152 The American Naturalist. [February, 
PETROGRAPHY. 
Geology of Angel Island, San Francisco Bay.— Angel Island 
in San Francisco Bay, Cal., consists essentially of a syncline of sand- 
stone interbedded with an intrusive sheet of fourchite and cut by a 
serpentine dyke and a second mass of fourchite. A radiolarian chert 
is associated with the sandstone.. The most interesting feature con- 
nected with the rocks is the discovery by Ransome? that both the four- 
chite and the serpentine have effected metamorphic changes in the 
sandstone and in the chert, and that in all cases the resulting product 
is the same, viz., a glaucophane schist. The serpentine and the four- 
chite are thus true eruptive rocks, neither being, as supposed by 
Becker, a metamorphosed sediment. The glaucophane schists are true 
contact rocks, and are not the result of a general oregional metamor- 
phism of pre-existing rocks. Not only do they occur as contact facies 
of the sandstones and cherts, but the former rock often contains peb- 
bles of schists, in their essential features similar to the contact schists’ 
The sandstone is made up of quartz, plagioclase and fragments of 
various rocks. The fourchite consists almost entirely of nearly color- 
less augite in rounded or irregular grains, and a small quantity of an 
interstitial substance composed of smaller granules of augite and a fine 
grained neatrix, which under high powers resolves itself into small, 
stout colorless crystals imbedded in a yellowish-green substance that is 
nearly isotropic. The crystals are thought to be zoisite, which may be 
an alteration product of plagioclase, although the author thinks this 
origin not probable. Often the augite is changed peripherally into 
glaucophane, which either replaces the pyroxenes, fills cracks in them, 
or occurs in the spaces between adjacent grains. A few of the speci- 
mens examined possess a glassy groundmass and others are porphyritic. 
Brecciated and spheroidal facies were also observed. The schist pro- 
duced by the alteration of the sandstones and cherts is sometimes com- 
posed of aggregates of glaucophane in a matrix of colorless albite. 
Brown mica, garnets and sphene are also present to some extent in the 
rock. Other varieties of the schist are essentially aggregates of quartz 
and glaucophane. Occasionally the glaucophane is in falrly well de- 
fined crystals, but usually it is in sheaf-like bundles of fine needles. 
The altered cherts now consist of spherules of cryptocrystalline silica 
! Edited by Dr. W. S. Bayley, Colby University, Waterville, Maine. 
? Bull. Dept. Geol. Univ. of Cal., Vol. 1, p. 198. 
