Raneome.] 



Geology of Angel Island. 



granular. The component minerals, with the exception of scat- 

 tered specks of pyrite, cannot be made out without the microscope. 

 In thin section ( PI. i 3, Fig". 1 ) the rock is seen to be made up almost 

 entirely of augite, in irregularly bounded grains, with a subordinate 

 amount of interstitial groundmass. 



The augite never shows complete crystallographic boundaries, 

 and is usually in the form of rather rounded, irregular grains. It is 

 colorless, or has a very slight brow nish tinge, and shows the usual 

 high relief. The double retraction is strong and its character posi- 

 tive. The dispersion is p ?>. The prismatic cleavages are never 

 very distinct, but can usually be detected in the larger grains, in 

 which cases the direction of extinction is nearly always inclined to 

 them, or bisects their angles. The maximum extinction angles 

 appear to lie between 40'"' and 45°. The grains are occasionally 

 twinned, sometimes repeatedly so, resulting in two halves, separated 

 by several narrow lamellae. Each grain generally possesses an 

 independent optical orientation, except in occasional cases in which 

 the separating material consists of a secondary blue amphibole pres- 

 ently to be described. 



The contrast between this nearly colorless augite of Angel 

 Island and the violet-red pleochroic augite of Point Bonita, is rather 

 noteworthy, when there is taken into consideration the fact that 

 the eruptive rocks of each locality are intimately associated w ith the 

 same sedimentary strata, and are accompanied by nearly identical 

 spheroidal facies. From the constant character, extending over 

 wide areas, generally possessed by the augite in the eruptive rock's 

 belonging to any one petrographic province,* it might have been 

 supposed, (7 priori, that violet-red pleochroic augite w ould be found 

 on Angel Island; but such has not turned out to be the case. 



The groundass, which forms quite a subordinate part of the 

 rock, is seen with low powers to consist of smaller granules of 

 augite, and a very fine polarizing matrix. With a No. 8 Hartnack, 

 however, this matrix resolves itself into a dense matted aggregate 

 of small, stout, colorless prisms, w hich are themselves sometimes 

 imbedded in a small quantity of a yellowish green substance which 

 appears nearly or quite isotropic. In section the prisms are usually 



*Iddings, Origin of Igneous Rocks, Bull. Phil. Soc, Wash., Vol. 12, p. 130. 



