2?8 University of California. [Vol.i. 



MICROSCOPICAL PETROGRAPHY OF THE MAIN DIKE. 



Owing to the exceedingly friable nature of the dark rock of the 

 main body, only one section was prepared from it. Under the 

 microscope this is seen to possess a holocrystalline diabase granu- 

 lar structure. The components, given in order of their separation, 

 are: magnetite, olivine, feldspar, augite, and, last of all, the constitu- 

 ent which probably once occupied the angular areas, now filled by 

 analcite, between the older constituents. 



The feldspar is present in short lath-shaped and tabular forms, 

 twinned according to the albite law. Twinning is seen in all except 

 some tabular ones which show a decided zonal structure, the angle 

 of extinction decreasing from the center outward. Many of the 

 crystals are dotted with sharply defined isotropic areas. These, on 

 farther investigation, proved to be analcite, and will be more par- 

 ticularly described in connection with the secondary dikes. The 

 feldspars are also being invaded by a green to brown fibrous or 

 granular material. This substance is particularly prominent in the 

 angular spaces between the feldspars from which it has spread. 

 Owing to the state of decomposition of the feldspars but few extinc- 

 tion angles were observed, and they were not generally symmetrical 

 to the twinning plane; but it would seem from the fact that sev- 

 eral were between twenty-five and thirty degrees that the feldspar 

 must be fully as basic as labradorite. 



Augite appears in allotriomorphic form, showing only slight 

 signs of decay. It occurs either in large masses penetrated by and 

 inclosing the feldspars, or in wedge-shaped areas between them. It 

 is distributed quite irregularly over the slide. The color is a pale 

 brownish green, no pleochroism being noticeable. Interpositions 

 of dark dust-like particles appear along irregular planes, and are 

 possibly of secondary origin. It is distinguished from the ortho- 

 rhombic pyroxene by the high extinction angle on 00P00 . 



A reddish brown almost opaque substance is present in consid- 

 erable abundance, occupying fully as much of the surface of the 

 slide as the augite. In places it is not sharply differentiated from 

 the matty green material, but in general has quite well-defined 

 boundaries, which in several cases are hexagonal. In some of these 



