1904.1 HAEHL AND ARNOLD — THE MIOCENE DIABASE. 43 



formation of analcite will be discussed under the head of 

 ''Chemical Characters." 



Lenses of high powers reveal the presence in the feldspars of many 

 dust-like particles, the nature of which is unknown. Usually they 

 are without definite arrangement. Inclusions of gas bubbles, patches 

 of augite, magnetite, and also serpentine and chlorite are noted. 



Augiie. — Augite is very plentiful in thediabasic faciesof the rock, 

 and is usually allotriomorphic with respect to the feldspars. It is of 

 a pale brown ccldr with a tinge of red, probably due to a small per- 

 centage of titanium — a supposition which the rock analysis appears 

 to verify. Its pleochroism is faint, changing the shade and not the 

 color. Extinction angles as high as 53° were noted, and zoned crys- 

 tals with undulatory extinction were occasionally seen. Twinning 

 is not uncommon. Cleavage cracks are very distinct, and the inter- 

 secting cleavage lines parallel to the prism of 87° 6' are frequently 

 observed. The augite is remarkably fresh and clear in this rock, 

 having withstood the effects of weathering better than the feldspars. 

 Smaller crystals of augite, occasionally included in the larger phen- 

 ocrysts, are often almost entirely decomposed into what appears to 

 be a yellowish-brown chlorite, the coloration being due to the iron 

 ores present. Frequent irregular patches of gas and fluid inclusions 

 occur in the phenocrysts, sometimes long and rope-like, and often 

 clustered around smaller included grains of augite. Irregular in- 

 clusions of feldspar are often found and are generally much kaolin- 

 ized. Magnetite and its decomposition products are also present in 

 the phenocrysts. 



Olivine. — Olivine is not abundant in the slides of the diabasic 

 facies. It would have been possible, however, to so choose the sec- 

 tions as to show considerable of this mineral, as its occurrence 

 seems to be in occasional local patches and segregations. It is pres- 

 ent, however, in very small quantity in the typical slides, usually in 

 minute clear patches, making up the centre of a mass of brownish 

 decomposition material, badly discolored by iron and showing no 

 characteristic optical properties. Its crystal forfti, where dis- 

 integration is complete, suggests its origin from olivine. In rare 

 instances, too, this secondary decomposition mass assumes a fibrous 

 structure, strong pleochroism and strong double refraction with 

 bright red and green polarization colors, suggesting iddingsite.^ 



1 "The Berkeley Hills," by A. C. Lavvson and Chas. Palache, Bull. Dept 

 Geol. Univ. Cal., Vol. II, No. 12, p. 430, Berkeley, 1902. 



