288 University of California Publications. [Geology 



PAGE 



Chemical Composition 321 



Eesume 323 



Porphyry at the Chainman Mine 323 



Quartz Blout ..... 324 



Term Defined 324 



Field Eelations 325 



Varieties of Quartz Blout 327 



Eelations to Copper Ores and Garnet Bock 329 



The Porphyry as a Copper Ore 330 



Minette 344 



Chemical Composition 345 



Ehyolitic Lavas and Tuffs 347 



Three members of Volcanic Series 347 



Petrographical Character 349 



Obsidian ; 350 



Other Metalliferous Deposits 351 



Faults 353 



Mineralogy of the District 356 



INTRODUCTION. 



The Egan Range is one of the larger members of the Basin 

 Range system of mountains traversing eastern Nevada with a 

 general north and south trend. The range has a breadth of 

 several miles and is separated from the neighboring parallel 

 ranges by broad, flat, alluvial valleys coextensive with the ranges 

 between which they lie. The chief formations which enter into 

 the structure of the range are, so far as at present known, sedi- 

 mentary strata ranging in age from the Cambrian to the Car- 

 boniferous. These strata have an aggregate thickness of many 

 thousands of feet, and the entire sequence has been compressed 

 in part into open, fairly symmetrical, folds, which locally 

 may be more acutely appressed and affected by dislocations. 

 The pre-Cambrian basement upon which the Palaeozoic rocks 

 rest is not exposed ; but, at various places in the range, there 

 are intrusive masses of granitic and monzonitic rocks and some of 

 these have been supposed erroneously to represent the Archaean. 

 It is probable that these granitic rocks date from the mid-Meso- 

 zoic revolution, which affected the Cordilleran region from the 

 Wasatch to the Pacific, although it is only positively known that 

 they are post-Carboniferous. These intrusions would thus be 

 contemporaneous, in a general way, with the granites of the 



