PETROLOGY: IDDINGS AND MORLEY 
531 
Thus at most seven /x's can be determined by the seven X's. If however 
G has one involutorial set, it has oo i such sets.^ If therefore F admits 
one A 7, it must admit oo i Ay^s. 
^ A variable system of sevens on two twisted cubic curves, these Proceedings, 2, 337 
(1916). 
^ In the sense that other analogous porisms can be based on the properties of an 
algebraic (2, 2) form. 
^ The number of poristic forms of the special type which factor into three bilinear 
forms was estimated at oo ^ instead of oo ^. 
^ Coble, Symmetric binary forms and involutions, Amer. J. Math., 31, 189. 
A CONTRIBUTION TO THE PETROGRAPHY OF THE 
PHILIPPINE ISLANDS 
By J. P. Iddings and E. W, Morley 
BRINKLOW, MARYLAND. AND WEST HARTFORD. CONN. 
Received by the Academy. Auguit 10, 1916 
A very large part of the volcanic rocks of the Philippine Islands 
occur as tuff-breccias with flows and dikes of massive lava, and a very 
great area of country is covered with vegetation, so that it will be many 
years before anything like a thorough knowledge of the igneous rocks 
of the region will be acquired. Fragmentary contributions to the 
petrography of the islands are the most that may be expected for some 
time. Already it is known that the principal volcanic rocks are ande- 
sites with much fewer basalts and dacites, and almost no rhyoHtes. 
A short visit to Luzon in 1910 enabled one of us to study the collections 
in the Mining Department of the Bureau of Science [iddings, J. P., 
Philippine J. Sci., 5, 155, 1910,] and to collect specimens from some 
of the more accessible localities on the island. From these specimens 
the accompanying chemical analyses have been made by Dr. Morley. 
The first three analyses are from basaltic lavas having andesitic 
characters. The rocks are porphyritic with andesitic habit to the 
groundmass and a variable amount of modal olivine, which is present 
in the rocks from Antipolo and Taal, but does not appear in the speci- 
men from Mayon which was analyzed, though it occurs in other lava 
from the same volcano. The normative plagioclase in each case is 
labradorite. 
The basalt from Antipolo, anal. 1, is massive lava from near the 
waterfall. It is magnophyric and seriate porphyritic with phenocrysts 
of labradorite and smaller ones of olivine. The groundmass consists 
of prismoid and anhedral feldspar, anhedral pyroxene and euhedral 
