Palachh .] 



Soda-Rhyolite North of Berkeley. 



63 



It represents an intermediate stage of crystallization, and will be 

 termed the "spherulitic facies." The third type is a volcanic glass 

 with spherulitic and porphyritic modifications, representing a very 

 slightly differentiated phase of the magma. It will be known as 

 the "glassy facies." 



The Porphyritic Facies. — The porphyritic facies is a hard, com- 

 pact, fresh rock, dull white in color with subconchoidal fracture. 

 Phenocrysts of quartz and feldspar are abundantly disseminated 

 throughout the ground-mass, which is in most cases cryptocrys- 

 talline, but occasionally fine granular. Some secondary veins of 

 quartz traverse the rock and the surface is in places stained yellow, 

 or brown by iron oxide. 



Under the microscope the ground-mass is seen to be a micro- 

 granular aggregate of quartz and feldspar. No glass could be 

 detected. The porphyritic feldspar is cloudy from decomposition 

 products, but retains traces of twinning structure which show it to 

 be a plagioclase. The porphyritic quartz exhibits in a few instances 

 pyramidal terminations, but for the most part has lost its original 

 form by corrosion and resorption by the magma; the intricate bays 

 thus formed are bordered almost continuously by a narrow band of 

 glassy material which merges insensibly into the surrounding gran- 

 ular ground-mass. 



Fig. 1, Plate 5, illustrates such a corroded crystal with its glassy 

 border. The quartz contains numerous small inclusions of various, 

 but mostly irregular, shapes, each containing a stationary bubble and 

 arranged in rudely parallel bands. No ferro-magnesian silicates are 

 present. 



Fig. 2, Plate 5, represents a typical appearance of this rock as 

 observed between crossed nicols. 



The Spherulitic Facies. — The spherulitic facies presents numerous 

 variations in detail of structure, but is in general white- or grayish 

 in color, with a mottled aspect, due to the presence of minute black 

 or gray dots; its structure varies from massive to finely laminated 

 or vesicular; its fracture is uneven to lamellar. There are two 

 varieties of this spherulitic facies, a massive and a laminated. 



The Massive Spherulitic Variety. — The variety which gave the 

 most definite results under the microscope, and of which a sample 



