University of California. 



[Vol. i. 



Panidiomorphic Gncissic Fades. — There are many variations in 

 the general appearance of the nepheline-pyroxene-malignite in its 

 exposures on Poohbah Lake. These are chiefly due to variaticns 

 in the coarseness of the rock, or to the degree of disintegration of 

 the rock. There is, however, besides these a distinct local facies 

 which deserves special notice, although, unfortunately, its relations 

 to the main mass were not determined beyond the fact that the one 

 graded with the other. It is a peculiarly foliated or platy rock, 

 which, although having the same mineralogical composition as the 

 normal facies, presents a very different structure. The constituent 

 minerals are, as before, orthoclase, aegerine-augite, nepheline, apatite, 

 titanite, and biotite. The relative abundance of these is, however, 

 somewhat changed. The orthoclase is relatively more abundant 

 and the aegerine-augite less so than in the non-foliated facies. 

 Nepheline occurs in about the same proportions, but the apatite is 

 more sparingly represented. Titanite is more plentiful than biotite, 

 but both play an accessory role. 



The structural differences between this facies and the normal 

 type of nepheline-pyroxene-malignite are radical. The orthoclase 

 instead of crystallizing in large allotriomorphic masses having a 

 poikilitic relation to all the other constituents is here idiomorphic. 

 It occurs in plates tabular to oo P 00(010) ranging in size from 3 to 

 7 mm. and from .5 to 1 mm. thick. The aegerine-augite commonly 

 shows idiomorphic forms or approximations to these, and appears 

 in general to have in part antedated the orthoclase and in part been 

 contemporaneous with it in crystallization. The nepheline is idio- 

 morphic usually, and inclosed in the orthoclase, but occasionally 

 is moulded against the earlier aegerine-augites. The apatite is 

 idiomorphic. but the titanite is allotriomorphicly interlocked with 

 the aegerine-augite and rarely shows its characteristic crystal outlines. 

 In structure, therefore, the rock may be said to be panidiomorphic. 

 This statement, however, fails to express the most striking struc- 

 tural feature of the rock. This consists in the parallelism of the 

 orthoclase plates. Since these plates are the chief constituent of 

 the rock, their disposition in one plane, with but a moderate amount 

 of overlapping, as with shingles on a roof, gives the mass a very 

 platy or scaly appearance, when viewed in fractures parallel to this 



