30 RESEARCH IN CHINA. 
limestone, which is often locally oolitic, and is for the most part dark gray 
in color with tinges of purple. 
The upper portion of the Ch’ang-hia formation is composed of a some- 
what variable succession of dark and light gray limestones which are occa- 
sionally oolitic, more frequently coarsely crystalline, but prevailingly dense 
and ringing. Many of these layers are indefinitely mottled with ocherous 
coloring matter, which is distributed in cloudy masses, regardless of rock 
structure. Harder layers in this group often produce a second cliff, but 
this is never so marked a feature as that made by the black oolite. 
Near the top of the formation the conglomeratic limestones appear. 
As a rule they are light gray, but are interbedded with thin layers of pale 
green earthy material, which also often incloses the pebbles. Many of 
the layers are strikingly like ordinary conglomerates when the exteriors 
have been exposed to weathering. 
Oolitic limestones.—The dark oolitic limestones, which are so charac- 
teristic of the Sinian system throughout China, deserve special mention 
in this connection. Von Richthofen noted and described these rocks,* 
and applied to them the name “‘globulitic limestone,’ being of the opinion 
that they were not true oolites. They should be distinguished from the 
conglomeratic phases of the Sinian limestones, which are quite different 
in aspect and are believed to be equally distinct in origin. 
In its commonest phase the dark oolite has a gray matrix of minute 
calcite crystals. The oolitic bodies are dark brown or black, ranging in 
diameter from a fraction of a millimeter up to three or four millimeters. 
They are usually spheroidal in shape, but may be oval or ellipsoidal. 
When examined under the microscope these spherules are distinguished 
from the matrix by a slightly darker coloration and a dark periphery; 
there is frequently a suggestion of concentric rings within. The material 
of the spherule is granular calcite like that of the matrix. Very few of the 
spherules in this variety of the oolite possess distinct nuclei, and where such 
features are present they are usually indefinite masses of somewhat coarser 
grained calcite. 
Another phase of the Cambrian oolites, represented by a specimen 
from the upper portion of the Man-t’o formation, shows the concentric 
structure conspicuously. In this rock the matrix is light-colored calcite, 
while the oolites themselves are deep red. ‘These red spherules usually 
contain nuclei which, in some cases, can be identified as bits of marine 
shells. Around these nuclei there are concentric rings of very finely 
divided calcite, more or less colored by a red earthy material. About 
* China, vol. 11, pp. 196, 223. 
