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University of California. 



[Vol. 3 



combination of all these characters appears to identify the mineral 

 beyond reasonable doubt as alkali feldspar. The lath- shaped 

 feldspars, on the contrary, are without exception triclinic. Their 

 composition was determined by Becker's method as about 

 Abi Ani. The structure, although on a much smaller scale, is 

 exactly analagous to that exemplified in the dioritic rock of the 

 Middle Fork (see p. 115), and strikingly illustrates the tendency 

 of the alkalies, especially potash, to remain in the mother-liquor 

 until the other bases have crystallized. 



Alkali-feldspar in the role of a cement has been noted by 

 Washington in the groundmass of certain Italian lavas of the 

 vulsinite family.* In some of those rocks, the alkali feldspar 

 borders, and is oriented upon, the phenocrysts, but in the present 

 case it appears to have crystallized quite independently of the 

 adjacent plagioclase. 



A few points of difference in the other pyroxene-andesites of 

 this locality may now be noted. Original hypersthene was not 

 observed in the other specimens, but was always replaced by 

 iddingsite. This material is found to vary in structure and color. 

 The fibres, while they sometimes have the parallel arrangement 

 above described, are often in spherulitic or irregular aggregates. 

 The color may become very pale, or it may change to a reddish 

 brown. The micropoikilitic structure occurs locally in the other 

 specimens, but all the slides show a large amount of glass-base. 

 The feldspar phenocrysts in the most weathered rock have under- 

 gone a partial alteration to zeolite, which will be described later 

 in a more typical example. 



The apatite in this last-mentioned specimen has a notable 

 peculiarity. The larger crystals, occurring in the groundmass 

 and included in the augite, are charged with a brownish pigment, 

 which in longitudinal sections is seen to be arranged in fine lines 

 parallel to the vertical axis. Rotated over the polarizing nicol, 

 these crystals are found to have a marked pleochroism, with the 

 greater absorption for rays vibrating parallel to the principal 

 axis (e>o). The identity of the mineral is proved by its high 

 refractive index, straight extinction, weak negative double 



*Journ. Geol. 189G, vol. 4, p. 833. 



