478 DESCEIPTIVE GEOLOGY. 



In its cliemical constitution, this rock is one of the most remarkable 

 granites in our collection. It stands from 12 to 20 per cent, lower in silica 

 than the normal composition of granite, while at the same time it is but 

 little higher in alumina, the increase being mainly in the other bases, 

 iron, lime, magnesia, and alkalies ; the soda and potassa together constitut- 

 ing more than one-tenth of the entire rock. 



Probably no decisive evidence will ever be produced to clearly show 

 the age of this isolated granite body. It has, however, in general, in its 

 mode of occurrence and structure, the character of an eruptive rock, while 

 its mineralogical habit places it in close proximity with those other bodies 

 in Western Nevada that have been regarded as of Jurassic age. 



On the northeast side of the granite, and just below the summit of 

 Melrose Mountain, occurs an intrusive rock of later age, an irregularly- 

 shaped dike or body of fine-grained diorite, with a prevailing pui-plish-gray 

 color. It is a dense rock, with an angular fracture and highly crystalline 

 groundmass. Both orthoclase and plagioclase feldspars are present, but the 

 latter appear to predominate, many of them occurring as brilliant acicular 

 needles. The hornblende is found in small, fibrous, green crystals dissem- 

 inated through the mass. 



The limestone of Castle Peak is a gray, compact, somewhat siliceous 

 rock, in which few fossils were found. The following Coal-Measure forms 

 however, were obtained: 



Produdus suhJiorridus. 

 AtJiyris Roissyi. 



Its beds in general dip easterly, resting directly against the granite 

 body, but, in the ridge to the east, dip northward, enclosing a partial syn- 

 clinal in Spring Canon. It is traversed, near the summit of the ridge, by 

 a dike of quartz-porphyry, having a north and south trend. This rock is 

 a pure, white, felsitic mass, containing rounded crystals of quartz, and white 

 feldspars porphyritically imbedded, its surface being stained with red, rusty 

 spots, surrounding small particles of partially decomposed magnetite, which 

 abound through the mass. The feldspars, which belong to the monoclinic 

 group, are generally opaque, especially on the surface, but, in the interior, 



