238 RESEARCH IN CHINA. 
The Pei-t’ai surface is capable of two interpretations, between which 
it is difficult to discriminate. It may either have been a well-developed 
peneplain, or an aged surface which was still relieved by monadnocks 
rising above broad and shallow valleys. If it ever was a nearly flat pene- 
plain it was later slightly elevated and suffered moderate erosion resulting 
in the development of low, rounded residuals, of which Pei-t’ai is the 
type. If it never was well planed, Pei-t’ai and other similar summits 
are monadnocks reduced from higher elevations. The distinction is one 
which belongs to a finer classification of physiographic history and can not 
be made without more adequate criteria than we now have. It is not 
important, for the region had in either case undergone prolonged erosion 
and had been reduced to the aspect of advanced old age. 
The aged surface of the Wu-t’ai-shan is covered by residual soil, a 
mantle of decayed rock or saplite. The material is a reddish sandy clay, 
which consists of the products of decomposition of the schists, in which 
there yet remain many angular joint-blocks of the obdurate rocks. The 
superficial aspect of the mantle is seen in Fig. A, Plate XXXII, where 
it covers a wind-swept pass at 8,000 feet, 2,400 meters, above sea; and its 
internal constitution may be inferred from the gullies and coarse deposits 
of rock fragments in Fig. B, Plate XXXIII. We regard this mantle of 
saplite as the lowest part of a deep zone of decayed rock, which accumu- 
lated during the Pei-t’ai stage, and which has been greatly denuded during 
subsequent stages. In most districts of eastern and central Asia it has 
been completely removed from its original areas, and has furnished much 
of the materials of the Huang-t’u formation; but on the Wu-t’ai-shan it 
remains within the surviving area of the Pei-t’ai form. 
Recognition of the Pei-t’ai stage and of the profound erosion which 
was accomplished before its close is important to an understanding of 
the relations of the present topography to the structure of the mountains. 
It follows from that recognition that whatever heights had existed before 
or during the beginning of the Pei-t’ai had vanished by its end, and 
therefore no mountains due to folding then survived. 
T’ANG-HIEN STAGE. 
The T’ang-hién stage corresponds with that phase of topographic 
development which is represented by the mature surface carved on the 
Pre-Cambrian schists and the Paleozoic strata. It is recognized by its 
topographic forms of which the most characteristic are the wide valleys 
associated with numerous and more or less extensive monadnocks. It is 
typically developed in the northern Loess Basins and near T’ang-hién, 
Chi-li, in the hills along the western margin of the Great Plain. Its name 
is taken from the latter locality. 
