ROCKS FROM NORTHERN AND CENTRAL CHINA. 445 
and micas. The micas are biotite and muscovite, with a little chlorite, 
all of them scattered as separate flakes with their axes approximately 
parallel; these flakes, it will be observed, are not interlaced with one 
another in the form of continuous seams, as in most of the schists. The 
chlorite is an alteration product after the biotite. The abundant grains of 
magnetite, which give the rock its color and magnetic powers, occur either 
singly or in irregular clusters, sometimes associated with a little pyrite. 
Many of them are more or less perfect octohedra. Grains of epidote, 
needles of rutile, and laths of apatite are present as rather common acces- 
sory minerals. 
It is probable that the minerals mentioned above are all of secondary 
origin, at least in the sense that they have been recrystallized under the 
conditions which prevail in the zone of anamorphism. From the presence 
of some aggregates of crystalline carbonates and quartz with grains of 
epidote, it is suspected that among the original minerals of the rock were 
certain complex silicates, such as feldspars, which by their decay produced 
these simpler compounds. 
Gneissic graywacke, No. 132.—The brownish graywacke is a member 
of the Han system which is dominant along the road from Pa-li-kuan to 
Pa-kua-miau in southern Shen-si. It appears to lie above the black slates 
and is probably a part of the K’ui-chéu schists. Specimen collected 2 
miles, 3 kilometers, north of the village of Pa-kua-miau. 
A very fine-grained green-gray rock streaked with slender lines of 
reddish-brown color. The rock is not truly laminated, but fractures un- 
evenly, having what has been termed “‘linear-parallel cleavage.’’* 
The minutely gneissic structure is very prominent in the thin section, 
the lighter bands containing quartz and feldspar with epidote, while the 
darker layers contain a large percentage of biotite, chlorite and iron oxides, 
with certain less numerous accessory minerals. All of the minerals show 
a well-marked parallel orientation. At irregular intervals one sees larger 
bodies of quartz and feldspar which were probably sand-grains not yet 
reduced in the process of metamorphism. ‘They are often set in ‘“‘eye- 
spots”? the corners of which are occupied by granulated material derived 
from the grains. The mica bands bend out around these porphyritic 
grains (Plate LVII, Fig. C). The biotite is present in the form of small 
dark-brown flakes, with which are associated minute scales of chlorite and 
granules of epidote and zoisite. All of these minerals are of secondary 
origin and the micas are distinctly oriented with reference to the schis- 
tosity. Small grains and scales of hematite, with little prisms of apatite, 
are scattered promiscuously through the rock. The section also reveals 
*Leith, C. K., Rock Cleavage: U.S. Geol. Surv. Bull. 239, Plate I. 
