ROCKS FROM NORTHERN AND CENTRAL CHINA. 435 
The schistose matrix of the rock consists of wavy streaks of muscovite 
and chlorite associated with more or less granular quartz, bits of decayed 
feldspar, and accessories. Any of these minerals may in one area or another 
predominate over the rest. Muscovite is by far the most abundant. Locally 
swarms of little violet tourmaline prisms are inclosed in the muscovite mass; 
and elsewhere there are clusters of iron ore and epidote granules. ‘The 
derivation of the muscovite and chlorite from the demolished portions of 
the feldspars is observable along cracks in the latter, and there is no reason 
to doubt that a large part of the schistose portion has had the same origin. 
The interpretation of the intensely metamorphosed gneisses is, in the 
present state of knowledge, to be attempted with great caution, and yet 
there are facts in this case which appear to warrant an expression of opin- 
ion. ‘The Pei-t’ai gneiss contains no hornblende, biotite, nor other common 
ferro-magnesian mineral, and in that respect is unlike most granites. 
Even the muscovite appears to be a secondary product derived from 
feldspar. Another peculiarity of this rock which appears to be incon- 
sistent with an igneous origin is the wide range in the composition of the 
feldspars. This diversity is more easily explained on the hypothesis that 
we have here an arkose, the grains of which have been obtained from various 
rocks. The presence of calcite in considerable abundance as an integral 
part of the mass further inclines me to the opinion that the Pei-t’ai gneiss is 
a metamorphosed arkose rather than a deformed granite. 
HU-T’O SYSTEM. 
The younger Algonkian rocks grouped in the Hu-t’o system differ 
from those of the Wu-t’ai in being only slightly metamorphosed. The 
processes of the zone of anamorphism have affected them only in minor 
degree. We find, therefore, not schists and gneisses, but slates, graywackes, 
limestones, and quartzites. Igneous rocks are even less numerous than in 
the older system. They comprise only dikes of basic and acid eruptives. 
Rocks OF SEDIMENTARY ORIGIN. 
PSEPHITES, 
Arkose conglomerate, Nos. 106-107.—This arkose lies beneath a quartz- 
ite of unknown thickness and forms most of the spur which extends out 
into the plain south of the village of Fang-lan-chén, Shan-si. 
Von Richthofen,* thinking that the conglomerate lay upon the Sinian 
limestones, mistook this series for the coal-measures and has so represented 
it on his map. In the field it seemed probable that it represented the base 
of the Hu-t’o system, but the fact was not established. The study of the 
specimens under the microscope shows that the rock is much less metamor- 

* China, vol. U, p. 371- 
