Ransome.] 



Geology of Angel Island. 



223 



a peculiar feature in the case of the former, is the intensity and 

 extent of the metamorphism on the western side of the dyke, and 

 the general absence of conspicuous alteration upon the eastern side. 

 The present hade of the dyke is to the south west, and consequently 

 the most intense metamorphism lias taken place on its upper side. 

 It is possible that the inclination was at one time greater than at 

 present, which might account for the observed difference. 



Following closely the western margin of the dyke, as it outcrops 

 on the surface, is a fairly continuous strip of dark- blue schist, of 

 varying width, and showing slight petrographic modifications from 

 point to point. A typical specimen taken close to the thumb-like 

 projection on the west of the main body of serpentine show s a fine 

 gray or brownish matrix crowded with slender glistening crystals 

 of dark steel-blue amphibole. These interlace in all directions, but 

 the majority have their longer axes nearly parallel and give to the 

 rock its schistosity. A section cut at right angles to the direction 

 of schistosity (PI. 13, Fig. 4) shows abundant cross sections, with a 

 few longitudinal sections of the amphibole prisms, imbedded in a 

 fine crystalline mosaic, which is probably largely albite, although 

 some quartz may be present. Thin, irregular flakes of brown mica 

 are abundantly scattered through the groundmass. The sections 

 of amphibole transverse to the prism show the usual hornblende 

 angles, and, when large enough, the characteristic cleavage of the 

 group in a series of fine-shaped lines. A peculiar and striking fea- 

 ture of these cross sections is a frequent zonal structure similar to 

 that described and figured by Palache.* When the shorter diag- 

 onal of the rhomb is lying parallel with the shorter diagonal of the 

 polarizing nicol, it shows an outer rim of a light greenish yellow 

 tint, surrounding a center which is either of lighter hue or colorless. 

 In a position at right angles to the above, the outer shell becomes a 

 deep blue or violet, with a strong absorption of light, while the 

 central portion becomes light green, with little or no greater absorp- 

 tion. The two portions of the crystal are generally well marked off. 

 The inner portion is characterized by abundant fluid inclusions, both 

 with and without bubbles. The zonal structure is also frequently 

 shown in the longitudinal sections, a strip of the green amphibole 



*Loc. Cit., p. 187, PI. 11, Fig. 3. 



