ROCKS FROM NORTHERN AND CENTRAL CHINA. 469 
The thin section shows a fine-grained mosaic of quartz, with a small 
amount of feldspar and bits of other minerals. The grains are now 
decidedly angular, and either interlock with each other after the manner of 
a crystalline rock, or are cemented together by a greenish clay-like material 
which occupies all interspaces. In addition to the predominant quartz, 
one sees scattered grains of orthoclase, microcline, and plagioclase feldspars 
and also small bodies of iron ores, tourmaline, rutile, zircon, etc. The 
zircons are very small, but they are unusually numerous in this rock. It 
is difficult to resolve the greenish cement-material into its components; 
they appear to be chlorite, delessite, and limonite. 
The rock was formerly an impure sandstone, the pores of which were 
filled with a greenish clay. The induration of the rock has been effected 
in part by the secondary enlargement of the quartz grains, but principally 
by the crystallizing of the earthy material filling the interspaces. The 
effects of compression are almost negligible. Some of the crystals show 
strain-shadows and the slide is crossed by a few irregular cracks along which 
earthy iron oxides have been concentrated. There is no evidence that the 
mass has been severely strained or that recrystallization has progressed 
beyond the merest beginnings. 
CARBONATE ROCKS. 
Black flinty dolomite, No. 151.—A prominent member in the lower 
portion of the Ki-sin-ling limestone, the horizon being about 200 feet, 
60 meters, above the glacial shales at the locality visited. The rocks are 
thin-bedded, and the nodules of flint are quite evenly distributed over 
the surface of the slabs. Specimen collected 0.75 of a mile, 1.2 kilometers, 
east of Nan-t’ou, Hu-pei. 
This is a dense coal-black limestone in which are embedded numerous 
disc-shaped or spheroidal nodules of black flint 1 to 3 cm. in diameter. 
These bodies are most prominent on the weathered surfaces, partly because 
they stand out in relief and partly because such surfaces take on a grayish 
color, while the flints themselves remain black. The rock is very hard 
and does not react with cold hydrochloric acid, except around the circum- 
ferences of the flint nodules. 
The great mass of the rock consists of minute rhombic crystals of clear 
dolomite associated with much dark bituminous matter. The flint nodule 
shown in the slide gives a circular section with a circumference of great 
regularity. The structure of this body is roughly concentric, the outer 
portions being unlike the inner (Fig. 70). The peripheral part is composed 
of relatively large crystals of calcite, some of which are distinctly rhombic 
in form; this calcite appears lighter than the rest of the rock, because it 
contains less carbonaceous matter. Toward the center of the nodule the 
