1894.) Mineralogy and Petrography. 419 
MINERALOGY AND PETROGRAPHY:' 
The Eruptive Rocks of Cape Bonita, Cal.—The eruptive 
rocks forming the main mass of Cape Bonita, the northern Cape separ- 
ating San Francisco from the Pacific Ocean, are spherical basalts and 
diabases, in addition to basic tuffs. The basalt is remarkable for the 
great spheroidal masses that characterise it. In many places the entire 
rock-mass is a closely packed aggregate of large bolster-like bodies, 
whose cross-section is approximately circular. These consist of a com- 
pact amygdaloidal rock, made up of lath-shaped plagioclases lying in 
a glassy base. In all cases the rock of the spheroids is much altered, 
and is of the same composition in the interiors as on the peripheries of 
the bodies. In a few cases augite may be detected as small grains that 
are younger than the plagioclases, but the rock on the whole is very 
uniform in character. The diabase is more interesting petrographic- 
ally. It is younger than the basalt ‘and has intruded this rock. Besides 
the usual constituents of diabase it contains iddingsite in large, rounded, 
idiomorphic forms. The augite varies in color from nearly colorless to 
a deep violet red, the latter varieties possessing a pleochroism in 
yellowish green and violet red tints. A qualitative test showed the 
Presence of titanium. Sometimes the augites of different colors are 
intergrown, when they are optically continuous, and not infrequently 
the mineral is intergrown with brown hornblende. The outlines of the 
iddingsite are strongly suggestive of olivine. It was one of the earliest 
Separations from the magma, being included in the augite and in the 
hornblende. Its own enclosures are magnetite and chromite or pico- 
tite. In some phases of the rock both green and brown hornblende are 
Present, Both of these are regarded as original and as of the same age 
as the augite, for they are frequently intergrown with the pyroxene as 
well as with each other. In one place the diabase is variolitic, with 
Variolites composed of tiny brushes and crystallites of various minerals, 
ying in a microlitie diabasic groundmass. Iddingsite occurs both in 
the groundmass and in the varioles. The pyroclastic rock associated 
with the basalt and the diabase is probably an ash of a basaltic charac- 
ter. Some of its component fragments resemble closely the material 
of the spheroidal rock. Analyses of the rocks discussed are given by 
Mr. me,’ in a recent number of the University of California 
n. - 
' Edited by Dr. W. S. Bayley, Colby University, Waterville, Me. 
Bull. Geol. Dept. Univ. Cal., Vol. 1, p- 71. 
