ROCKS FROM NORTHERN AND CENTRAL CHINA. 447 
The rock shows little evidence of metamorphism, except that the 
quartz may be recrystallized. No secondary minerals can be distinguished, 
nor are there any microscopic folds or thrusts. A few fractures which cut 
the rock in several directions have been healed with fresh quartz. 
Black clay slate, No. 139.—This is a characteristic but thin member 
of the middle Paleozoic shales on the head-waters of the Nan-kiang in 
southern Shen-si. The entire formation is evidently the equivalent of the 
Sin-t’an shales of the Yang-tzi valley. The specimen comes from the gorge 
of the Nan-kiang 5 miles, 8 kilometers, south of the town of Chén-p’ing-hién. 
A soft, coal-black clay slate in which the cleavage is well developed, 
but somewhat irregular, 7. e., the rock splits up into plates of uneven thick- 
ness and with somewhat rough surfaces. The only visible constituents 
of the slate are certain small bodies of pyrite and other minute grains which 
are not identifiable in the hand-specimen. On account of the high percent- 
age of coaly material which this slate contains, even the thin section appears 
almost black; and other components are rarely recognizable. Here and 
there are scattered minute bits of calcite and quartz, and occasional flakes 
of a colorless mica, too minute to be identified. Only a suggestion of par- 
allel arrangement can be noted with regard to the constituents of the rock, 
and it is evident that the recrystallization of the clay has only begun. 
It is highly improbable that the slaty cleavage of the rock is due to 
this trace of schistose arrangement of the minerals, which is so incon- 
spicuous a feature. It is evident that we have here an example of fracture 
cleavage as defined by Leith.* 
Crumpled black slate, No. 131.—The crenulated black slate is a local 
phase of the Wu-shan coal-bearing slates on the Han river above Hing-an-fu. 
Good exposures occur near Siau-tau-ho, and our specimen came from an 
outcrop near the coal-dump on the north bank of the Han near that village. 
A blackish aphanitic slate which has been deformed along two planes 
at a large angle to each other in such a way as to develop countless little 
parallel folds or crenulations. This gives the rock a striated appearance 
which is unusual and striking; the cleavage is of the type described by 
Leith} as ‘“‘intermediate between plane-parallel and linear-parallel cleav- 
ages.’’ The only visible minerals are a few oxidized grains of magnetite. 
The rock consists mainly of fine-grained quartz, the usual graphitic 
matter and minute micaceous flakes (kaolin?), the orientation of which 
corresponds to the schistosity in all its vagaries. The only other mineral 
which is not directly connected with the shear-zones seems to be tourmaline, 
which is scattered in small prisms. 

* Leith, C. K., Rock Cleavage, U. S. Geol. Surv. Bull. 239, p. 119. 
} Ibid., Plate I. 
