696 • DESCRIPTIVE GEOLOGY. 



It will be seen that the rock has the normal composition of a quartz- 

 sanidin-rhyolite rich in ferrite. 



On the east side of the easterly-dipping Triassic limestones, -south of 

 McKinney's Pass, and lying directly upon the backs of the upturned beds, 

 is a long outburst of diorite, and extending about 8 miles in a north and south 

 line, forming what were at one time the foot-hills of the range. As in the 

 Triassic rocks to the west of it, the lines of erosion strike northwest and 

 southeast, dividing the dioritic mass into parallel ridges. The rock itself 

 consists of a very coarse-grained combination of hornblende, biotite, a vary- 

 ing but small amount of quartz, and both monoclinic and triclinic feldspars. 

 It is difficult, upon the first examination of this rock, to determine whether 

 plagioclase or orthoclase is the prevailing form of feldspar, and, conse- 

 quently, whether the rock is a diorite or syenite. The microscope, how- 

 ever, reveals more or less unchanged nuclei, which are distinctly striated 

 in nearly all the feldspars, and the rock becomes a mixed hornblende, bio- 

 tite, and quartz-bearing diorite. It bears a family likeness to the diorites 

 farther south in the range, in the region of Chataya Peak, and only differs 

 from them in the presence here of a considerable amount of biotite replacing 

 a part of the hornblende. 



Along its east margin, this diorite occurrence is overflowed by a broad 

 n^ass of rhyolite, which overlaps it upon the north, coming in direct contact 

 with the limestones. At the foot of McKinney's Pass, in the rhyolite, and 

 very near the limestone, may be observed included fragments and boulders 

 from Triassic beds, but they are very local and always within a few feet of 

 the point of contact, showing conclusively, however, that the intrusive beds 

 have come up- through the Mesozoic strata. These rhyolites form in gen- 

 eral a more or less oval-shaped mass, 8 miles from north to south and 4 

 miles in breadth from east to west, and known as the Sou Spring Hills. 

 The most striking feature of this group is its marked difference lithologically 

 from the group on the west side of McKinney's Pass. There, in a micro- 

 crystalline groundmass, was a surprising amount of well-developed crys- 

 tals of quartz and sanidin. Here the rock consists of an extremely fine 

 base, which is essentially poor in crystallized minerals, being, in general, 

 microfelsitic, and in places absolutely porcelainous. Such differences pro- 



