ROCKS FROM NORTHERN AND CENTRAL CHINA. 365 
of plagioclase and bits of magnetite. Such fragments may have been 
incorporated in the lava in the process of extrusion. 
It is evident that this porphyry formerly had a glassy ground-mass, 
but devitrification has set in and the whole mass is now crystalline, although 
a high power of the microscope is required to reveal the fact. The struc- 
tures which have just been described indicate that it was, in all proba- 
bility, a superficial lava flow. Since its extrusion it has been subjected 
to no metamorphic process other than those which are characteristic of 
the belt of weathering. 
WESTERN SHAN-TUNG. 
Roughly speaking, this district includes the western mountainous 
portion of the province centering about the holy mountain, T’ai-shan. 
It extends west and north to the plain of the Yellow river. It is separated 
from the mountainous district of eastern Shan-tung by a belt of lowlands 
in which the surface rocks are largely weak strata of Carboniferous and 
later age. Within the district the rock formations range from the oldest 
Pre-Cambrian to the early Mesozoic (?), and they consist of igneous, meta- 
morphic, and sedimentary rocks. 
T’AI-SHAN COMPLEX. 
The following descriptions represent most of the prominent varieties 
of igneous and metamorphic rocks which occur in the basal Pre-Cambrian 
system.* The slopes of the T’ai-shan itself offer an excellent opportunity 
for the collection of fresh material and for the study of the mutual relation- 
ships of the rocks in the field. Other valuable localities may be found in 
almost any part of the district where the ancient rocks are exposed. 
Rocks OF DOUBTFUL ORIGIN. 
GNEISSES AND SCHISTS. 
The oldest rocks which have been discriminated in the Archean complex 
are certain gneisses and schists. They have been so severely metamor- 
phosed that their origin is no longer demonstrable by present methods. 
The gneisses are suggestive, however, of altered granites, and there is a 
significant lack of the types of schists which commonly develop from sedi- 
mentary rocks. We have not observed any which might not be derived 
from igneous material. 
Gray biotite gneiss, No. 30.—This is one of the coarser phases of the 
gray gneiss which forms a large part of the Pre-Cambrian complex. It 
is believed to be a deformed granite. It is not possible at present to 
define its relation to the other phases of the gneiss described in succeeding 

*Von Richthofen mentions and briefly describes the more prominent members of the T’ai-shan system 
(China, vol. 11, pp. 195-220), 
