Palache.] 



Soda Rhyolite North of Berkeley. 



65 



perfectl)' laminated forms the lamellae are very thin and easily sepa- 

 rated. More often they are not readily separable, being defined by 

 color modifications or by the presence or absence of the spherulitic 

 dots. The lamination planes sometimes extend without curvature 

 or interruption over considerable areas, giving rise, in weathered 

 masses of the rock, to sharp, angular forms. More commonly the 

 fluxion lines are extremely contorted, twisted into knots, merged 

 with one another or interrupted by shear planes. In some cases 

 the rock has been completely shattered, the angular fragments hav- 

 ing been recemented originally by glass and secondarily by silica into 

 a compact breccia. 



This phase of the rock is usually vesicular, the vesicles ranging 

 from minute clustered pores to cavities an inch in diameter, of irreg- 

 ular shape, drawn out in the direction of flow, and lined or rarely 

 filled with iron-stained silica. The flow lines pass around these 

 larger cavities showing that they were formed, not by solution but 

 while the rock was still viscous. 



In thin section this variety of the spherulitic facies is found 

 to be composed largely of glass, portions of which have become 

 devitrified. The spherulites are but imperfectly developed, but are 

 arranged in rows or groups along the well-marked flow lines. No 

 phenocrysts were detected in this variety. 



Intermediate forms are found between the spherulitic facies 

 and the porphyritic facies, in which the spherulitic structure dis- 

 appears, the laminated texture and glassy ground-mass still remain- 

 ing in greater or less proportion. 



On the other hand the spherulitic facies passes by less regular 

 gradations into the third form, or glassy facies. 



Tlie Glassy Facies. — This facies includes several varieties, of 

 which the simplest is a glass of greenish black color and subcon- 

 choidal fracture. Under the microscope it is seen to be a very uni- 

 form glass with imperfect perlitic structure. Numerous slender 

 microlites are scattered through the glass without any definite 

 arrangement. 



Slightly more differentiated than this glass is a porphyritic form, 

 dark green to light green or gray in color and of a more lithoidal 

 aspect. Under the microscope it is seen to have a glassy ground- 



