ROCKS FROM NORTHERN AND CENTRAL CHINA. 393 
BASALTIC ROCKS. 
Altered basalt porphyry, No. 24.—Although this rock was found only 
as an intrusive in the Archean rocks of the T’ai-shan, where no Paleozoic 
rocks are present to act as time-indicators, it is so much like other basic 
intrusives, which are known to penetrate the Sinian strata in this part of 
the province, that the rock has been placed in this category rather than 
with the T’ai-shan complex. Specimen obtained from a vertical dike, 7 
feet thick, on the east side of the summit of the T’ai-shan, at an elevation of 
about 4,300 feet, 1,280 meters. 
A very dark aphanitic rock in which the only visible features are certain 
pale-greenish blotches with indefinite outlines. 
The microscope shows the texture of the rock to be rather fine and 
roughly ophitic. The small but somewhat irregular laths of feldspar are 
embedded in a mass of augite containing grains of iron ores. The greenish 
blotches seen in the hand-specimen prove to be altered phenocrysts of 
plagioclase. 
The essential minerals of the rock, in the order of their crystallization, 
are labradorite, ilmenite (with magnetite and pyrite), and augite. As 
decomposition products, there are also kaolinitic materials, chlorite, biotite, 
and leucoxene. 
The feldspars vary in size from minute lath-shaped crystals to idio- 
morphic phenocrysts which are several millimeters in breadth. The decay 
of the feldspars, and particularly the larger crystals, to clear fibrous kaolin- 
itic aggregates has gone so far as to render the twinning bands invisible in 
most cases. 
The pale augite crystals are prevailingly allotriomorphic and frequently 
twinned. Much of this pyroxene is fresh as compared with the feldspars, 
but in places it has been altered, either marginally or more or less completely, 
to a pale greenish or yellowish fibrous material (probably delessite) with 
numerous specks of the iron ores. 
The ilmenite occurs in irregular grains and clusters which are, in most 
cases, difficult to distinguish from magnetite, the presence of which is indi- 
cated by the quadratic shapes of certain crystals. Some of the ilmenite 
grains are bordered with leucoxene, but this feature is by no means 
prominent. 
The biotite occurs now only in very small irregular scales. It seems to 
have been more abundant formerly and to have contributed largely to the 
production of the chloritic decomposition products. From the frequent 
association of this biotite with pyroxene and iron ores, it is thought that the 
mica may have developed secondarily in contact with these two minerals— 
the so-called biotite “‘reaction rims’’ of certain petrologists. 
