4i8 



University of California. 



[Vol. 2. 



nated as a coloring matter of the serpentine and other minerals, in 

 all forms of the andesite. 



Analcite has been mentioned as forming interesting replace- 

 ments of feldspar. This has taken place in a brecciated andesite, 

 the fragments of which have been largely cemented by analcite, 

 which has also crystallized in vugs and open cracks as brilliant and 

 perfectly-formed trapezohedrons. Thin sections of the rock show 

 that most of the feldspars are wholly or in part replaced by anal- 

 cite, the alteration having taken place from either the center of the 

 crystal, leaving at times a narrow peripheral rim of undecomposed 

 feldspar, or more commonly by an irregular invasion of the crystal 

 from various points, in which case isolated cores or shreds of feld- 

 spar can still be detected. This change affects both phenocrysts 

 and microlites, and appears to be accompanied by the formation of 

 calcite, presumably from the calcium liberated from the plagioclase. 

 The analcite is water clear and wholly isotropic. It was readily 

 decomposed by acid applied to the surface of the thin section, but 

 did not gelatinize, as analcite is said to do by Rosenbusch. The 

 silica separated as a powder which would not hold the stain of the 

 fuchsine solution applied to it. One of the crystals was found to 

 have a specific gravity of 2.256. Analcite is also found in amygda- 

 loidal cavities in one facies of the rock. 



The remaining minerals mentioned as occurring in the ande- 

 sites, calcite, chalcedony, opal, quartz, and natrolite, all occur as 

 the amygdaloidal fillings of cavities in a single facies of the rock, 

 and will be best described under that head. 



The Amygdaloidal Andesite. — We may now proceed to the 

 description of the structural and other microscopic features of the 

 andesitic rocks, not, indeed, dealing with each particular lava flow, 

 but rather characterizing the chief varieties into which they may be 

 classed. 



One of the most prominent of these varieties is that which, 

 under the designation of the "amygdaloidal andesite," has been 

 described as the basal member of the volcanic portion of the Lower 

 Berkeleyan. The general macroscopic features of the rock have 

 been already sketched in the description of its field occurrence. It 

 is a dark gray to black, compact rock of amygdaloidal habit, 

 which is readily distinguished from the other lavas of this field. 



