ROCKS FROM NORTHERN AND CENTRAL CHINA. 409 
of the city of Chéu-ts’un and the basalt in this case appears to overlie 
red and gray sediments of the Sin-t’ai series. 
A stony-black lava which at first sight seems to be entirely aphanitic. 
A closer examination, however, reveals numerous small bits of olivine, 
and upon the brownish weathered surfaces pyroxene crystals also become 
visible. 
The base of this rock is obviously glassy. In this glass are embedded 
abundant narrow microlites of feldspars, grains of magnetite and pyroxene, 
and small phenocrysts of olivine and a few of pyroxene. Most of the augite 
occurs in minute grains. The forms and arrangement of the feldspar 
prisms give the rock a very marked ophitic texture. 
With the exception of the iron ores, olivine was the first mineral to 
crystallize. It occurs in small bodies which are frequently bounded by 
crystal faces, and also in minute granules which form a part of the ground- 
mass. Inclusions of magnetite and brown glass are rather numerous. 
In some cases the crystals have been somewhat corroded and the pits filled 
with the material of the ground-mass. The common alteration to green 
serpentine has only begun and is almost confined to cracks and edges. 
The feldspar is labradorite, which appears uniformly as small lath- 
shaped microlites. A suggestion of flow structure is observed in the rude 
parallel arrangement of the microlites in certain areas of the slide. 
The augite, which occurs occasionally in small irregular phenocrysts, 
is pale olive-brown and slightly pleochroic. By far the greater part of 
the augite, however, is in the ground-mass in the form of abundant rect- 
angular or shapeless grains which appear yellowish in color. It seems 
to have been the last mineral to crystallize. 
Although magnetite preceded the other minerals in time of crystal- 
lization, it occurs only in the form of minute grains distributed abundantly 
throughout the rock. 
The glassy base which forms the matrix in which all the other con- 
stituents are embedded is usually nearly colorless; in certain areas, how- 
ever, it has a brownish tinge. 
PERIDOTITES. 
Altered peridotite, No. 35.—The altered peridotite occurs as a dike, 
40 feet thick, which cuts vertically across red sandstones belonging to the 
Sin-t’ai series, about 4 miles, 6.5 kilometers, south-southwest of the town 
of that name. It is exposed in a barren lowland which is considerably 
dissected by ramifying gulleys. 
Contact phenomena.—Along the contact between the green dike and 
the adjacent red sandstones there is a zone, 2 or 3 feet in breadth, through 
