ROCKS FROM NORTHERN AND CENTRAL CHINA. 457 
size, while the clay-like impurities have been wholly reorganized into such 
minerals as mica, tourmaline, etc. 
White dolomitic marble, No. 120.—A portion of the gray limestone of 
the coal-bearing series. It probably owes its unusual crystallinity to the 
fact that it has been strongly metamorphosed by the intrusion of igneous 
rocks (Nos. 126 and 127) which lie adjacent to it. Specimen from the 
right bank of the Han river, 6.5 miles, 10.5 kilometers, west of Shi-ts’tian- 
hién. 
A medium-grained, uniformly crystalline marble of grayish-white 
color. No accessory minerals are visible, and there is no evidence of schis- 
tosity. By the rapid weathering of the small amount of calcite which is 
present, the dolomite crystals are left upon the surface and form a friable 
exterior coating. 
The rock is composed of interlocking crystals of dolomite with a small 
amount of calcite. As these minerals occur here in almost the same condi- 
tion as to size, shape, and position of crystals, it is almost impossible to 
distinguish the two except by treatment with acids. All of the crystals 
are relatively irregular, the rhombic outlines being rarely perceptible. The 
opaque white color of the rock is apparently due to the fact that the larger 
dolomite crystals contain many small inclusions of carbonate; these serve 
to break up and reflect incident light. 
This is a very pure dolomitic marble, containing no bodies of quartz 
nor any other foreign minerals. Since the complete recrystallization of 
the mass the rock has been fractured and in places brecciated. The cracks 
thus formed are now sealed with veins of fine-grained calcite, the trans- 
parency of which contrasts with the somewhat cloudy body-material of the 
rock. 
Rocks OF IGNEOUS ORIGIN. 
The igneous rocks of the Han river district readily divide themselves 
into two groups. The first group contains the granites, and of these we 
have no specimens.* The other division comprises holocrystalline rocks 
of basic composition, the majority of which may be classed as gabbroids. 
In all cases the rocks occur as rather deep-seated intrusions. Neither 
surface volcanics nor porphyry dikes were observed at any point along our 
route in western China. 
Black amphibolite, No. 125.—This rock occurs as a dike in the meta- 
morphosed Carboniferous strata, in the valley of the Han river near Shi- 
ts’iian-hién. Specimen obtained in the tributary ravine on the right bank 
of the Han river, about 1 mile, 1.5 kilometers, below Shi-ts’tian-hién. 
*For a description of Post-Carboniferous granite of Tsin-ling mountains, see page 440. 
