346 



University of California. 



[Vol. r. 



absorption does not satisfactorily explain why the pleochroism 

 should be so much more pronounced in sections transverse to c 

 than in all other sections. The dispersion is p< v. 



In thin sections the planes of the prismatic zone are fairly, 

 sharply defined. Terminal planes, however, are absent, or are 

 represented occasionally by blunted outlines. In size the prisms 

 range from very small dimensions to a length of about 5 mm. and 

 a breadth of from .5 to 1 mm. The average size of the prisms is 

 probably about 2.5 x .5 mm. There is no pronounced extension of 

 the crystals in the plane of the orthopinacoid. Twinning is not 

 uncommon according to the usual law in which coPoo is the twin- 

 ning plane. Minute interpositions and secondary products arising 

 from alteration of the pyroxene are entirely lacking, but there are 

 occasional inclusions of the other constituents of the rock, such as 

 apatite, and sometimes, also, grains of magnetite, the latter mineral 

 not being otherwise represented in the rock. Chemically the 

 presence of sodium in addition to the magnesium, calcium, and iron 

 was proved by treating minute grains of the pyroxene, with hydro- 

 fluosilicic acid. An abundant deposit of the characteristic hexa- 

 gonal crystals of fluosilicate of sodium was obtained. From the 

 above-noted characters it is evident that the pyroxene is an 

 iegcrine-augitc. 



The nepheline occurs in the rock under different conditions of 

 crystallization. For the most part it is scattered through the ortho- 

 clase in idiomorphic, cloudy white crystals which rarely exceed 

 1 mm. in diameter, the average size being about .5 mm. These 

 crystals show well-defined crystal boundaries in the cross sections, 

 which are commonally hexagonal, pentagonal, rectangular, etc., in 

 shape. They have evidently preceded the orthoclase in the crystal- 

 lization of the magma. Other occurrences are allotriomorphic with 

 reference to the prisms of aegerine-augite. These, too, have evi- 

 dently antedated the orthoclase. There are, however, in the slides, 

 areas of nepheline which have blurred and hazy or irregular out- 

 lines interlocking with the orthoclase, in which they are imbedded 

 as if the mineral had in these cases crystallized simultaneously with 

 the orthoclase. Finally, there are occasional areas where the nephe- 

 line and orthoclase present a most remarkable micropegmatitic in- 



