353 



University of California. 



[Vol. i. 



exposed along the northern shores of the lake, and which is here 

 termed amphibole-malignite, may be best described by comparing 

 it with the garnet-pyroxene-malignite. In its general aspect as 

 viewed in the hand specimens, the rock differs from the garnet- 

 pyroxene-malignite in the fact that the orthoclase crystals are 

 smaller, and less sharply defined, while the dark green to black 

 matrix in which they are imbedded is much coarser, owing to the 

 predominance of a lustrous black amphibole in grains of large size. 

 The feldspars have the same habit as before, except that their edges 

 are ragged in detail, and that a considerable proportion of the 

 groundmass is involved with or inclosed in the crystals, particu- 

 larly on their periphery. There is also a less pronounced parallel- 

 ism in the disposition of the orthoclase, although the rock is still 

 distinctly gneissic. When viewed in thin sections, other mineralogi- 

 cal differences become apparent. As a result of the preponderance 

 of the amphibole, the cegerine-augite is quite subordinate in amount, 

 and is intergrown with the amphibole. The melanite, which is so 

 characteristic a feature of the slides of the garnet-pyroxene-malig- 

 nite, is wholly wanting. Biotite plays about the same role. Tita- 

 nite is less abundant, and apatite remains about the same. The 

 black amphibole proves to be difficult of investigation in thin sec- 

 tions, owing to its remarkably strong absorptive powers. As near 

 as could be ascertained, it has moderately low extinctions on c, viz.: 

 12° to 2i°, although it can not be said with confidence that 21° is 

 the maximum angle. The disposition of the axes of elasticity 

 could not be established, but if we assume them to be as in arfved- 

 sonite, the pleochroism is o=deep green blue, b=deep greenish 

 brown, c=dull yellowish green. Absorption, a>b>c. The horn- 

 blende cleavages are strongly developed, and the characteristic value 

 for the prismatic angle was obtained by measuring cleavage fragments 

 on the goniometer. Leaving aside the large orthoclases (microper- 

 thite), the structure of the rock is like that of the garnet-pyroxene- 

 malignite hypidiomorphic granular, the titanite and apatite being 

 the only idiomorphic constituents in the coarse groundmass in 

 which the porphyritic orthoclases are imbedded. 



Chemical Analysis. — The following analysis of the rock was very 

 kindly made for the writer by Mr. J. W. Sharwood, instructor in 

 chemistry in the University of California. 



