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University of California. 



rvoL. 2. 



Vesicular or brecciated varieties of the basalt are rare. In the 

 massive flows of the Upper Berkeleyan occasional white dots are 

 visible, which in thin section are seen to be amygdules filled with 

 opal and natrolite or calcite. In the basalt quarry above Pie Knob, 

 where the basalt is seen resting upon gravel, portions of which it 

 has picked up as it flowed along, were found fragments of highly 

 scoriaceous lava, presenting unaltered the aspect of the vesicular 

 scoria which accompany modern lava flows. But at no other 

 point were forms of this character found, and they are undoubtedly 

 exceptional. 



The large dyke which cuts the gravels north of Orinda Hill 

 exhibits an interesting variety of structural features. On the edges 

 of the mass the rock is fine grained to aphanitic in texture, dark 

 gray in color, and distinctly laminated. The structure changes 

 rapidly as the distance from the edge increases, till the rock 

 assumes a distinctly granular aspect, with a rough fracture and 

 reddish gray color. In thin section the coarse-grained rock is 

 seen to possess hypidiomorphic granular structure, the idio- 

 morphic feldspar and olivine being surrounded by allotriomorphic 

 augite, and little or no ground-mass present. Here are found 

 the beautiful reticulated aggregates of magnetite described above. 

 In this, the most coarsely crystalline rock found in the area, 

 was found a structure the nature of which is problematical. At 

 irregular intervals small spherical amygdules of calcite occur. In 

 a section cut through one of these there was seen to be, in addition 

 to the filling of granular calcite, a narrow band of dark semicrystal- 

 line material intervening on one side of the space between the cal- 

 cite and the normal rock substance. Further examination revealed 

 other areas of a similar material not associated with amygdaloidal 

 fillings, but forming dark-colored, sharply-defined oval spots about 

 a quarter of an inch across, in the mass of the rock. These spots 

 are scarcely visible in the hand specimen, but in thin section they 

 stand out prominently from the surrounding coarsely crystallized 

 material. They exhibit thin, lath-shaped feldspars, and augite 

 grains contained in a brownish glass full of minute microlites of 

 feldspar and magnetite. Minute areas of calcite and serpentine 

 replace portions of the ground-mass. None of the constituents of 



