386 H. W. TURNER — GEOLOGY OF MOUNT DIABLO. 



constructed by Charles Hoffmann as Pyramid hill (locally called Mount 

 Zion), Black point and Eagle point. 



It is believed to be of igneous origin from its being a holocrystalline rock, 

 apparently indistinguishable from known eruptive diabases ; from its no- 

 where exhibiting any trace of clastic structure ; from its sharp contact with 

 surrounding rocks, whether other crystalline rocks or unaltered sediments ; 

 and from its being practically uniform throughout in composition and micro- 

 scopic structure, the main variation being a complete replacement of the 

 augite by green fibrous hornblende in certain considerable portions of the 

 area. No dikes have been seen, however, extending from it into the sur- 

 rounding rocks. 



In an unpublished paper on the Almaden quicksilver mine in Spain, Mr. 

 G. F. Becker has traced a coincidence between the quantity of ilmenite in 

 the diabase and the degree of fluidity which the phenomena indicate. He 

 calls attention to the well-known fact that, in a blast furnace, slags are ren- 

 dered practically infusible by a small percentage of titanium, and infers that 

 this substance has a similar effect on rock magmas, rendering them less fluid 

 and less likely to penetrate surrounding masses as narrow dikes. The abun- 

 dance of titaniferous iron ore (ilmenite) in the Mount Diablo diabase may 

 account for the lack of dikes. 



The following description of the diabase is taken from slides numbered 43 

 and 143 of the series collected at Mount Diablo, now preserved in the labora- 

 tory of the United States Geological Survey in Washington. 



Macroscopically, the rock is dark-greenish, compact, evidently crystalline, 

 and of even texture. 



Under the microscope it is seen to be a typical diabase, holocrystalline, 

 ophitic in structure, composed of augite, plagioclase and ilmenite, with 

 uralite and chlorite as secondary constituents. 



The plagioclase is nearly all twinned after the albite law, the extinctions 

 on the twinning plane 3/(010) being large (26°-25°, 30°-26°), suggesting 

 labradorite. Zonal structure was noted in two plagioclases. 



The augites, in many cases fresh, give large extinction angles (42°-54°) 

 with characteristic cleavage, and are colorless in thin section. The augite 

 is frequently twinned, sometimes more than once, as is shown by the different 

 extinctions of different portions of the same mass ; the different portions 

 showing, as well, different directions of the dominant cleavage. In one 

 individual an irregular middle portion extinguished differently from the 

 remainder of the individual, although the dominant cleavage lines of the 

 entire crystal were continuous. The interference colors of the augite are 

 usually bright blue to green. Clear, doubly refracting grains in the centers 

 of some of the augite individuals do not show interference colors. The pres- 

 ence of these grains in the augite may be related to the presence of chlorite 

 in the interior of the uralite. 



