118 RESEARCH IN CHINA. 
The thickness of the Nan-t’ai group, measured from the lowest con- 
glomerate of the Si-t’ai group to the overthrust fault, appears to be about 
2,000 feet, 600 meters. Within this section obscure folds may cause 
reduplication of strata, and thus necessitate a reduction in our estimate of 
thickness. A further correction, which can not be calculated, is demanded 
by the changes in thickness attendant upon the development of schistosity 
in certain layers. 
Strata of the Nan-t’ai group extend northeastward beyond the T’ai- 
shan-ho toward Tung-t’ai, a reddish quartzite assigned to the group forming 
the ridge 24 miles, 4 kilometers, south of the peak. 
The Shang-ho-miau section consists of coarse white marble, garnet- 
schists, biotite-schist, chlorite-schist, and dark quartzites, which, though 
mineralogically similar to the Nan-t’ai strata, are nevertheless separated 
from them by an overthrust fault, and are especially distinguished by the 
purity of the marble. 
The overthrust between these two bodies of strata was observed in 
the canyon of the western branch of the T’ai-shan-ho and in the valley 
of the eastern tributary, north of Shang-ho-miau, and appears to be an 
important structural feature of considerable displacement. ‘The strata on 
the overthrust side, the lower ones of the Nan-t’ai group, are probably older 
than those in the underthrust mass. Both groups are characterized by 
the presence of dark quartzites, marble, and schists. The degree of meta- 
morphism is similar in the two cases. While each group contains rocks 
not present in the other, nevertheless the similarities seem to be more 
pronounced than the contrasts. Therefore we have mapped the rocks of 
the Shang-ho-miau section as Nan-t’ai, with a question. 
On the northern slope of the Wu-t’ai-shan dark ferruginous quartzites 
were observed by von Richthofen and Willis in the O-shui valley. From 
the summit of Pei-t’ai similar resistant rocks were seen flanking the north 
side of the mountain. Here, as in the section just described, the quartzite 
appears to lie beneath the green schists of the Si-t’ai group. 
Si-t'ai group.—The typical rocks of this group are the green chlorite- 
schists and phyllites of the central mass of the Wu-t’ai-shan. Along the 
southeast side of the outcrop they grade downward into schistose quartzites 
and arkoses, beneath which lie coarse conglomerate-schists. The chlorite- 
schists have the composition of non-ferruginous shales, and as they pass 
into the derivatives of quartzites we infer that they are of sedimentary 
origin. 
The transition from shale downward through sandstone and arkose 
into conglomerate is highly suggestive of basal strata lying upon an uncon- 
formable contact. This contact is now so metamorphosed that it was 
