ROCKS FROM NORTHERN AND CENTRAL CHINA. 471 
From the former it differs not only in lacking the ferruginous material, 
which brings out so prominently the structures in those specimens, but in 
being slightly more advanced in crystallization. On the other hand, by 
an increase in the size of the crystals this would readily pass over into such 
a rock as No. 17. 
Gray dolomitic oolite, No. 152.—The exact horizon which is character- 
ized by this limestone is not known, although it will doubtless be found 
within 500 feet, 150 meters, of the base of the Ki-sin-ling formation. The 
specimen was taken from talus blocks which had rolled down from the 
cliffs 0.75 of a mile, 1.2 kilometers, east of Nan-t’ou, on the Yang-tzi. 
A light-gray rock in which a granular texture can be detected even 
with the unaided eye. It contains abundant small rounded bodies of 
various shapes and sizes, most of them like the ground-mass in color, but 
in some cases either darker or chalky white. In no other specimen, among 
the Chinese oolites, do these bodies exhibit such diversity of shape as we 
find here. They are rarely spherical, but are more often irregularly rounded 
or flattened, the surface being lumpy or pustulate. In no cases, however, 
are they either elongate or angular. 
In this slide the majority of the globules are distinguished from the 
matrix only by the dark borders which surround them. Both within and 
without these borders the rock consists of a medium-grained mosaic of 
clear calcite, with which are associated irregular grains of dolomite which 
may be distinguished by treatment with acids. 
In addition to the clear bits of granular calcite, there are others of 
similar shapes which appear dense and grayish under the microscope and 
remain aphanitic even when magnified more than 400 diameters; these 
are the bodies which appear white in the hand-specimen. On the weathered 
surface they stand out as porcelainous granules with pitted surfaces. The 
chalky material is a carbonate, probably dolomite, and the pits doubtless 
represent the locations of calcite grains which have been dissolved. That 
these opaque bodies are not intrinsically different from their more granular 
neighbors is indicated by the fact that the slide exhibits all gradations 
between the two, and in several cases both the granular and the aphanitic 
materials are present in the same globule. 
Whether clear or opaque the oolitic bodies rarely show any trace of 
distinctive structure. Among the former one occasionally sees compound 
globules like those in No. 17,* and in a very few cases faint concentric lines 
are visible near the outer edges of the bodies. No trace of fossils can be 
seen. 
* See page 381. 
