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U)iiversity of California. 



[Vol. 2. 



doubt primary, showing characteristic basal sections and a pale 

 greenish brown color. Quartz occurs filling interstices between the 

 other components, being the last to separate. Magnetite is quite 

 abundant in skeleton crystals. 



The question arises in regard to the proper designation for a 

 rock of this kind. A silica determination gave 66 per cent, which 

 is rather acid, even for a diorite, as the term is used. The distinc- 

 tion which Brogger* has proposed seems a very good one, that is, 

 to confine the use of the term diorite to the medium acid abyssal 

 rocks of granitic structure, while the term diabase is to be used for 

 the hypabyssal rocks. These to be farther divided into medium and 

 basic rocks corresponding to the deep-seated ones diorite and gab- 

 bro. He considers the classification according to the ruling dark 

 constituent of very little value. 



In accordance with this the rock will be designated simply an 

 ophitic quartz-diorite. A new name, it would seem, must ultimately 

 be adopted for medium acid rocks of diabasic structure. 



Another dike farther east contains greenish brown hornblende 

 in granules and broad plates. The latter are thickly dotted in a 

 poikilitic manner with exceedingly minute feldspar laths. The 

 greater number of the diabase dikes east of the Lion's Head contain 

 a greenish, shreddy hornblende and lath feldspars. The former 

 often incloses cores of granular augite, showing that a part of the 

 hornblende at least has been derived from augite. The primary 

 nature of a part of the hornblende is evident from the form of the 

 basal sections and the manner in which it incloses the feldspars, a 

 similar structure not having been seen in the case of the augite. 

 As a general thing, the feldspar of these rocks is more decomposed 

 than in any of the other feldspathic rocks. 



A rather interesting dike of diabase occurs intrusive in the ser- 

 pentine south of the Head. It is a fine-grained rock, containing 

 poikilitic crystals of primary brown hornblende. In these crystals 

 are numerous inclusions of feldspar, augite, and hypersthene. The 

 hornblende and augite are slightly more abundant than the hyper- 

 sthene. The feldspar laths probably belong to anorthite. Small 



* Die Eruptionsfolge der Triadischen Eruptivegesteine in Siidtyrol. 



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