ROCKS FROM NORTHERN AND CENTRAL CHINA. 405 
of this plagioclase indicate that it is a labradorite having approximately 
the composition of Ab,An,. 
The few irregular slabs of brown biotite seem to be primary constituents 
of the gabbro. Where the magnetite is contiguous to pyroxene, biotite 
is frequently present in what appears to be reaction rims between the two 
minerals. If this relation is a genetic one, as it is said to be in many rocks, 
some of the necessary material for the biotite has probably been furnished 
by the feldspar, or has been introduced from outside. 
In comparison with other gabbros, magnetite is not common in this 
rock. In addition to its occurrence in the form of inclusions in the augite 
and its association with the decay of olivine, it appears somewhat rarely 
as irregular bodies in the interstices between other minerals. 
ANDESITES. 
Gray augite andesite, No. 49.—This specimen comes from the borders 
of a mass of basic intrusives 3.5 miles, 5.5 kilometers, southeast of Yen- 
chuang. The greater part of the rocks are of darker color and presumably 
of more basic composition. 
A rather light-gray densely crystalline porphyry in which the pheno- 
crysts are black augites and white feldspars. The feldspars, being so nearly 
the color of the ground-mass, are inconspicuous. 
The ground-mass of the rock consists of stout crystals of lime-soda 
feldspar interlaced with augite and a little biotite. The feldspars and the 
pyroxenes together probably compose 90 per cent of the rock. It may 
be mentioned that quartz is present, but is comparatively rare. 
The feldspars are zonally built and are rather calcic in composition; 
the average is a labradorite. They began to crystallize earlier than the 
pyroxenes, but are not sufficiently idiomorphic to produce a well-defined 
ophitic texture. Although the feldspars are relatively fresh, alteration 
has begun, especially in the interiors of the crystals. In such cases the 
altered area usually develops along zones in the crystals and takes on 
forms concentric to crystal outlines. 
The phenocrysts of augite are comparatively rare and require no special 
mention. The great majority of the larger crystals are composed of a 
pyroxene-amphibole intergrowth. When examined in the thin section 
these phenocrysts are shown to be extremely complex in constitution and 
irregular in outline. In the majority of cases they consist of augite and 
labradorite intricately intergrown and usually including irregular bodies 
of hornblende, biotite and iron ores. ‘The biotite and hornblende are 
usually concentrated near the center, but without any definite orientation, 
while the network of pyroxene and feldspar forms the periphery. In other 
cases, however, a clear body of augite is surrounded by a narrow and very 
