43° 



University of California. 



L 3L. 2. 



stance like leucoxene surrounding many magnetite grains indicate 

 that a portion, at least, of that mineral is titaniferous. 



Apatite is always present as inclusions in the other constitu- 

 ents in the form of elongated colorless prisms with characteristic 

 optical properties. 



The properties of the serpentine, which is so abundantly devel- 

 oped in the basalts, are identical in all respects with those already 

 described for the serpentine of the andesites. 



The secondary mineral which has been noticed in both basalt, 

 and olivine-bearing andesite, is believed to be iddingsite. It is 

 quite abundant in forms of the basalt which exhibit the most alter- ' 

 ation, but in fresher rocks it was seen in process of development 

 from olivine. Macroscopically it is not very prominent, but may 

 sometimes be seen as a bronzy or yellowish mineral with good 

 cleavage. In thin sections it has a pale green to greenish or brown- 

 ish yellow color, and either the characteristic hexagonal outline of 

 olivine or a sharply rectangular form also seen in olivine crystals 

 in the same slides. Its distinguishing features are a very dis- 

 tinctly laminated or fibrous structure, the fibers parallel to the 

 prismatic axis of the crystal; distinct pleochroism in shades of 

 greenish yellow, the absorption greatest in the direction of the 

 fibration; straight extinction; and strong double refraction, the 

 mottled polarization colors showing bright green and crimson tints. 

 An imperfect parting transverse to the prismatic axis was observed 

 in a few crystals. Crystals showing these characters were seen 

 still retaining unaltered cores of olivine, which is doubtless the 

 parent mineral. No observations were obtained which established 

 the orientation of the optical axis with reference to the cleavage of 

 the mineral. In all observed properties it appears to agree with the 

 mineral described * as iddingsite. If further study should demon- 

 strate its identity with that mineral, it will be of interest as 

 proving that iddingsite may be formed by alteration of olivine, as 

 previously suggested but not confirmed. 



- "Geology of Carmelo Bay," Bull. Dept. Geol. Univ. of Cal., Vol. [, No. i, 

 p. 31, et seq. 



Eruptive Rocks of Point Bonita, Ibid., Vol. I, No- 3, p. 90. 



