ROCKS FROM NORTHERN AND CENTRAL CHINA. 361 
Rocks OF IGNEOUS ORIGIN. 
RHYOLITE PORPHYRIES. 
Buff rhyolite, No. 64.—Our specimen is a badly decayed piece of one 
of the numerous small dikes which invade the Yung-ning sandstone (Cam- 
brian?) between Ch’au-kia-tién and Li-kuan-ts’un. Its associates are gray, 
greenish, and black andesitic and basaltic porphyries of notable variety. 
The pinkish buff ground-mass of the rock was feldspathic, but in this 
specimen is completely decayed. The numerous phenocrysts, which are 
of smoky quartz and buff feldspar, are remarkable for their perfect crystal 
forms. Many of the quartzes are short prisms with complete pyramidal 
terminations. 
The rock has suffered no changes except those which characterize the 
belt of weathering. The former existence of flow structure is suggested 
by parallel dark and light bands which cross the specimen. 
Buff hornblende rhyolite, No. 66.—The small dike from which the 
specimen was taken cuts dark porphyry and an included mass of marble 
4 miles, 6.5 kilometers, south of Siung-yué-ch’6ng. The marble is Algon- 
kian, but the dike is probably of the same age as the last, 7. e., Post- 
Cambrian. . 
This variety differs from the last chiefly in containing many small 
phenocrysts of green hornblende. The decayed feldspathic ground-mass 
and the idiomorphic crystals of quartz and feldspar are similar in the two 
cases. 
BASALTS. 
Hornblende basalt, No. 63.—The specimen came from a small dike in 
the Yung-ning sandstone less than 3 miles, 5 kilometers, north of Fu-chdéu. 
The rock is dark gray with indistinct greenish blotches. There are 
no phenocrysts and the texture is densely aphanitic. 
The microscope reveals a matted network of long prisms of feldspar 
and hornblende set in a feldspathic paste. The feldspars are zonally 
built, but the average composition is near labradorite. The brown amphi- 
bole is scarcely pleochroic and may be largely decayed. The identification 
of it rests largely upon the occurrence of numerous rhombic basal sections. 
Abundant grains of magnetite are scattered through the rock. 
Calcite is present in large quantity as a secondary mineral. It occurs 
in part irregularly and in part as a replacement of small olivine phenocrysts. 
The former presence of the olivine is indicated by the characteristic forms 
of the calcite pseudomorphs. Chalcedony locally forms a part of these 
calcite bodies. 
