452 
PETROLOGY: IDDINGS AND MORLEY 
A CONTRIBUTION TO THE PETROGRAPHY OF JAPAN 
By J. P. Iddings and E. W. Morley 
BRINKLOW, MARYLAND. AND WEST HARTFORD. CONNECTICUT 
Read before the Academy. April 17. 1916. Received. June 17, 1916 
It has been known for many years that the volcanic lavas of the 
Japanese islands are chiefly pyroxene-andesites, and that other lavas 
are more siHceous and some less so, but the actual chemical composi- 
tion of most of them has long remained unknown, though chemical 
analyses of a few varieties have been published. This was the case to 
within a fe\v years, when some of the principal volcanoes were visited 
by one of us in order to study various forms of craters of a few of the 
many active volcanoes of this region, and to collect material for chemi- 
cal and microscopical study. Since that time a number of excellent 
chemical analyses of igneous rocks have been made in the laboratory 
of the Imperial Geological Survey of Japan, some of which have been 
published in volume 2 of Igneous Rock by J. P. Iddings. 
The accompanying seventeen analyses, by E. W. Morley, furnish new 
chemical data relating to lavas of several of the most prominent Japan- 
ese volcanoes, and form a series from the basalts, or basaltic andesites, 
of Fuji yama and Aso to dacites and rhyolites of less well-known localities. 
The habit of the pyroxene-andesites is much the same in all the 
varieties collected, except that some are compact rocks, others vesicular. 
They have abundant small phenocrysts, that is, they are sempatic and 
seriate minophyric. The most abundant phenocrysts are strongly calcic 
feldspars. Pyroxenes are inconspicuous and are in large part hyper- 
sthene, the relative amounts of augite and hypersthene varying con- 
siderably. Olivine occurs as phenocrysts in few of the rocks analyzed, 
and quartz phenocrysts in a few. Magnetite is abundant in most of 
the andesites, chiefly as microscopic constituents of the groundmass 
together with microlites of feldspar and pyroxene. It also forms small 
phenocrysts. 
While the lavas at each volcano vary somewhat in composition, the 
variation is within narrow limits so far as observed. The material 
analyzed represents characteristic rock from each locality, but not all 
that occurs there. Analysis 1 is of the rock from the Hoyei crater on 
the east slope of Fuji yama. The rock is basalt with andesitic habit, 
and contains abundant oHvine and is the lowest in silica of the rocks 
analyzed. Nevertheless, the calculated norm contains a small amount 
