Ransome.] 



Geology of Angel Island. 



207 



Becker,* In describing the alteration oi theaugiteof his "pseudo- 

 diabase," which he regarded as ,1 metamorphosed sediment, men- 

 tions the fact that "the uralite often lias a bluish tint approaching 

 that of glaucophane." There can at present he little doubt of the 

 true eruptive character of the so-called "pseudo-diabases," in which 

 ease the mineral observed by him was very likely the same as 

 that just described. 



The fourchite is characterized by being remarkably free from 

 the usual accessory minerals of basic eruptive rocks. Man}- slides 

 .show no magnetite, and apatite has not been detected, although the 

 chemical analysis (I of the table ol chemical analyses, p. 23 1 ) show s a 

 small amount of phosphoric acid. Its specific gravity is 3.20. 



In spite of the general uniformity of the fourchite composing the 

 sill, there are, nevertheless, a few departures from what has been 

 described as the typical rock. A specimen taken from the western 

 limb of the outcrop, at a point about two hundred yards southeast 

 of where the line of section C C (Plate 12) crosses it, shows a 

 greenish, decomposed rock, which under the microscope is seen to 

 consist of a glass}- groundmass with felted lath-shaped feldspars, in 

 which are imbedded large phenocrysts of plagioclase, apparently 

 much clouded and decomposed. Under high power these large 

 feldspars are seen to be crowded with extremely minute prisms of 

 the mineral which has been referred to zoisite. No augite is visible 

 in the slide. Also on the 360-foot contour, due north of the cen- 

 tral summit of the island, and therefore on the upper edge of the 

 .V-shaped band of fourchite above Hospital Cove, a decomposed 

 glassy facies was observed, which, however, was too much obscured 

 by decay for satisfactory microscopic stud}-. 



Turning now to the area of eruptive lying west of the serpentine, 

 no such general persistence of character is to be found as exists in 

 the mass just described. Immediately under the capping of chert 

 on the south side of Point Stuart Hill, and along the beach immedi- 

 ately north of Point Knox, the rock is macroscopically identical 

 with the fourchite of the sill. Under the microscope they differ 

 very slightly from the typical facies of the latter, the augite exhibit- 



* Quicksilver Deposits of the Pacific Slope, Monog. XIII, U. S. Geol. 

 Surv., p. 75. 



