484 DESCRIPTIVE GEOLOGY. 



of thicknesses varying from 10 to 20 feet, were observed in a little hill just 

 cast of the main ridge. 



The marble is a remarkably beautiful, pure, fine-grained crystalline 

 rock. Under the microscope, it is seen to have a crystalline-granular 

 structure, the crystals of calcite having distinct twin striations, together 

 with numerous fluid-inclusions containing a moving bubble which does 

 not disappear at 100° C. A partial analysis of a specimen of this lime- 

 stone made by Mr. B. E. Brewster shows it to be a normal dolomite, con- 

 taining 56.537 per cent, carbonate of lime and 41.119 per cent, carbonate 

 of magnesia. It occurs in beds of varying thickness, some of which 

 would furnish immense blocks, and are admirably situated for quarrying 

 purposes. In general, they present the fine grain and purity of color of a 

 statuary marble, but are sometimes shaded with wavy bands of bluish color 

 running through the mass. The porphyries occur at comparatively regular 

 intervals on the sides of the hill, seeming to be in every way conformable 

 with the beds of marble. 



This interesting group of porphyries, while having a general generic 

 resemblance, presents in its extreme types considerable difference of appear- 

 ance. The upper bed is a greenish-white felsitic rock, having much 

 the appearance of a quartz-porphyry. It contains white, mostly opaque 

 crystals of- monoclinic feldspar and distinct rounded crystals of quartz in a 

 light-green micro-crystalline groundmass. Only a few rare crystals of horn- 

 blende and mica can be distinguished through the mass, which, like the 

 porphyry of the Wachoe Mountains, shows stains of iron oxide, though no 

 grains of magnetite are visible ; Zirkel refers the origin of these stains to 

 the decomposition of hornblende. The other extreme is a dark-gray granit- 

 oid rock, rich in comparatively large crystals of hornblende and mica, and 

 well-defined orthoclase crystals, while the quartz can with difficulty be 

 detected by the naked eye. A middle member is more distinctly a felsitic 

 poi-phyry, showing only comparatively few white feldspar crystals, and a few 

 sparse hornblendes and micas in dark-gray micro-crystalline groundmass. 



Under the microscope, these exterior differences are lost in the closely 

 similar mineralogical behavior of the rocks. The groundmass is seen 



