LAWSON ~] 



I'alacheJ 



The Berkeley Hills. 



417 



gonal sections, grayish in color and showing distinct pleochroism 

 in tints of bluish gray and pale yellow, the absorption being e > o. 

 Its color seems to be due to the presence of minute rod-like 

 inclusions arranged parallel to the prismatic axis of the crystal. 

 These pleochroic crystals occur alike, included in feldspar and 

 augite, and embedded in the ground-mass. 



Glass is a variable constituent of the andesites. Its color varies 

 from colorless, through greenish and yellowish tints, to a deep 

 brown or almost black. The opacity is due to clouds of minute- 

 dust-like particles, which, as they occur likewise in adjacent feld- 

 spar crystals, probably consist of submicroscopic individuals of 

 magnetite. They may, however, in part be of globulitic character. 

 Besides forming a constituent of the ground-mass of some facies of 

 the rock, glass is widely distributed in the form of inclusions of 

 porphyritic crystals of feldspar and augite. 



Of the secondary minerals whose origin may be clearly traced 

 to the original constituents of the rock, serpentine is the most 

 abundant. In color it is greenish to bright or brownish yellow and 

 is non-pleochroic. In ordinary light it appears structureless, but 

 in polarized light it breaks up into a finely-fibrous felted aggregrate, 

 polarizing in pale bluish tints, or sometimes quite isotropic. The 

 color appears to be due to the presence of ocherous iron oxide, 

 probably separated at the time of the formation of the serpentine 

 from its parent mineral. It is found partially or wholly replacing 

 pyroxene and olivine pheuocrysts, in cracks and cavities in feldspar 

 and replacing portions of ground-mass. 



Occasional crystals are seen of a mineral of serpentinous aspect 

 which appears to be allied to iddingsite; it will be fully described 

 in another place, it being more characteristically developed in the 

 basalts. 



Chlorite and calcite have been shown to occur very sparingly as 

 alteration products of augite. 



Limonite occurs abundantly in one facies of the andesite in 

 grains and shreds throughout the ground-mass, and as the filling or 

 lining of cracks, where it forms mammillated coatings. It is clearly 

 secondary, but its immediate source is not evident, as it permeates 

 all the constituents of the rock. It also occurs widely dissemi- 



