72 GEOLOGY OF THE COMSTOOK LODE. 



Section 2. (Chapter III.) 

 THE DECOMPOSITION OF THE EOCKS. 



Such facts as have been established with reference to the decomposi- 

 tion of the Washoe rocks are necessarily mentioned in connection with the 

 lithological description of each species. The subject, however, is one of 

 such great importance in the geology of tlie District that it appears advisa- 

 ble to consider the observations bearing upon it as a whole, and in some 

 detail. 



Area of extreme decomposition. — While few absolutcly fresli rocks occur in the 

 region surveyed, decomposition so great as to oppose a serious obstacle to 

 lithological determinations is confined to a smaller area. In the nature of 

 things this area is incapable of precise definition, but it is shown as nearly 

 as may be in its relation to the Comstock and the Occidental lode by the 

 accompan5'ing sketch majD, page 73. From this it appears that precisely 

 the area which is of the most importance in a discussion of the vein-geology 

 is that profoundly decomposed. 



Effects of decomposition on various rocks the same. While tllC physical cliaraCtCr of 



the difterent rocks has to some extent modified the physical results of decom- 

 jjosition, the chemical and mineralogical changes and the degree of alter- 

 ation observed in the rocks of the decomposed area seems almost wholly 

 independent of their age or species. Granular diorites, porphyritic diorites, 

 the two diabases, earlier hornblende-andesite, and augite-andesite appear to 

 have been subjected to the same influences, with the same results. Quartz- 

 porphyry and younger hornblende-andesite come within the limits of the 

 chief area of decomposition only to a slight extent, either above or below 

 ground, but to that extent they show the same eff"ects, as does also the met- 

 amorphic-diorite in limited spots more or less nearly related to the focus of 

 action. Only basalt and granite have escaped with mere traces of decom- 

 position, while the quartz-porph3'ry as a whole appears to have been sub- 



