STRATIGRAPHY OF CHI-LI AND SHAN-SI. 123 
meters, southwest of Shi-tsui, Sargent brought a specimen of typical gray 
Hu-t’o limestone, and reported that the beds dip gently southwest. The 
basal portion of this mountain is known to consist of highly inclined 
Wu-t’ai schists, and it is reasonable to infer that the unconformable con- 
tact of Hu-t’o series upon Wu-t’ai occurs in the upper slopes of the peak. 
We have sketched the line of unconformity upon the map, according 
to the observed structure and inferences from distant views. 
HU-T’O SYSTEM. 
General statement.—Between the highly metamorphosed schists of the 
Wu-t’ai system and the unmetamorphosed strata of the Sinian system, we 
find in northern Shan-si a succession of strata consisting of slightly altered 
argillites and flinty limestones, which we have called the Hu-t’o system,* 
on account of the typical development in the hills near the Hu-t’o river, 
from Hin-k’ou to Tung-yti (atlas sheets B I and CI). The terrane is also 
wide-spread in the vicinity of Wu-t’ai-hién, which is the central point of 
the district in which we saw it. 
It is distinguished from the Wu-t’ai schists by the very much more 
intense metamorphism of the latter, and the two are without doubt uncon- 
formable. From the Sinian or earliest Paleozoic it is separated by an 
unconformity, which marks a prolonged interval and various geologic 
changes. It is thus a Pre-Cambrian series, and of those which came under 
our observation it is the latest. 
In its typical district we divide the Hu-t’o system into two groups, 
based on the probable structural relation of the detached and incomplete 
sections we were able to observe. The lower, consisting chiefly of slates 
or argillites interbedded with subordinate members of quartzite and crys- 
talline dolomite, we name the Téu-ts’un group, since the strata occur in the 
hills about that village. The upper group, which is composed of massive 
gray flinty limestone with thinner beds of argillite and quartzite, we call the 
Tung-yii group from its typical occurrence in the ridge east of that village. 
The Téu-ts’un group of the Hu-t’o system is known only in its typical 
locality in northern Shan-si. The Tung-yiti group also is as yet recognized 
only in that district, unless we regard the Ta-yang limestone as its repre- 
sentative. Outside of the Wu-t’ai-hién region the Ta-yang limestone is of 
wide-spread occurrence. Judging from the descriptions given by von 
Richthofen, it is found not only in the mountains west of Peking, in the 
vicinity of the Nan-k’ou pass, but also toward the northeast, probably 
- extending beyond the Great Wall; and in the vicinity of Téng-chéu-fu, 
in Shan-tung, similar rocks appear to be well developed. 
*In all probability the Hu-t’o is the littoral phase of the siliceous limestones observed by von Richt- 
hofen in Nan-k’ou pass, northwest of Peking. If so, Nan-k’ou system has priority as a systematic term 
of general application, and Hu-t’o should be restricted to the local development of the strata. 
