466 RESEARCH IN CHINA. 
Rocks OF IGNEOUS ORIGIN. 
GRANITIC Rocks. 
Gneissotd quartz-diorite,* No. 148.—This quartz diorite is known from 
a single locality only, namely, the village of Nan-t’ou at the head of the 
I-chang gorge of the Yang-tzi river. Here it forms the basement upon 
which the lowest Paleozoic sediments were deposited. Although its rela- 
tion to the other Pre-Cambrian rocks is wholly unknown, it must be 
supposed that it was intruded in Pre-Cambrian times into other rocks of 
still greater age. 
This is a medium-grained black-and-white granite in which a distinctly 
banded structure is visible, especially when a large body of the rock is 
inspected; in small hand-specimens the feature is by no means prominent. 
The lighter minerals appear to consist of quartz and colorless feldspars, 
while biotite and hornblende, in nearly equal proportions, constitute the 
darker portions of the rock. 
The feldspar probably makes up more than half of the mass. The 
mineral is a sodic plagioclase, whose composition appears to be approx- 
imately Ab,An,, and is therefore intermediate between oligoclase and 
andesine. The crystals are more or less obscured by irregular patches of 
alteration products which apparently consist of kaolin and zoisite, with a 
few flakes of muscovite and bits of epidote. These decayed areas appear 
under the microscope as a pale grayish aggregate with irregular polariza- 
tion colors. 
As is usual in granitic rocks, the quartz was one of the last minerals to 
crystallize and now occurs in very irregular masses. The mineral requires 
no special description in this case, except the statement that it not infre- 
quently includes trichites and other crystals of minute size. 
The hornblende is the dark-green variety which is usually found in 
the granitic rocks. The crystals are irregular in shape and in orientation. 
For the most part, the hornblende is relatively fresh, yet alterations of 
several sorts have made some progress. In most cases chlorite and epidote 
are the products of the change—an alteration which has evidently taken 
place while the rock was in the zone of katamorphism. In other cases the 
hornblende has altered into longitudinal streaks of a brown serpentine-like 
product which alternate with seams filled with pleochroic greenish fibers 
which are probably uralite. 
The biotite is somewhat less abundant than the amphibole. Its color 
is rich olive-brown. The origin of this mica is not obvious, but the occur- 
rence of the flakes in seams and streaks, which are roughly parallel, and 

*Pumpelly has described this and two other related varieties from the same region. (Smithsonian 
Contributions to Knowledge, vol. xv, Geological Researches in China, Mongolia, and Japan, p. 4.) 
