PLATE LVI. 
PRE-CAMBRIAN GNEISSES. 
A. Bent feldspar in gneiss. The left side of the field is crossed by a shear-zone in which 
the quartz and feldspar have been crushed and dragged out into streaks. The large 
striated crystal of plagioclase is undisturbed except where it comes into contact with 
this shear-zone; there the twinning bands are bent, and the bent portion does not 
extinguish with the rest of the crystal. (Gneiss in the T’ai-shan complex, near T’ang- 
hién, Chi-li. Specimen 71, slide (c) crossed nicols, x 50, page 414.) 
B. Rutile in altered biotite. The dark crystal occupying the center of the field is a flake 
of biotite surrounded by feldspar and quartz. The darker bands in the mica are 
unaltered biotite, while the lighter streaks indicate the extent to which the alteration 
to chlorite has progressed. The numerous little prisms embedded in this chlorite are 
rutile, which has probably been produced from the titanium content of the original 
biotite. (Gneissoid quartz-diorite from the Pre-Cambrian at Nan-t’o, Hu-pei. Speci- 
men 148, slide (a), polarized light, x 65, page 466.) 
SEDIMENTARY ROCKS FROM WEST CHI-LI. 
C. Flint-conglomerate. The minutely crystalline portion on the right of the figure is 
part of a pebble of black flint. This was embedded in rounded sand-grains of quartz, 
several of which appear on the left. The grains and the pebbles are completely 
cemented by cherty silica which is identical with that of the flint pebble. (Conglom- 
erate probably at the base of the Cambrian near Nan-t’ang-mei, Chi-li. Specimen 
73, crossed nicols, X 50, page 421.) 
D. Banded gray limestone. The bands of dark and light color are due to the presence of 
exceedingly minute carbonaceous impurities which appear to have been distributed in 
layers in the original calcareous mud. The material has since crystallized, but the 
color bands have not been affected by the growth of the crystals, for they pass through 
them without deviation. (Typical phase of the Ta-yang limestone at Ta-yang, Chi-li. 
Specimen 88, ordinary light, x 55, page 420.) 
ROCKS OF THE WU-T’AI SYSTEM. 
E. Magnetite-quartzite. The clear interlocking crystals of quartz probably represent 
sand-grains and cement which have entirely recrystallized. The black spots are idio- 
morphic crystals of magnetite which doubtless have been derived from ferruginous 
impurities in the original sandstone. The upper part of the field is crossed by a vein 
of magnetite which is of later age. The cloudy translucent bodies in this vein and 
scattered through the slide are siderite, now partly altered to iron oxide. (A thick 
member of the Shi-tsui series northwest of Shi-tsui. Specimen 91, ordinary light, 
X 40, page 424.) 
F. Garnet-schist. Two small garnets and several biotite flakes are embedded in a biotitic 
schist in which the micas have a distinct parallel arrangement. In the vicinity of the 
large crystals the micaceous bands are bent and thus form eye-spots resembling those 
in conglomerate-schists. (Garnet-staurolite-schist associated with white marble in 
the Shang-ho-miau section, Shan-si. Specimen g5, slide (a) ordinary light, x 20, 
page 428.) 
