LAWSON "I 

 PALAO J 



The Berkeley Hills. 



Chlorite is sparingly present in some sections of the basalt as 

 an aggregate of pale green color, weak pleochroism and weak- 

 double refraction. 



Varieties. — The basalts show structural variations, depending 

 chiefly on the degree of development of crystallization and the 

 corresponding coarseness of grain of the rock. Such variations 

 a - e local in character, and the same flow may exhibit many of 

 them within a very small area 



The most typical form of the basalt, characteristic of the flows 

 leposing on the Siestan formation, is a very fresh-looking dark iron- 

 jray rock, compact and massive, with a fracture varying from even 

 to splintery and uneven. It is distinctly porphyritic, crystals of 

 glassy feldspar, with striated faces, and yellow grains of olivine, the 

 latter sometimes the more abundant and conspicuous, being readily 

 distinguishable with the unaided eye. Pyroxene in well-cleaved 

 plates is also sometimes visible. In thin sections this rock pre- 

 sents the features of a typical doleritic basalt. Phenocrysts of 

 feldspar, augite, and olivine are imbedded in a granular or micro- 

 litic ground-mass consisting of feldspar, augite, and magnetite. In 

 rare cases, a typical ophitic structure prevails, the lath-shaped 

 feldspars forming an interlacing network filled in with large allo- 

 triomorphic individuals of augite. The ground-mass often exhibits 

 the effects of differential movements of the rock previous to con- 

 solidation in the parallelism of the microlites or their tangential 

 grouping about the phenocrysts. 



Scarcely less common than this form is an almost aphanitic 

 facies found in portions of flows and the edges of the dykes. It is 

 deep iron black to reddish black in color, with even to subconchoi- 

 dal fracture, and has a characteristic basaltic aspect. The texture 

 is compact and is either perfectly aphanitic or uniformly and 

 finely crystalline, with occasional crystals of porphyritic dimen- 

 sions. In thin section these fine-grained basalts show a more or 

 less uniform admixture of lath-shaped feldspars, grains of augite 

 and magnetite, and small idiomorphic crystals of olivine, with gen- 

 erally some glassy matter in the interstices. Flow lamination is 

 more marked in the» fine-grained forms of the porphyritic facies, 

 and is often macroscopically visible in an irregular and imperfect 

 lamellar parting parallel to the plane of bedding. 



