Ransome.] 



Geology of Angel Island. 



213 



clinopinacoidal faces. The usual prismatic cleavage is distinct in all 

 sections, those perpendicular to c showing the characteristic horn- 

 blende angle. The maximum extinction angle observed was about 

 7° to the prismatic cleavage, the angles in general being much 

 smaller. The relation of the axes c: C = 7°, and b = b was deter- 

 mined by means of the quartz wedge, and confirmed by the mica 

 plate. The strong and beautiful pleochroism is very notice- 

 able, a being light greenish yellow, c indigo to ultramarine blue, 

 and b a rich violet. The absorption is c>b>n. Sections from the 

 orthodiagonal zone, showing blue and violet pleochroism, give a 

 biaxial interference figure in convergent light, with a small axial 

 angle. The optical character is negative. The interference colors 

 are only fairly brilliant in rather thick slides. Some crystal frag- 

 ments were picked out from the powdered rock and treated with 

 hydrofluosilicic acid in accordance witli Boricky's method, and 

 yielded abundant crystals of the sodium fluosilicate, together with 

 some gelatinous alumina, and possibly a very few crystals of the 

 silicofluoride of calcium. From the foregoing optical and chemical 

 properties the mineral may be safely classed as glaucophane. 



Dr. Chas. Palache* in a recent number of this Bulletin has 

 described a new mineral from the vicinity of Berkeley, intermediate 

 in chemical composition between glaucophane and riebeckite, and 

 having an optical orientation similar to the latter. The paragenesis 

 of the two minerals is practically the same in both cases, which 

 lends additional interest to the fact that they are distinct. It may 

 be stated that the general appearances of the two minerals under 

 the microscope are very similar. The)' differ slightly, however, in 

 shades of pleochroism and in the colors between crossed nicols. 



The colorless groundmass of the rock is seen, with crossed 

 nicols, to be composed of closely-fitting grains of plagioclase. 

 These grains are perfectly fresh and clear and generally without 

 twinning. When twins occur it is most commonly in the form of 

 simple halves, or rarely three or more lamellae are present, twinned 

 according to the albite law. The extinction angles average' about 

 1 5° as a maximum on either side of the composition plane in sym- 



*On a Rock from the Vicinity of Berkeley, Containing a New Soda Amphr 

 bole. This Bulletin, Vol. I, pp. 1S1-192, Pis. 10 and 11. 



