ROCKS FROM NORTHERN AND CENTRAL CHINA. 403 
thickly sprinkled with minute specks of dark-brown micaceous ilmenite (?). 
The exact nature of the various grains of iron ore scattered through the 
rock is difficult to determine, but the fact that borders of leucoxene and 
sphene are frequently associated with such grains indicates ilmenite, and 
the inference is also supported by the presence of the independent bodies 
of titanite which show that the rock is rather rich in titanium. 
Hornblende-syemte porphyry, No. 52.—Another of the volcanic rocks 
associated with the Permo-Carboniferous sediments in the region northeast 
of Yen-chuang. Specimen from a 2-foot, 0.6-meter, dike in tuffs and 
shales 2 miles, 3 kilometers, northeast of the town of Yen-chuang. 
A violet-gray porphyry in which needles of hornblende are the only 
conspicuous phenocrysts. In addition there are also numerous pale-yellow- 
ish spots of decomposition products which are probably derived from 
other phenocrysts. The ground-mass itself is entirely aphanitic, and the 
microscope reveals in it alkali feldspars with the usual darker minerals 
and numerous small phenocrysts of dark-green hornblende. A suggestion 
of flow structure is seen in the roughly parallel arrangement of the feldspars 
at many points in the slide. 
The dark-green crystals of hornblende are rendered even more con- 
spicuous in the thin section by the black borders of magnetite with which 
they are invariably surrounded. These appear to be resorption rims devel- 
oped before the solidification of the lava, rather than subsequent decompo- 
sition products. A few of the hornblende crystals inclose areas of colorless 
pyroxene. They are but little altered. 
From their low index of refraction, small extinction angles and the 
absence of twinning bands it is evident that the feldspars of the ground- 
mass are largely orthoclase with a subordinate amount of albite. These 
feldspars occur in the form of lath-shaped prisms, but they are so intimately 
intergrown that their outlines are irregular. A peculiar feature of this 
rock is the wreath-like clusters of feldspar prisms, which surround many 
of the phenocrysts and even grains of magnetite; the laths are tangential 
to the inclosed body and so form whorls in the ground-mass. 
The darker constituents of the ground-mass are hornblende with a 
little pyroxene and abundant small bodies and flakes of iron ores. 
GABBROS. 
Olivine-hypersthene gabbro, No. 2.—This rock, referred to by von 
Richthofen* as ‘‘diorite or hyperite,’’ forms several low conical hills in 
the vicinity of Tsi-nan-fu. The hills are surrounded and partially buried 
by alluvial deposits, so that the relation of the gabbro to other rock 

* China, vol. 11, pages 198 and 222. 
