370 University of California Publications in Geology [Vol. 10 



2. The second variety is of a light gray, green or bluish color, 

 except where stained with iron oxide ; it has a subconchoidal fracture, 

 appears massive, and is very hard. Under the microscope the ground 

 mass appears to be cryptocrystalline or microlitic, and in it are scat- 

 tered numerous groups or aggregates of spherulites, which are of two 

 kinds : those that are in circular groups imbedded in a dense brown 

 glass, and those that are sporadically scattered through the ground 

 mass, and are also present in the altered phenocrysts. The former are 

 microscopic in size, spherical in shape and show no distinct radiating 

 fibers, but give a black cross between the crossed nicols, as shown in 

 plate 25, figure A. According to Iddings, 8 the number of plates of 

 feldspar composing such spherulites increases to such an extent that 

 the outline of each individual plate is lost ; and when no distinct trace 

 of any single fiber can be seen the ideal spherulite is produced. The 

 second kind of spherulite is composed of fibers and plates, whose out- 

 lines can be seen under the microscope. In some cases they appear 

 to radiate from a small grain at the center, probably quartz. These 

 spherulites do not show a black cross between the crossed nicols. They 

 appear to be as numerous in the altered phenocrysts as in the glassy 

 matrix. According to Iddings, spherulites of this type are composed 

 of radiating plates or prismoid crystals. An example is shown in plate 

 25, figure B. This variety of the microlitic facies does not show the 

 flow lines as distinctly as that first described, but both kinds of spheru- 

 lites are present in most of the sections examined of this facies. 



DISTRIBUTION OF THE DIFFERENT FACIES 



The four types of rhyolite have a rather definite distribution along 

 the extent of the outcrop. The rock grades from a more glassy facies 

 at the northern end of the belt to a more crystalline facies at the south- 

 ern end. Both varieties of the glassy facies were obtained in that por- 

 tion of the belt between Berkeley and Leona Heights. At Leona 

 Heights some of the specimens collected were glassy, while others 

 showed a distinct porphyritic character. From the vicinity of Leona 

 Heights to Lake Chabot the rock is predominately porphyritic in text- 

 ure. South of Lake Chabot to the town of Haywards the felsophyric 

 facies predominates; that is, the ground mass in the rhyolite becomes 

 much more granular and the glassy material is not so common in the 



s Iddings, J. P., Igneous Rocks, vol. 1, p. 229, 1909. 

 9 Op. cit., p. 229. 



