142 



University of California. 



[Vol. ^ 



anomalous results. These may be summarized as follows : Basal 

 cleavage flakes gave angles varying from 0° to 3°, with an 

 average of about 1° in a dozen measurements. This average 

 would indicate an acid andesine. The angle on the brachypinacoid 

 measured in two flakes, was 11°, corresponding to a more basic 

 andesine. Five sections nearly perpendicular to the bisectrix a 

 were discovered, which gave extinction angles, measured from the 

 trace of ccPoo toward the obtuse angle oP: go Poo, of 56° to 66° 

 with an average of 60? This angle, according to Pouque,* indi- 

 cates labradorite. The Michel Levy method on two doubly 

 twinned crystals gives concurrent angles corresponding to 

 andesine. While the results thus seem to indicate something 

 abnormal in the optical character of the plagioclase, it is evi- 

 dently of a more basic character than the general appearance of 

 the rock would indicate. 



Of the associated tuffs one is a soft cream-colored rock, com- 

 posed mainly of fine pumice fragments, with their cavities lined 

 with microscopical crystals of secondary origin. A grain or two 

 of quartz was the only original mineral seen. 



The second is apparently of a type much richer in iron. The 

 mass of the rock is rather dark green, and composed of fragments 

 evidently of decomposed pumice, with considerable more or less 

 earthy green or greenish brown matter. Numerous bipyramids 

 of quartz are imbedded in this matrix, and less conspicuous 

 crystals of glassy feldspar. 



For microscopical study only very imperfect slides could be 

 prepared, but their examination revealed a few interesting facts. 

 The feldspars prove to be mainly oligoclase, with a smaller 

 amount of orthoclase. The quartzes have preserved their crystal- 

 lographies boundaries in great part, but have generally suffered a 

 slight magmatic corrosion. They usually contain a few large 

 glass inclusions, each with a bubble, and often clouded with 

 crystallites. The material in which these large crystals are 

 imbedded contains, besides small fragments of feldspar, and 

 original quartz, only secondary minerals. The pumice fragments, 

 recognized by their vesicular structure, have been thoroughly 

 devitrified. Secondary quartz is abundant, and a zeolite occurs in 



*Bull. Soc. Pr. de Mineralogie, 1894, No. 17, p. 428 



