Kansome.] 



Geology of Angel Island. 



227 



ingly tough. On freshly fractured surfaces the lens shows 

 numerous cleavage faces of striated feldspar, and the rock would 

 be tentatively classed in the field as a fairly fresh diabase. Near 

 the margins the rock becomes finer grained, and at the contact is 

 quite dense and cryptocrystalline in texture. This would ordinarily 

 be taken as fair evidence of its intrusion into the serpentine; but 

 the latter rock shows no perceptible alteration, and microscopic 

 study of the inclusions seems to throw much doubt upon the intrusion 

 hypothesis. 



In thin section the rock shows a holocrystalline granular struc- 

 ture, being made up of large allotriomorphic plates of augite, in 

 a groundmass of plagioclase. The augite is nearly colorless, with 

 a slight brownish tinge, is not pleochroic, and resembles very closely 

 the augite of the fourchite. The plates are frequently twinned, 

 occasionally polysynthetically, and show the usual cleavages of 

 monoclinic pyroxene. Many of the plates and grains are sur- 

 rounded by a dark cloudy border, while others are fringed with 

 hornblende, usually in the form of delicate needles of pale green 

 actinolite. The most remarkable thing about the rock, however, is 

 the feldspathic base in which the augites lie. This is made up of 

 clear, perfectly fresh plagioclase, in grains of considerable size, and 

 with no regular boundaries. The grains do not abut sharply and 

 decisively against each other but shade off in a wavy, uncertain 

 manner between crossed nicols. When twinning occurs it is usually 

 in the form of broad bands, or the grain consists of two simple 

 halves. Observations made upon a number of plates showing a 

 symmetrical extinction on either side of the composition plane, in 

 the great majority of cases afforded extinction angles of about 15 , 

 or less, and the plagioclase is almost certainly albite. It is gener- 

 ally remarkably fresh and clear, but has a clouded appearance under 

 very low powers, due to the presence of included needles of actinolite, 

 which pierce it in all directions, and are identical in every respect 

 with those already mentioned as fringing the augite. Under high 

 powers the cross sections of the smaller needles are irregular in 

 shape, but the larger ones give the characteristic rhombic forms of 

 the amphibole group. 



A slide from the rock in the upper of the two little quarries, 



