Ransome.] 



Geology of Angel Island. 



22 I 



Under the microscope the rock shows a somewhat cloudy, 

 colorless substance, speckled with grains and irregular aggregates 

 of magnetite. With crossed nicols, the colorless groundmass is 

 resolved into a confused aggregate of serpentine in lamellae, fibers, 

 and grains, polarizing in low tints of gray and yellow. Occasionally 

 the fibers are arranged at right angles to each other, giving sugges- 

 tions of the "grate structure," but it never goes farther than a sug- 

 gestion. A few remnants of large crystals of some brightly polariz- 

 ing mineral with pronounced cleavage and inclined extinction are 

 also present, and have been determined in fresher specimens as 

 diallage, An analysis of the serpentine of one of the harder nod- 

 ules is given in column II of the tableof analyses (p. 23 1 ). During the 

 chemical examination, it was found that some of the supposed grains 

 of magnetite were not decomposed by acids, and gave a distinct 

 chromium reaction in the borax bead. On the other hand, the 

 microscopic slides show none of the brown translucent chromite that 

 is to be seen in the serpentine from Tiburon. 



Although the foregoing may be taken as a general description 

 of the serpentine, there is more or less variation observable in the 

 mass. Some slides consist almost wholly of serpentine in finely 

 felted fibers, others show varying amounts of magnetite and perhaps 

 chromic iron, and some contain remnants of pyroxene crystals. 

 No olivine has been detected in any of the sections, nor can the 

 former presence of rhombic pyroxene be shown with certainty. 



At the northern extremity of the dyke, down on the beach, the 

 rock exhibits a different facies from that previously described. It 

 is here much less sheared and the nodular structure is absent. On 

 fresh fracture it shows numerous brilliant cleavage faces of some 

 light-colored pyroxene, in crystals of 10 mm. or so in length. This 

 rock is clearly but slightly serpentinized, and represents a compara- 

 tively little altered facies of the original dyke rock from which the 

 serpentine has been derived. 



Under the microscope it shows a holocrystalline allotriomor- 

 phic granular structure, and is made up of large interlocking areas 

 of diallage, which is colorless in thin section, and shows the usual 

 brilliant interference colors and high relief. Some few sections 

 show all three cleavages, but the prismatic cleavage is usually 



