority for my suggestions, I offer the fol- 

 lowing explanation.* Baron v. Richthofen regarded Mount Da^^dson as 

 syenite and the visible plagioclases as accessory. The rock does indeed 

 more nearly resemble ordinary sj-enite in its general appearance than ordi- 

 nary diorite, and the error was never detected until Professor Zirkel exam- 

 ined it microscopically. In the porphyritic diorites v. Richthofen saw a 

 plagioclase rock, but the triclinic character of the feldspars in the porphyry 

 aroused no known doubt in his mind as to those of the mass of Mount 

 Davidson. Porphyritic syenites are very rare, while the relation of the 



' It would be 8Uperflaou8 to remind geologists that iu 1?05 the science of microscopical lithology 

 was undeveloped. 



