TEN ABO PEAK. 571 



expose numerous walls, which offer fine views of the granitic mass. The 

 granite is characterized by its tough, dense appearance, with but little ten- 

 dency to crumble or wear away by exfoliation. It varies very considerably 

 in texture, from a fine to a medium-grained rock, and from a pearl-gray to 

 a rather dark-gray color, due partly to the differences in texture and in part 

 to the more or less quantity of dark mica present. The granite is made up 

 of quartz, in translucent grains, usually quite small, both monoclinic and 

 triclinic feldspars, and dark biotite plates. In the region of Shoshone Wells, 

 the rock is perhaps somewhat lighter in color than the main body of Tenabo 

 Peak, but otherwise shows but little difference. 



Between the forks of Upper Mill Creek, on the west side of Tenabo, 

 occurs a rock-mass of later age than the granite just described, as it pene- 

 trates the latter as an intrusive body, with sharp lines of contact. It may 

 probably be best classed as a diorite, but at the same time it presents many 

 features of a fine-grained granite which has broken through the older body. 

 It is a compact, dense rock, breaking with great difficulty under the ham- 

 mer, with an uneven surface and angular fracture, behaving like a fine 

 granite, or, more accurately, like a rock in which fine quartz is a predomi- 

 nant constituent of the groundmass. Few granites, however, possess so 

 fine a texture as this rock, the groundmass being almost crypto-crystalline, 

 with clearly-defined plagioclase and fibrous hornblende, while a more care- 







ful search indicates some orthoclase and quartz. The microscope reveals 

 the presence of biotite in small flakes, and that the triclinic forms of feld- 

 spar prevail over monoclinic. So much hornblende with triclinic feldspar 

 places the rock among the diorites, while on the other hand mica, asso- 

 ciated with so considerable a proportion of quartz, would suggest a granite. 

 It is of special interest that the diorite of the Agate Pass Region to the 

 north also carries a large amount of quartz as an accessory constituent. 



Resting upon the granite, and forming the summit of the peak, oc- 

 curs a body of limestone which dips with an angle of 30° to 35° to the 

 eastward, and strikes south 10° east. The limestone extends but a short 

 distance north of the summit, but thickens rapidly to the south. It is 

 underlaid by a belt of pure white quartzite 420 feet in thickness, in turn 

 underlaid by a second limestone formation. This quartzite has a saccha- 



