730 DESCBIPTIVE GEOLOGY. 



andesitic dikes traversing the Archaean body. Their mode of occurrence 

 appears to be quite similar in all cases, and they are found skirting the base 

 of the range as massive eruptions, where the Triassic strata rise steeply out 

 of the plain, seldom reaching more than a few hundred feet in altitude, 

 although they frequently form nearly continuous masses for several miles. 

 The largest exposures are found near Eldorado, Santa Clara, and Buffalo 

 Canons, and, as might be supposed, along the main northwest and southeast 

 fault, which nearl}^ severs the range into two portions, and is evidently a 

 line of great weakness. An interesting feature of these outbreaks is 

 their close resemblance in general lithological habit. They are all fine- 

 grained, but distinctly crystalline, the only easily distinguishg^ble mineral 

 being olivine, which in places is very abundantl}^ scattered through the 

 groundmass. Under the microscope, the plagioclase and augite are charac- 

 teristically shown. In a vesicular basalt directly east of Buffalo Peak, the 

 feldspars are rendered quite impure by the presence of numerous glass- 

 inclusions. Professor ZirkeP calls attention to the microscopical structure 

 of a variety of basalt from Eldorado Canon, rich in olivine, containing 

 well-developed crystals of picotite, a mineral abundant in the olivine of 

 European basalts. 



The northern end of the West Humboldt Range has in former years 

 been the scene of considerable mining activity, mineral-bearing veins occur- 

 ring throughout all the formations of the Star Peak Triassic, and being found 

 on both sides of the range in nearly every cailon. Those of the most impor- 

 tance thus far developed traverse the limestones, or the calcareous and argil- 

 laceous shales lying between the limestones and quartzites, and in gen- 

 eral conform in dip and strike with the sedimentary strata. In " Mining 

 Industry",^ Mr. J. D. Hague has given with some detail descriptions of the 

 most important mining developments in the range up to the time of his 

 visit, showing the modes of occurrence, the amount of exploita,tion accom- 

 plished, together with the methods and costs of mining and milling. Miner- 

 alogically, these veins are of more than ordinary interest in comparison with 

 most mining regions of Western Nevada, where usually the ore consists 



^ Microscopical Pelrogiaphy, vol. vi, z45. 

 ^Mining Industry, vol. iii, 308. 



