ROCKS FROM NORTHERN AND CENTRAL CHINA. 385 
rounded. In numerous places, however, the surfaces are rough and cav- 
ernous, indicating that the pebbles have been somewhat corroded since 
they were formed (Plate LIV, Fig. D). 
Almost all of the pebbles are composed of very dense limestone, usually 
of a gray color, and, except in a few instances, devoid of fossils. In some 
of the specimens little rhombs of siderite and dolomite are scattered through 
this calcareous mass. ‘The vast majority of the pebbles possess no distinct- 
ive structure of any kind; but in a few cases there are parallel lines sug- 
gestive of stratification. None, however, are zonally or radially built or 
show any traces of concretionary structures. There is also no clear evidence 
of organic structure. In one specimen (40) several pebbles contain hori- 
zontal layers of fragments of small trilobites and brachiopods, but they do 
not impart to the pebbles their distinctive features. 
The color of the pebbles varies notably among the specimens. Ordi- 
narily they are gray, frequently reddish, and less often of a greenish or 
yellowish hue. In one common variety (No. 5) the nodules are dark red; 
but when one of these bodies is broken, the color is found to be only super- 
ficial, the interior being gray like the matrix. It is as if the pebbles con- 
tained finely divided carbonates of iron, the peripheral portions of which 
had been altered to iron oxides by exposure to weathering. 
In any one specimen the pebbles may show wide differences in color 
and composition. In some the calcite grains are plainly visible, while others 
have so dense a texture as to be opaque. 
In the majority of cases the position of the pebbles is the same as in 
other conglomerates, 7. e., most of them lie with their longer axes parallel to 
the bedding planes of the formation, but a few, especially the shorter ones, 
stand at various angles to the stratification. Willis observed a slab of this 
limestone near Yen-chuang in which the stratified arrangement is not 
apparent, but rather the pebbles seem to diverge from foci as in rough 
radiate arrangement, and they then lie on edge, with the shortest and 
longest axes in the plane of bedding and the intermediate axis steeply 
inclined thereto. The accompanying sketch (Plate LI, Fig. E) was drawn 
in the field and represents a portion of the slab mentioned. 
The matrix.—In general the matrix may be described as a finely crystal- 
line mass of calcite, associated with various impurities; but in the different 
specimens the texture, shapes of the grains, and the abundance and nature 
of the extraneous material are subject to considerable variation. In the 
gray varieties (No. 48) the cement contains little besides the finely crystal- 
line calcite. In the dark red rocks (No. 157) calcite is intimately associated 
with finely divided hydrohematite and little rhombic crystals of dolomite. 
In still others siderite is an important constituent of the cement, becoming 
