700 DESCRIPTIVE GEOLOGY. 



the range, between 5 and 6 miles south from this mass, the hill-flanks for 

 several hundred feet are composed of a rather finer-grained diorite, which 

 is interesting as reproducing the occurrence of mica-diorite, the biotites in 

 this case far exceeding numerically the hornblendes. Quartz is wanting, 

 but apatite is abundant. There are in this range three types of diorites, : 

 one south of McKinney's Pass, containing biotite, hornblende, and quartz, 

 with triclinic feldspar; the Chataya Peak group, which is essentially a 

 true hornblende-diorite, without mica, but rich in apatite; and the western 

 foot-hill group, which consists, besides the numerous plagioclase-feldspars, 

 of mica and hornblende, in which the mica predominates, considerable 

 apatite, but no quartz. 



Lying between the Chataya Peak diorites and the mass of rhyolite 

 which lies to the east of it, and well exposed in Pine Nut Canon, is an 

 obscure trachytic mass having more or less affinity to the rhyolites. The 

 relations between these two types of rock is well shown here, as in their 

 mode of occurrence and general habit they resemble the trachytes, but in 

 microscopical structure of groundmass, and in the amount of silica present, 

 which is far too high for normal trachyte, are allied to the rhyolites. 

 Although the silica in the rock analyzed reaches as high as 75 per cent., 

 free quartz is almost entirely wanting. The rocks possess a reddish-gray 

 color, a rough trachytic texture and fracture, and with no well-developed 

 mineral constituents except feldspar, which gives the mass a porphyritic 

 habit. In speaking of the groundmass of these rock, Professor Zirkel says:^ 

 "One variety contains a groundmass which consists of undulating, twisted, 

 and entangled axially-fibrous strings and bands, between which is a little 

 felsitic substance that is nearly structureless, but rich in heaps of ferrite and 

 opacite. This type of groundmass is as common in rhyolites as it is rare 

 in trachytes. In another specimen, this structure is wanting; the gi'ound- 

 mass being here, as in most other trachytes, an aggregation of feldspar, 

 opacite, and ferrite, which are in the usual manner accumulated in little 

 heaps with rounded outlines." Local solfataric action has attacked some of 

 these trachytes decomposing the feldspars, the microscope revealing in the 

 porcelain-like product grains of calcite with rhombohedral cleavage. The 



^Microscopical Petrograpby, vol. vi, 149. 



