Ransome.] 



Geology of Angel Island. 



2i5 



which is thick!) - speckled with flakes of brown mica. The gran- 

 ular matrix lias not, however, the dazzling whiteness of the albite, 

 but has the more vitreous appearance of quartz. Glaucophane is 

 here very subordinate, occurring' only in occasional radiating tufts 

 of small crystals. 



Under the microscope the colorless groundmass, or matrix, 

 turns out to be composed entirely of interlocking grains of clear 

 quartz, without any feldspar. Brown mica is abundant, sometimes 

 in isolated scales but more often in sheaf-like bundles. Garnets are 

 particularly plentiful, and in certain parts of the slide cluster into 

 dense aggregates. They are all of small size, the largest observed 

 being about o. i mm. in diameter. Titanite occurs very sparingly. 

 One acutely rhombic section was noted about 0.2 mm. in length, of 

 a faint brown color, and showing a perceptible absorption parallel 

 with the shorter diagonal of the rhomb. Orystallographic bounda- 

 ries are, however, very rare, the mineral usually occurring in small 

 highly refractive grains, traversed by comparatively coarse and 

 irregular cracks. 



In the single strip of schist of which the foregoing rock forms a 

 part, there is considerable variety to be observed. Near its middle 

 part a schist outcrops having a green color, and brilliant w ith scales 

 of white mica. The microscope shows it to be mainly composed of 

 a colorless mica, having the optical properties of muscovite, and a 

 green pleochroic mineral in granular aggregates, which appears to 

 be hornblende. It is biaxial, and appears to have oblique extinc- 

 tion. The pleochroism is from a bright green to yellow green. 

 The colors between crossed nicols are about equal to those of 

 hornblende. The absence of crystal form and of distinct cleavages 

 renders the determination doubtful. Chlorite, brown mica, and 

 calcite are also present. It was thought that the colorless mica 

 might be paragonite, but it gave a distinct flame reaction for potas- 

 sium and is probably muscovite. 



About a hundred yards west of the foregoing rock several 

 pieces of schist were picked up containing" visible garnets, but the 

 latter were wholly decomposed, and this particular fades was not 

 exposed in situ. 



The discovery of perfectly crystalline schists apparently result- 



