756 DESCRIPTIVE GEOLOGY. 



in the strata extending westward to the Black Rock Desert. The strike of 

 the beds is from north 20° to 35° east, and the dip where observed invari-. 

 ably to the westward. 



In places, the strata are penetrated by narrow dikes of diorite and 

 syenite, throwing the dip nearly vertical. The dikes, however, are on too 

 small a scale and of too local a nature to be represented on the map. On 

 the high hills just to the north of the granite, the slates stand but little beloAv 

 the vertical. Barren of vegetation, devoid of moisture, monotonous in out- 

 line, and of a dull gray color, the hills present a dreary, desolate aspect. 

 It is to be regretted that no organic remains have been found in these beds, 

 their reference to the Jurassic age being based solely upon structural grounds 

 and their resemblance to Jurassic strata in California, as well as to the slates 

 overlying Jurassic limestone on the opposite side of the Humboldt Valley 

 in the West Humboldt Range. 



To the southwestward, in the Truckee Range, and in Eldorado Canon, 

 Virginia Range, just south of the limit of the map, the Triassic formations 

 have been identified, but in neither case are the overlying slates exposed, 

 while from the Montezuma Range westward to the Sierra Nevada the 

 Jurassic strata are found in isolated patches resting directly uj^on the 

 granite, poorl}^ exposed, except at the northern end of the Lake Range, and 

 probably of no great thickness. 



Breaking through the slates of the Montezuma Range is a body of por- 

 phyry, which, from its mode of occurrence and peculiar habit, requires 

 some special mention. It occurs just south of Willow Spring on the north 

 side of French Canon, rising over 1,000 feet above the stream-bed in a 

 beautiful conical-shaped peak, and is so entirely unlike the other outbreaks 

 of the older pre-Tertiary rocks as to suggest a later origin. On the other 

 hand, although it resembles some varieties of rhyolite, its groundmass is 

 quite unlike those of the region, and at the same time none of the feldspars 

 are sanidin. The geological position of the rock affords no clue to its age. 

 The rock withstands atmospheric agencies remarkably well, and breaks 

 with a rough, hackly fracture. It possesses a light coffee-brown color, with 

 a fine crystalline feldspathic groundmass, in which can only be detected 

 well-developed orthoclase and large grains of translucent quartz, frequently 

 more than one fourth of an inch in length. The value of the microscope 



