ROCKS FROM NORTHERN AND CENTRAL CHINA. 425 
Gneissic graywacke, No. 92.—This resembles the last rather closely and, 
if the structure of the Shi-tsui section is synclinal, it may represent a 
recurrence of that very stratum. The specimen comes from quartzitic 
layers interbedded with schists about 4 miles, 6.5 kilometers, up the T’ai- 
shan-ho from Shi-tsui (Plate XVIII, stratum 20). 
The rock has a pale pinkish color modified by black scales of mica on 
the cleavage planes. The texture is fine, as in the last variety. In com- 
position the rock is closely similar to No. 84, but it has been meta- 
morphosed. The original grains of quartz and feldspar have been reduced 
by mashing to a granular banded aggregate in which a few of the deformed 
sand-grains have still escaped complete granulation. Secondary flakes of 
muscovite and biotite lie in streaks parallel to the cleavage, except where 
interfered with by the remaining sand-grains. Abundant calcite bodies 
form a part of the mosaic. The ease with which this mineral recrystal- 
lizes enables. it to adjust its forms readily to changing conditions during 
metamorphism. As it occurs in this rock, therefore, it exhibits no strain 
phenomena or other evidence of the anamorphism through which it has 
passed. 
Gneissic arkose, No. 98.—Certain arkoses and arkose schists lie upon 
the conglomerate schists of the Si-t’ai group. Specimen collected 3 miles, 
5 kilometers, south of Wu-t’ai-shan village from beds equivalent to the 
top of stratum 30 in Plate XVIII. 
This is a gray massive rock with an obvious parallel structure, but 
not distinctly banded. The surfaces are dotted with glassy and red quartz 
and feldspar grains like phenocrysts in a porphyry. ‘The internal consti- 
tution of the rock is similar to the ground-mass of the conglomerate (No. 
97) and needs but little additional mention. The relatively coarse grains 
of quartz and alkali feldspar have been greatly deformed and through 
granulation have contributed largely to the formation of the schistose 
ground-mass. Here again the superior resistance of feldspar to deforma- 
tion is exemplified; quartz is readily granulated, while feldspar often 
remains in eye-spots. ‘The micas of the ground-mass are chiefly minute 
flakes of sericite with some chlorite. 
Quartz-sericite schist, No. 99.—The transitional zone between the 
green schists and the arkoses of the Si-t’ai series comprises gray quartzose 
schists and gneisses which grade into the rocks both above and below with- 
out clear distinction. This specimen was found not far below the green 
schists, 2 miles, 3 kilometers, south of Wu-t’ai-shan village. 
A pale greenish-gray rock with imperfect slaty cleavage. The surfaces 
have the silvery sheen of sericite, but most of the constituents are too 
minute to be seen with the unaided eye. The microscope shows that the 
