PalacheJ LJierzolite-Serpe)itiiie. 169 



very small masses, chiefly the spheroids included in the serpentine. 

 There is, however, a much more abundant form of the rock which 

 serves to connect more closely the rock of the spheroids with the 

 serpentine. In it serpentinization has proceeded so far as to have 

 practically invaded the whole mass, a few scattered grains of ensta- 

 tite alone remaining of the original constituents. These enstatite 

 crystals retain their pristine freshness, and the serpentine has not 

 been subjected to the crushing and shearing evidenced in the slick- 

 ensided facies. Veins of chrysotile and marmolite traverse it in 

 places, and in many respects it shows its relation on the one hand 

 to the typical serpentine of the area, ami on the other to the rock 

 of the spheroids. 



The evidence above presented proves that the serpentine is 

 derived from a crystalline rock' whose original constituents were 

 olivine, enstatite, diallage, chromite, and magnetite. It thus has 

 the composition of a lherzolite, and is accordingly so designated. 



No chemical investigation of this group of rocks was undertaken 

 by the writer. But the peridotite described by Mr. Turner,* from 

 Mt. Diablo, presents so many points of similarity to the Potrero 

 lherzolite that its analysis by Dr. Melville is quoted. Dr. New- 

 berryt has also described the serpentine of the Presidio, San Fran- 

 cisco, which is geologically continuous with that of the Potrero, 

 and of similar nature. He gives an analysis by Dr. J. D. Easter 

 of a specimen which appears to be similar to the serpentine 

 described in this paper, and this analysis is therefore appended:- — 



I. II. 



Si O, 53.25 39.60 



Cr, :1 .54- .20 



Al, O, 2.80 1.94 

 Fe 2 3 .69 



Fe 5-93 l 84 , 



Mn O .09 • j 45 



Ca O 16.22 



Mg O 19.9 1 36.90 



Na, O .19 



H, O .29 1 2.91 



99.98 1 00.00 



* Loc. cit., p. 389 and p. 406, analysis 242. 



t Pacific R. k. Repts., Vol. VI. Pi. II, p. it, 1S55. 



