130 RESEARCH IN CHINA. 
to the ridges surrounding the deeply filled loess basins. Our observations 
necessarily covered but a small part of even these available exposures, 
and in mapping we have delineated the general distribution only. 
The best section of the Téu-ts’un group is that observed between 
the Ts’ai-shi-ling and Wu-t’ai-hién. Strata similar to lower members of 
that section occur east of Liu-ytian. 
The Tung-yti limestones are exposed in the ridge east of Tung-yti, 
and their stratigraphy is given in the description of that section. They 
were observed in isolated outcrops elsewhere, but no other continuous 
section was seen. 
PRE-TA-YANG UNCONFORMITY. 
The Ta-yang limestone, which, in Chi-li, is the probable representative 
of the Hu-t’o system, lies directly upon the T’ai-shan complex, the Wu-t’aj 
system being lacking. The general characteristics of the Ta-yang and 
T’ai-shan would lead us to anticipate an unconformity, and the fact was 
established by the observation of very good exposures. 

Fic. 26 (Blackwelder).—Wan-hién, Chi-li. Unconformable contact of Ta-yang limestone 
(Algonkian) upon T’ai-shan complex (Archean). 
One occurrence is near Wang-k’uai-chén in the F 6u-p’ing district 
(Fig. 30). The basal shaly member of the limestone, dipping at a moderate 
angle, lies in manifest unconformity upon the weathered and eroded surface 
of the gneiss. The high hill immediately east of Féu-p’ing-hién consists 
mainly of gneiss, but is crowned by nearly horizontal shales and flinty 
limestone of the Ta-yang. 
Six miles, 9.5 kilometers, northwest of Wan-hién, a castellated butte 
affords an admirable exposure of the unconformity (see Fig. 26). The top 
of the butte consists of 110 feet, 33 meters, of the typical flinty limestone of 
the Ta-yang formation, beneath which are ro to 15 feet, 3 to 4.5 meters, of 
gray calcareous shales. The shales lie directly upon the gneiss, without the 
intervention of any conglomerate or arkose. The exposure is too limited 
to show any considerable extent of the surface of the basal complex, but 
the ancient topography seems to have been only slightly undulatory. The 
plain extends across the truncated edges of the layers in the gneiss. 
In the trip to Nan-t’ang-mei, Willis observed the cherty limestone 
within 16 feet, 5 meters, of mica-gneiss of the T’ai-shan complex. The 
limestone dips gently northwest and lies at the base of the great thickness 
