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University of California. 



[Vol. 2. 



readily with hydrochloric acid. When associated with them it was 

 formed after calcite and opal, and before chalcedony. It was not 

 seen in the same cavity with anal cite, though the two minerals 

 occur in adjacent amygdules. Analcite occurs either alone or 

 crystallized simultaneously with calcite. Its properties have 

 already been described. Opal forms the filling of the veinlets 

 already mentioned as frequently traversing the rock, and is the 

 earliest deposit in some amygdules. It is colorless and slightly 

 doubly refracting, the spherical masses exhibiting a dark interfer- 

 ence cross.* A bright green mineral was observed in some amyg- 

 dules, forming an exceedingly thin external coat A thin section 

 of one of these showed the outer layer to be of opal stained with a 

 green coloring matter of undeterminable character. Chalcedony 

 is, as above stated, very abundant in the amygdules, and exhibits 

 characteristic optical properties. It was deposited after opal, natro- 

 lite, and some calcite, and before quartz and some calcite. 



It has been shown that the amygdaloidal facies occurs over a 

 large area, and is characterized throughout by the presence of 

 secondary minerals, of which silica in various forms is far the most 

 abundant. As the rock itself is comparatively unaltered, it could 

 not have furnished the great amount of silica thus present, and it 

 is a matter of interest to seek the extraneous source of supply 

 capable of furnishing that substance in sufficient quantity and 

 uniformly over so large an extent. Such a source is probably to 

 be found in a bed of very acid volcanic tuff which commonly over- 

 lies the amygdaloidal rock. Circulating waters would readily dis- 

 solve silica from this tuff whose porous structure would facilitate 

 such action, and under slightly different conditions of temperature 

 or pressure they would deposit the mineral in the vesicles of the 

 underlying bed of andesite. 



The Grizzly Peak Andesite, PorpJiyritic Facies. — The next most 

 important variety of andesite is that which we have described in 

 the account of the stratigraphy as the "Grizzly Peak andesite." 

 There are two rather distinct facies of this andesite lava; the lower 

 flows, being holocrystalline and laminated, are referred to as the 

 holocrystalline facies, and the upper flows, being distinguished by 

 *Rosenbusch Mik. Phys., pp. 122 and 89. 



