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University of California. 



[Vol. 2. 



will be convenient to describe the various forms under separate 

 heads, designating each by a term descriptive of its structural 

 peculiarity. The most typical of these facies will be known as, 1st, 

 the amygdaloidal facies; 2d, the porphyritic facies; 3d, the holo- 

 crystailine laminated facies; 4th, the aphanitic laminated facies. 

 To avoid unnecessary repetition, the minerals composing the rocks 

 will first be described, and then the rocks in their various forms 

 will be considered. 



Constituent Minerals. — The andesites are composed essentially 

 of plagioclase feldspar and augite. In addition there occur as 

 accessory constituents in varying amount, orthoclase, hypersthene, 

 olivine, apatite, and portions of glassy base. The secondary 

 minerals include serpentine, chlorite, calcite, iron ores, chalcedony, 

 opal, quartz, analcite, and natrolite. Plagioclase feldspar occurs in 

 two generations. The porphyritic crystals are generally of idio- 

 morphic outline, in stout to long prismatic forms. They vary in 

 size from the smallest dimensions up to a quarter or half inch in 

 length. Frequently several crystals occur in juxtaposition, forming 

 allotriomorphic aggregates or groups of crystals whose outer 

 boundaries only are idiomorphic. The feldspars are clear and 

 glassy, and exhibit distinct cleavage cracks and irregular shrinkage 

 cracks in abundance. Measurements of extinction angles on 

 sections of albite twins whose alternate lamellae extinguished 

 symmetrically on the trace of the twinning plane, gave maximum 

 results varying from 13 to 32 degrees. As the angles from 13 to 

 23 degrees were obtained from the feldspars of one facies of the 

 andesite, and those from 24 to 32 degrees from the feldspars of 

 another facies, it is probable that the plagioclase is of variable 

 composition, but it is all embraced within the limits of the labrado- 

 rite group. Twinning after the albite law is the rule, sections of 

 phenocrysts which showed no twinning lamellae being very excep- 

 tional. The lamellae vary much in breadth and number, but seem 

 to hold a somewhat constant relation to the size of the crystal, the 

 largest crystals having the fewest and broadest lamellae. Twins 

 formed according to the pericline law and the carlsbad law were 

 also observed in a few crystals. Zonal structure is very common 

 in the feldspars. The zonal bands are sometimes broad and 



