118 



University of California. 



[Vol. 3. 



These angles correspond respectively to a basic oligoelase near 

 Ab2 Ani and an acid oligoelase near Abs Ani. 



The orthoclase, generally twinned on the Carlsbad law, 

 exhibits a faint zonal banding. The extinction angles measured 

 from the basal plane are not high, indicating that the percentage 

 of soda is rather low. 



Both kinds of feldspar contain liquid inclusions, and a few of 

 apatite, zircon, and magnetite. Of more interest, however, is 

 the frequent inclosure in the peripheral portions of the pheno- 

 crysts of small quartz and feldspar grains similar to those com- 

 posing the groundmass. This of course demonstrates that the 

 growth of the feldspar phenocrysts continued after the ground- 

 mass was at least in part crystallized. The quartz, which occurs 

 with the usual corroded outlines, exhibits no characters of especial 

 interest. 



The micaceous secondary mineral above referred to, although 

 somewhat paler, is similar in color to ordinary biotite. It further 

 resembles biotite in having strong pleochroism and strong double 

 refraction. In sections normal to the cleavage, the trace of 

 the cleavage plane is found to be the direction of greatest 

 absorption and the direction of least elasticity. The interference 

 colors are the brilliant mottled greens and reds that are shown 

 by the micas. The secondary character of the mineral, however, 

 is quite evident from its occurrence in small, irregular aggre- 

 gates and in veinlets that often follow cleavage cracks in the 

 feldspar. Its properties are very similar to those of the mineral 

 which Professor Lawson* has named iddingsite, and it is prob- 

 ably closely related to, if not identical with, that species. In 

 describing other rocks, we shall have frequent occasion to note 

 the presence of similar material. 



Pyroxenites of Spanish Gulch and Beach Creek. — The speci- 

 mens from these two localities are so similar that they may be 

 described best under a single heading. Both rocks are very coarse 

 allotriomorphic granular aggregates of greenish gray pyroxene, 

 which is the only mineral that can be identified macroscopically. 

 The grains often attain a length of one or two inches. The 

 pyroxene always shows one very nearly perfect pinacoidal 



*Bull. Dept. Geol. Univ. Cal., Vol. I, 1893, p. 30, et seq. 



