532 
PETROLOGY: IDDINGS AND MORLEY 
magnetite. The norm contains a small amount of quartz, no olivine, 
and plagioclase which is approximately Ab2 Ana. 
The basalt from the south base of Taal volcano, anal. 2, is massive 
lava on the shore of the lake which surrounds the volcano, and is over- 
laid by tuffs from more recent eruptions. It has small phenocrysts of 
plagioclase, augite and olivine, and has more mafic minerals than the 
rock from Antipolo, but the feldspar is less calcic. The rock is transi- 
tional to andose. 
The lava from Mayon volcano, anal. 3, is labradorite-andesite, 
with augite and hypersthene and without noticeable olivine. The 
norm contains 5% of quartz, but the rock is hessose, closely similar to 
the basalts from Taal and Antipolo. Other lavas from Mayon contain 
olivine. 
The rock from Benguet Road, anal. 4, is holocrystalline hornblende- 
andesite or andesite-porphyry, slightly altered. The phenocrysts of 
plagioclase are zonal with alternating shells which differ in composition, 
more or less calcic. The hornblende is brownish green, in part altered 
to chlorite, epidote and calcite. The rock is intruded in andesitic 
breccias and lavas. The hornblende-andesite from Olongapo, anal. 5, 
is seriate magnophyric with phenocrysts of zonal plagioclase with 
alternating zones differing in composition as in the rock from Benguet 
Road, the outermost zone being strongly calcic instead of strongly 
sodic as is commonly the case. The hornblende is brown with black 
border, and opaque paramorphs of hornblende are abundant. 
The dacite from Corregidor Island in Manila Bay, anal. 6, is a dense 
white porphyry forming a large massive body on the sea coast near 
the landing. It is dopatic and seriate mediophyric, with phenocrysts 
of glassy plagioclase, quartz and biotite. The groundmass is holocrys- 
talline and seriate microporphyritic, consisting of minute phenocrysts 
of prismoid plagioclase and abundant microscopic euhedral bipyramidal 
quartzes in a microcrystalline matrix which besides quartz and alkalic 
feldspar must contain some kaolin, judging from the chemical analysis 
and norm, for there is 5% of normative corundum, a large amount of 
normative quartz and 2.5% of water. The calculated kaolin is 13% 
which is probably more than is actually present, as some of the normative 
corundum and water belongs to the modal biotite. The rock is als- 
bachose, and is the most siliceous volcanic rock from the Philippines 
so far analyzed. It is interesting to note that the volcanic rocks in 
these islands are Hke many in Japan and in western America. 
