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University of California . 



[Vol. 3. 



increase to 9.6. This appears sufficiently high to agree with the 

 estimate arrived at by a careful microscopical survey. The 

 peculiar structure of the groundmass is thus seen to be merely the 

 necessary result of its slow cooling, without disturbing move- 

 ment, allowing complete crystallization of the elements in their 

 natural order. 



Hornblende Andeaite of Hold's Canon. — This rock shows the 

 usual porphyritic structure. In the hand-specimen may be 

 seen, imbedded in a pink, rough-textured groundmass, abun- 

 dant phenocrysts of dull white feldspar and black hornblende 

 prisms. 



On examination of thin sections, the phenocrysts are found to 

 lie in an altered andesitic groundmass, which contains amygdal- 

 oidal cavities filled with a variety of secondary minerals. Apatite 

 and magnetite are rather abundant accessories. 



The phenocrystic feldspar always shows albite striation, often 

 combined with pericline and generally with Carlsbad twinning. 

 They are optically determined to be mainly near acid labradorite 

 (Abi Aui), though certain shells of the zonally banded crystals 

 are more acid. Glass inclusions are characteristically abundant. 

 Their alteration consists in the formation of minute veins appar- 

 ently of quartz and zeolites. 



The hornblende, which is always sharply idiomorphic, is of a 

 brown variety, rich in iron. The pleochroism is extremely 

 intense. The axes colors are: 3, clear greenish yellow; b, 

 greenish brown; C, deep reddish brown. The interference colors 

 are brilliant, and indicate a double refraction in excess of .050. 

 The maximum extinction angle is about 5°. The heavy borders 

 of magnetite are mingled with opaque hematite dust. 



The groundmass contains many slender rods of rusty iron 

 ore, which apparently represent a second generation of horn- 

 blende entirely resorbed. The feldspars of the second gener- 

 ation are all of plagioelase, determined as basic andesine, and 

 therefore somewhat more acid than the phenocrysts. Filling the 

 interstices between these laths and microlites is a crystalline 

 paste composed in considerable part of zeolites. It is evidently 

 of a secondary character, and perhaps represents an originally 

 glassy base. The groundmass is interspersed with fine parti" 



