STRATIGRAPHY OF CHI-LI AND SHAN-SI. 131 
of strata exposed in the mountain for 3 miles, 5 kilometers, in that direction. 
The outcrop of this unconformity continues west by south, past Si-ta-yang, 
and thence off under the alluvial plain. West of Ta-yang both the gneiss 
and the limestone have been reduced to low hills, and the contact is largely 
concealed beneath the Huang-t’u. 
TA-YANG FORMATION. 
General statement.—In western Chi-li we found extensive occurrences 
of a massive flinty gray limestone, the Ta-yang formation, which we 
named after the village of Si-ta-yang in the T’ang-hién district. It is 
separated from the Archean below and from the Cambrian above by 
pronounced unconformities, and belongs in the Algonkian. We regard the 
Ta-yang as equivalent to the Hu-t’o system of Shan-si and the Nan-k’ou 
limestone of von Richthofen. 
The Ta-yang formation is dominantly a limestone of gray color and 
dense texture. Von Richthofen found the formation in the Nan-k’ou 
mountains in northern Chi-li, lying beneath the known Sinian system in a 
position which led him to think that there was no stratigraphic break 
between them.* He therefore believed it to be a great marine phase of the 
SE 







SES 
eS 2 SIR BIO SLER gS AIRE ATL 
ESS PEL SEES SIRS EG 
SS SELES BEG 


Fic. 27 (Willis. Atlas sheet FI, section AA).—T’ang-hién, Chi-li. Ta-yang (Algonkian) limestone 
lying upon T’ai-shan (Archean) gneiss. Cambrian strata and unconformity on the left. 
lower Sinian. When, however, we had established the difference between 
them by observation of the unconformity, we recognized certain persist- 
ent characteristics, which should render them easily separable. Lime- 
stones of the Ta-yang (Nan-k’ou) system are rich in lamine and nodules 
of flint, in strong contrast to Sinian limestones, which are almost totally 
devoid of flint. Quartzite also appears at many horizons in the Ta-yang, 
but it has not been observed in the Sinian. Conversely, the dark oolitic 
and conglomeratic limestones, which are so characteristic of certain hori- 
zons in the Sinian, are not known in the older series. Fossils are abundant 
in the lower part of the Sinian, but none have been found in the Ta-yang. 
DISTRIBUTION. 
The T'a-yang limestone occurs in the low mountains which border the 
alluvial plain north of Wan-hién, and trends thence southwestward to the 
Sha-ho and beyond. It is well exposed in the mountains between T’ang- 
hién and Nan-t’ang-mei, and also between Si-ta-yang and Ning-shan. Small 

*Von Richthofen, China, vol. 0, page 317. 
