18 ''-'^■^ ■ CRANDALL— SAN FRANCISCO PENINSULA. [January 4, 



Massive Fades. — Fresh specimens of the massive facies were 

 taken from the Potrero and the Presidio. 



From the Potrero no sections were obtained in which serpentine 

 was not present. Other constituents or original minerals present 

 in the peridotite are as follows: orthorhombic pyroxene enstatite, 

 monoclinic pyroxene diallage, olivine, magnetite, and chromite. 

 These constituents give the serpentine the name of Ihezolite, the 

 original rock being a peridotite. 



The largest constituent of this rock appears to be the pyroxene 

 diallage. The crystals show^ no definite form but are in long irreg- 

 ular shapes, possessing marked cleavage in the long direction. 

 Basal sections occasionally show the parting characteristic of dial- 

 lage. The relief is medium and the crystals are colorless in ordi- 

 nary light. Under cross nicols the colors are high, most commonly 

 upper first and second order. The extinction angle varies with the 

 different sections from 18° up, the largest angle measured being 

 48°, the commonest about 43°. 



Enstatite, the orthorhombic pyroxene, occurs in lesser quanti- 

 ties than the diallage and is indistinguishable from the diallage 

 in ordinary light, the relief, color and form of the sections being 

 similar. 



Under cross nicols the colors are low, blue grey of the first 

 order in a slide of proper thickness. Being orthorhombic the 

 extinction is parallel and so may be distinguished from the diallage. 



In one section fine multiple twinning was observed in the ensta- 

 tite ; the extinction was almost simultaneous for the two sets of 

 lamellae, making it not especially noticeable. The lines of twinning 

 made an angle of over 30° with the direction of extinction, but 

 because of lack of similar sections the face of twinning was not 

 determined. Rosenbusch remarks that the twinning of enstatite is 

 not common. Both the pyroxenes showed in most of the slides 

 with serpentine formed through them with lattice work effects. 



Olivine is present only in small irregular grains wholly sur- 

 rounded by serpentine due to its having weathered faster than the 

 pyroxenes. 



In ordinary light it is clear and colorless and is distinguished 

 from the pyroxenes by its higher relief and lack of cleavage. In 



