ROCKS FROM NORTHERN AND CENTRAL CHINA. 397 
grains of magnetite and ilmenite, little rods of hornblende, and irregular 
bits of yellowish-green alteration products form the mass of the rock. 
The feldspars occur in small lath-shaped crystals, the great majority 
of which are Carlsbad twins. By far the commonest variety is orthoclase. 
The few striated feldspars seem to be oligoclase. Alterations to kaolin (?) 
and similar products have not proceeded far in either case. 
The original ferro-magnesian mineral of the ground-mass was doubtless 
a green hornblende, numerous traces of which still remain in the form 
of little irregular rod-like prisms. It is now, however, almost entirely 
replaced by a yellowish-green chloritic substance. The large phenocrysts 
of hornblende are likewise thoroughly decayed. Their places are now taken 
by pseudomorphs of coarse-grained calcite and other minerals; along the 
margins of the phenocrysts numerous radiating bunches of fibrous acti- 
nolite penetrate the calcite, and various irregular strips and patches of 
yellowish and greenish alteration products are also scattered about. Among 
these are distributed bits of the original hornblende, which are embedded 
in the calcite, preserving their former orientation and cleavage. These 
fragments are still pleochroic. 
Altered syenite porphyry, No. 23.—Essentially similar in its occurrence 
to the last rock described. Specimen from a thin sill in Man-t’o shales 
5 miles, 8 kilometers, east of Ch’ang-hia. 
Differs from No. 16 in no essential feature. The feldspars are less 
altered; they have low refractive indices and also low extinction angles. 
The striated feldspars are probably albite or oligoclase, but there can be 
little doubt that orthoclase is present in greater abundance. 
Quartz-syemte porphyry, No. 3.—The only known occurrence of the 
rock is in the foothills southwest of Tsi-nan-fu, where it cuts across the Tsi- 
nan limestone. The specimen comes from a dike 2 feet wide on the north 
side of the col, 2.5 miles, 4 kilometers, WSW. of the west gate of Tsi-nan-fu. 
The composition of this rock would permit its being placed in any one 
of several groups, as those groups are usually defined. If the granites are 
considered to be alkali-feldspar rocks, this might well be grouped among 
them, except for the scarcity of quartz. By others it might be considered 
a monzonite, on account of the very considerable amount of oligoclase 
and the scarcity of quartz. From the description the reader will perceive 
that with a slight decrease in the percentage of plagioclase and quartz, 
this rock would approximate in composition the syenites (16 and 23), 
while a slight increase in the amount of lime would bring it close to the 
dacites (42 and 44). Thus it appears that all of these intrusions might 
readily have sprung from a single general magma, in which there were 
slight variations of composition. 
