SUMMARY. 369 



LITHOLOGY. 



Importance of the rock determinations. — In areas SO largely covered by massive 

 rocks as the Washoe District, lithological determinations form the neces- 

 sary preliminary to geological investigation, for few points in the history or 

 the structure of such a region are independent of the character of the rocks 

 involved. Moreover, the economical importance of the District, the ob- 

 scure character of some points in its geology, and the great weight of the 

 authorities whose investigations had already been published, made it essen- 

 tial that the work done under the new United States Geological Survey 

 should be supported by the strongest and most detailed evidence. The 

 collections embrace over 2,600 specimens and 500 microscope slides. The 

 locality of each specimen was fixed with great care on the maps at the 

 time of collection, and no time or pains was spared in preparing the geo- 

 logical maps and sections. In laying down the various formations the 

 microscope was in constant use, slides being ground as the occasion arose, 

 and the results obtained from them finding immediate application in the 

 extension of the work. 



The area in which the Comstock lies is characterized by a wide-spread 

 and profound decomposition of the rock masses, and a study of the lithology 

 of the District resolves itself primarily into an investigation into decom- 

 position. In spite of the most painstaking choice of specimens, there is not 

 one in fifty of those collected underground which contains a particle of 

 either of the characteristic bisilicates or of the lithologically equivalent 

 unisilicate, mica, secondary minerals replacing them throughout. Even the 

 feldspars are rarely intact, and are sometimes wholly decomposed. When 

 the steps of these processes of degeneration are once understood, however, 

 it is comparatively easy to infer the original composition and structure of 

 the rock. Some of the results obtained are the following: 



Decomposition. — Hombleude, augite, and mica generally pass into a chlo- 

 ritic mineral, which, so far as can be judged by any optical tests now known, 

 is almost without exception the same, from whichever of the ferro-magne- 

 sian silicates it may have originated. This chlorite is generally green, but 

 in especially compact masses appears greenish-brown under the microscope. 

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