676 DESCRIPTIVE GEOLOGY. 



tbat have been regarded as most probably of Jurassic age, and this reference 

 is, in a measure, borne out by both the physical and mineralogical habit of 

 the rock. This association of two distinct types of granite has considerable 

 interest, as in our observations in Nevada the occurrences of one granite 

 breaking through, and in close contact with, an earlier mass of very differ- 

 ent composition was noticed in only few localities. 



This granite dike is a comparatively fine-grained rock of a uniform 

 texture, breaking with difficulty under the hammer with an uneven sur- 

 face and angular fracture. The constituent minerals all have a fresh 

 undecomposed appearance. In color, the rock is dark gray. It is made 

 up of quartz, feldspar, mica, and considerable hornblende as essential min- 

 erals, while under the microscope may be detected distinct crystals of 

 titanite. The rock, indeed, comes under the head of those classed as horn- 

 blende-titanite granites, so common in Western Nevada. The quartz is 

 abundant in clear translucent grains. Both monoclinic and tri clinic feld- 

 spars are present, the latter in a relatively large proportion, white in color, 

 and of a vitreous lustre. Biotite and hornblende crystals are associated 

 together; the former seem almost black, while the latter are dark green, 

 with a fibrous structure. Under the microscope, the quartz crystals of this 

 rock show an immense number of fine hair-like microlites, a common phe- 

 nomenon, but very marked in this case. Zirkel has estimated that within 

 the limits of one square millimetre, almost in the same plane, there are one 

 hundi'ed and twenty of these hairs, which w^ould give, in the small space of 

 one cubic millimetre, no less than ten thousand of these microlitic bodies, 

 really a remarkable observation. Besides the above, the quartz holds many 

 liquid-inclusions, carrying very perfect crystals of chloride of sodium, and in 

 one section, figured on Plate 1, fig. 4, accompanying Professor Zirkel's report, 

 may be seen both salt cubes and the dark microlites in the same inclusion. 

 Narrow dikes of diorite also penetrate the large body of granite. Those 

 observed were within two or three miles of Summit Springs. They are 

 usually fine grained, and composed mainly of dark-green hornblende and 

 tri clinic feldspars, with but little variety either in texture or composition. 



On the west side of Ragan's Valley, the granite occupies a somewhat 

 smaller area than to the east, and attains a much lower altitude, reaching in 



