ROCKS FROM NORTHERN AND CENTRAL CHINA. 465 
of a grayish amorphous material, which may be leucoxene, only one other 
mineral appears in the slide. This is a pale-greenish muscovite, a few 
small flakes of which are scattered through the ground-mass. In the 
arrangement of these flakes there is a marked parallelism to the banding 
of the ground-mass, and it is, therefore, evident that the mineral is one of 
those developed during metamorphism of the rock. Certain square cavities 
in the slide strongly suggest that magnetite is present in the rock, but that 
all the grains have been rubbed out in the process of grinding the section. 
This rock has the mineral composition of an aplite. Originally the 
texture was densely felsitic, but the primary texture has been somewhat 
modified by the mechanical deformation of the mass and the recrystalliza- 
tion of a small part of its constituents. Although the parallel orientation 
of mineral particles is perceptible in the thin section, no tendency toward 
schistose cleavage is to be noted in the rock. 
YANG-TZI GORGE DISTRICT. 
In a general way this is the region long known as the ‘‘ Yang-tzi Gorges.”’ 
We shall include in this district the valley of the Yang-tzi river, from 
I-chang-fu (in the province of Hu-pei) westward to Wu-shan-hién, together 
with all of the drainage slope north of the river. It thus comprises not 
only the valley of the great river, but also several northern tributaries 
such as the Ta-ning-ho, a stream which we followed from one of its sources 
to its mouth. 
The rocks of this district are for the most part of Paleozoic age with 
local exposures of Mesozoic and Pre-Cambrian terranes. They are strongly 
but not intensely folded, and with the exception of the oldest formations, 
they are not notably metamorphosed. In the numerous deep canyons, 
which have been cut by the Yang-tzi and its tributaries, these formations 
are exposed with a clearness which is surpassed in very few parts of the 
world. 
PRE-CAMBRIAN. 
The granite which underlies the Paleozoic sequence of stratified rock, 
at the head of the I-chang gorge, is probably only a part of a complex 
Pre-Cambrian mass which is composed of gneisses, schists, and igneous 
intrusives of various kinds. Rocks of this character were observed by 
Pumpelly, at the mouth of the Lukan gorge,* and in the glacial beds at the 
base of the Paleozoic limestones at Nan-t’ou we found numerous boulders 
of clay slates, siliceous limestones, dark schists, granite, and porphyry. At 
present, however, the Huang-ling granite appears to be the only member of 
this complex of which specimens have been collected and studied. 
*Smithsonian Contributions to Knowledge, vol. xv, Geological Researches in China, Mongolia, and 
Japan, p. 4. 
