10 CALIFORNIA ACADEMY OF SCIENCES. 



the absence of porphyritic quartz, this rock contains 74.25^ 

 Si02. 



2. A type similar to this, but with much porphyritic quartz, 

 was collected in the hills east of San Carlos. Macroscopi- 

 call}^ it has a violet-brown grounclmass, with numerous small 

 quartz grains and feldspar tables. In thin section, some of 

 the feldspar crystals show no twin structure, but are other- 

 wise similar to those in the type just described. Numerous 

 isolated fragments showed, however, when tested with liy- 

 drosilico-fluoric acid, that a plagioclase, nearly related to 

 oligoclase, predominated. 



Quartz occurs porphyritic in a few larger rounded and 

 corrroded grains with numerous fluid inclusions, some of 

 which show moving bubbles. The groundmass consists of 

 allotriomorphic quartz and unstriated feldspar grains, be- 

 tween which lies a still finer holocrystalline quartz-feldspar 

 mass. No glass or microfelsite. In the groundmass lie 

 numerous clearer bent and curved streaks, consisting of fine 

 fibres arranged perpendicularly to the length extension of 

 the bodies: the fibres which are not very strongly doubly 

 refracting often join in the middle along a seam parallel 

 to the length extension. These peculiar forms, calling 

 to mind the axiolites of the rhyolites (Zirkel), would 

 here, how^ever, appear to consist of feldspar. Fine 

 Fe.Oa and opacite are distributed throughout the ground- 

 mass. 



In the foothills of Sausal Yalley similar rocks are found, 

 in which, however, the groundmass is coarser, microcrys- 

 talline, and consisting of interlaced quartz and feldspar 

 grains, with a slight approximation to ejranophyric struct- 

 ure. Some of the feldspar grains in the groundmass are 

 striated. 



The porphyrites from the Punta Banda Kange differ partly 

 from those already described. The largest proportion of 

 them appear to be quartz-hornblende porphyrites, but be- 



