PROPYLTTE. 89 



diorite-porphyries of Washoe to the granitoid diorite is peculiar. Any 

 but a very tliorougli inspection would lead to the belief that the porphyries 

 are younger than the granular diorite. v. Richthofen had reason to suppose 

 that Mount Davidson was post-Jui-assic, and the plagioclase porphyries were 

 therefore in his eyes younger than that period, and older than the andesites 

 which cap the range. As I have endeavored to show, the dioritic porphy- 

 ries, when in a certain stage of decomposition, are scarcely distinguishable 

 from the thoroughly crystalline andesites, when the latter are also somewhat 

 decomposed. These v. Richthofen found at considerable distances from his 

 "syenite," and so associated with Tertiary rocks as to prove them members 

 of that series. Tertiary leaves were also discovered in similar rock at no 

 great distance. These wackenitic andesites, too, stood in such a relation to 

 the fresher rocks that they appeared to precede them, and the chain of proof 

 seemed complete of a pre-andesitic Tertiary rock. The extension of the 

 propylite to the mines was natural and easy. 



If propylite were older than andesite, where should we look for it but 

 in depth? And if there was no distinct lithological reason assignable for 

 pronouncing the underground rock, mostly in the last stages of decomposi- 

 tion, identical with that on the surface, there was next to no reason, macro- 

 scopically speaking, for supposing it different. This mistake having once 

 been committed, I do not believe it could ever have been corrected in oppo- 

 sition to even a far less weighty authority than Baron v. Richtliofen, had 

 not fresher rocks been opened up by the extensive lower workings, and had 

 the microscope not been sought as an auxiliary. That an association 

 between propylites and mineral veins should have been observed is natural, 

 for in mineralized districts we expect general decomposition. 



Propylites from other districts. — By thc courtcsy of thc gcologists of the Fortieth 

 Parallel Survey, I have been permitted to examine the specimens and slides 

 from all the localities laid down in their publications as propylite. Captain 

 Button, too, has kindly furnished me with specimens and slides from his 

 propylite localities in Utah. Rocks which are indeterminable in the field 

 are very apt to give uncertain evidence under the microscope, and as all 

 propylites are decomposed, I do not feel absolute confidence in my deter- 

 ^ minations of the propylites occurring outside of the district which forms the 



