THE ROCKS OF TDE WASHOE DlSTItlGT. 33 



but little advance. Geologists who do not deal with the phenomena of ore 

 deposits are commonly satisfied with determining the species ot the rocks 

 with which they have to do, and recording the mere fact of decomposition. 

 They therefore select oidy the freshest specimens for microscopical exami- 

 nation. If the resources of the microscope are to be fully brought to bear 

 upon the study of ore deposits, mining geologists must pursue a differeni 

 method. They must trace the mineralogical course of decomposition-jn'o- 

 cesses, and learn to recognize highly altered rocks, even when fresli speci- 

 mens are unattainable. 



Disputed character of Washoe rocks. — Tlicrc is a furtlier reasou for tlic Consider- 

 able and, as it may seem to some readers, the undue space which this chap- 

 ter occupies. Baron von Richthofen based the independence of the new 

 rock propylite largely upon the occurrences in the Washoe District. Later 

 investigators in the same field without exception have adopted his views. 

 Professor Zirkel's characterizations of the microscopical peculiarities of pro- 

 pylite were also founded chief!}' on the Wasiioe occurrence. Though at 

 the beginning of the })resent investigation I was fully persuaded of the inde- 

 pendence of propylite, I subsequently found reason to doubt it; but to prove 

 a negative is notoriously difficult, and the great authority of my predeces- 

 sors made the task still more onerous. It was necessary to demonstrate that 

 the whole superficial area and all the accessible mine-workings were occu- 

 pied by other rock-species, and to give in this report a sufficient number of 

 instances, with detailed descriptions, to enable geologists to decide for them- 

 sehes whether the elimination of propylite and the redetermination of some 

 (itlier i-()cks is justified by the facts.^ 



' Special localities — The rocks of the Washoe DISTRICT may be advantageously studied in the fol- 

 lowing localities : Granular diorite in nearly all varieties occurs along the line of the Virginia Water 

 Company's flume within a distance of a thousand feet north of Bullion Ravine. Porphyrillc dhrites can 

 l)e satisfactorily examined either in the McKibhen Tunnel or in Ojihir Ravine, between the most west- 

 erly point of the flume and the more southerly of the bluflfs marked " croppings" on the map. Earlier 

 diabase, in all varieties, is to be found from the Savaf/e couTiection with the Sidro Tunnel to the junc- 

 tion of the mail! tunnel with the Xorlh Lateral, and from this point to the Mint connection. Tountjcr 

 diabane (" black dike") is well seen on the west wall of the Belcher associated with black graphitic slates. 

 The foregoing are the rocks most important to miners ou the LODK. 



Granite is well developed close to the lied Jacket, C. D. 6, and on the dump of that mine. Quartz- 



Itorphyry is excellently exposed by a little quarry about 2,000 feet southwest of the Justice. The felsitic 



variety occurs ne.ar the drainage of Gold Canon (American Flat Canon tS the name given on former 



niapb), just east of Roux' Ranch. The little basalt mesa in the same locality is very accessible. Meta- 



3 C L 



