372 GEOLOGY OP THE COMSTOGK LODE. 



erosion often exposes the fresh core, which then, offering greater resistance, 

 projects as a "cropping," or, if it has an elongated form, it protrudes hke a dike 

 above the surrounding country. As the tendency of the mere action of atmos- 

 pheric agencies is to the production of ferric hydrate rather than of chlorite 

 from the bisilicates, the first impression which such a mass produces is that 

 of an older and a younger rock in conjunction. Nevertheless, sufficiently 

 thorough examination will reveal a transition. When the rock is not solid, 

 but brecciated or loose-grained, sufficient space often seems available to 

 permit the requisite increase of volume without disintegration. Large and 

 often prominent masses of very strongly cohesive decomposition-products 

 derived from breccia are common in the District. 



The mineralogical character and the microscopical phenomena of de- 

 composition seem to be identical in the different rocks. Those refined mani- 

 festations of physical character by which it is so often possible to discriminate 

 between older and younger rocks, and between the various rock species 

 when fresh, are nearly or quite obliterated by the decomposition process, 

 which impresses its own character on the product. 



Rocks of the District. — The rocks occurring in the Washoe District are gran- 

 ite; metamorphic schists, slates, and limestone; eruptive diorite of three 

 varieties; metamorphic diorite ; quartz-porphyry ; an older and a younger 

 diabase; an older and a younger hornblende-andesite; augite-andesite, and 

 basalt.' Chapter III. contains a discussion of each of these rocks and a 

 detailed description of about seventy-five slides, and is well illustrated. 

 Hei"e they can be dismissed with a very few remarks. 



'The signification attached to these names has varied somewhat as the science of lithology has 

 jirogrcssed. Some of the main points of their definitions adhere understood are as follows: 



Granite, pre-Tertiary non- vitreous crystalline rock, of which the principal constituents are ortho- 

 clase, quartz, and mica or hornblende. 



Diorite, pre-Tertiary non- vitreous crystalline rock, of which the main constituents are plagioclase 

 and hornblende. It may or may not contain quartz. 



Quartz-porphyry, pre-Tertiary glass-bearing porphyritic rock, of which the main constituents 

 are orthoclase, quartz, and hornblende or mica. 



Diabase, pre-Tertiary, more or less porphyritic rock, of which the main constituents are plagio- 

 clase and augite. 



Andesite, Tertiary or post-Tertiary, glass-bearing, more or less porphyritic rock, of which the 

 main constituents are i)lagioclase and hornblende, mica, or augite. The andesites in which augite is 

 the characteristic bisilicate appear to be separate eruptions, while mica and hornblende replace one 

 another to a variable extent in the same eruption. In the andesites feldspar predominates. 



Bas.alt, Tertiary or post-Tertiary plagioclase augite rock, with predominant augite, usually char- 

 acterized by the presence of olivine. 



