ROCKS FROM NORTHERN AND CENTRAL CHINA. 451 
slates and form a continuous sequence with them. They are, however, so 
thoroughly metamorphosed that it is difficult to judge whether they have 
been produced from pelites or from psammites such as graywackes. ‘The 
complex mineral composition of these schists indicates at least that they 
have been derived from sediments which contained a large variety of con- 
stituents. Since these rocks contain nothing suggestive of sand-grains or 
other coarse particles of an original nature, they will be considered provi- 
sionally as metamorphosed shaly strata. 
Brown quartz-mica schist, No. 123.—This is one phase of the spotted 
gray schists which were so characteristic of the region north of Shi-ts’tian- 
hién. The present specitnen exhibits the least advance in metamorphism 
of any of the three represented in our collection.* The rock is exposed 
in the canyon of the Han river, about 1 mile, 1.5 kilometers, above Shi- 
ts’tian-hién. There it is overthrust by the schists, gneisses, and graphitic 
limestones which are regarded as the Wu-shan formation. 
A brownish-gray rock of medium grain, in which the schistosity is 
only moderately well developed. On account of this imperfection of the 
schistosity, and the banded appearance of the edges it might as properly 
be classed with the gneisses. The micas do not form continuous sheets 
as in typical mica schists, but are merely sprinkled thickly in small flakes 
over the cleavage surfaces. It is noticeable in the hand-specimen that 
many of the mica crystals lie transverse to the cleavage: this probably 
explains in some measure the breakage of the rock in directions other than 
along the usual planes of schistosity. 
The lighter bands of the rock are composed very largely of quartz 
mingled with small parallel flakes of muscovite. Most of the quartz has 
recrystallized and forms a closely interlocking mosaic; there are a few 
lenticular bodies which show strain-shadows and appear to be crushed 
original grains. 
The darker layers of the rock contain biotite and muscovite and 
certain rather rare accessories in addition to the abundant quartz. The 
biotite occurs in large rich-brown flakes which are frequently not parallel 
to the cleavage. It is noteworthy, however, that the smaller flakes are in 
all cases parallel to the schistosity. The large biotites are very ragged in 
their outlines and include numerous small crystals of quartz, rutile, mag- 
netite, and zircon. The last mineral is always surrounded by the familiar 
dusky halos. Evidently these large micas have grown at the expense of 
the smaller flakes which accentuate the schistose cleavage, and are subse- 
quent in age to the dynamic metamorphism of the rock, which produced 
the structure.f 
*See Nos. 115 and 118. 
{For an illustration of this feature see Leith: Rock Cleavage, U.S. Geol. Surv. Bull. 239, Plate x11, B. 
