38 CALIFORNIA ACADEMY OF SCIENCES. [PROC. 30 SER. 



very fresh, lath-shaped feldspars in two generations, besides 

 amber-colored iddingsite and pale green augite. The 

 specific gravity is 2.770. 



The opal has the usual appearance of that mineral macro- 

 scopically. On one face of the specimen there is a con- 

 siderable crust of hyalite showing a distinctly botryoidal 

 surface when viewed with a lens. It is colorless and nearly 

 transparent, with a vitreous luster and a hardness of about 

 5.5. It is infusible, dissolving in soda, with effervescence, 

 to a clear glass. It is in large part soluble in caustic pot- 

 ash, and in the closed tube gives water. The areas of 

 opal in the slides are all isotropic. The sections readily 

 take a stain after heating with concentrated hydrochloric 

 acid, which, however, scarcely attacks the powdered mineral. 

 The color of the opal in thin section is light brown. 

 Cavities occur in it, occasionally lined with chalcedony. 



Both generations of feldspars appear to be labradorite, 

 and the larger ones form the most prominent feature of the 

 slide, being rather numerous and of considerable size, 

 ranging in length from about 2 mm. to 3.5 mm. They are 

 allotriomorphic, and contain, as inclusions, considerable 

 iddingsite, besides a little glass, an occasional smaller 

 feldspar in the largest sections, and rarely augite. The 

 small included feldspars always show more or less resorp- 

 tion. 



The augite occurs in small grains with very irregular 

 boundaries. These contain many cracks which give them 

 a granular appearance. The mineral shows no alteration 

 and contains as inclusions occasional grains of magnetite, 

 besides partially included small feldspars. Several sections 

 were seen with a few small feldspars wholly enclosed. 



The most characteristic mineral of the rock is the iddings- 

 ite. In amount it slightly exceeds the augite, and equals 

 about one-third of the feldspar. It varies in size from .06 

 mm. to nearly .4 mm. It was the second mineral to separate 

 from the magma, preceded by the magnetite. It occurs as 

 usually elongated grains with very irregular boundaries, 

 frequently marked by bays due to magmatic corrosion. 



