STRATIGRAPHY OF CHI-LI AND SHAN-SI. 119 
not distinguished in the field, but the evidence of the unconformity seems 
strong, and we regard the conglomerate as the base of the Si-t’ai group. 
In the southern part of the Wu-t’ai section the Si-t’ai rocks appear to 
lie in a monocline, dipping steeply to the north. The conglomerates occur 
on the northern slope of the range south of Ts’a-pu, where they dip but 23° 
northwest, and, overlying magnetic quartzite, are themselves overlain by 
chlorite-schist. Thus the normal sequence is repeated in the same order 
and direction, without, so far as we observed, any intermediate occurrence 
in reversed order representing a southern dip. It is indeed highly probable 
that the chlorite-schists form a syncline in the crest of the range, compris- 
ing the summits Si-t’ai, Chung-t’ai, and Pei-t’ai, and that the above-noted 
repetition is due to an overthrust from the north, which cuts out the anti- 
cline that would otherwise lie adjacent on that side. 
If this interpretation of the structure be correct, the uppermost strata 
are to be found along the summit of the range and may include the Pei-t’ai 
gneiss, if that rock be of sedimentary origin. 
This group of conglomerate arkose and chlorite-schists, which makes 
up the mass of the Wu-t’ai-shan, is called the Si-t’ai group from the western 
peak of the range. 
The original thickness of the Si-t’ai group can not be determined; the 
absence of distinctive strata and the intricate structure, involving schistos- 
ity, folding, and, thrusting, render ‘even an estimate worthless. The mass 
is, however, great, and probably was originally several thousand feet thick. 
Correlation of the Wu-t'ai groups.—Of the three groups which make up 
the Wu-t’ai system, as we have divided it, the Shi-tsui group is apparently 
the oldest. It lies adjacent to and above the T’ai-shan complex; contains 
psammitic deposits derivable by erosion from that complex, and in all prob- 
ability so derived; and exhibits a normal sequence of sediments, from the 
base upwards through coarser and finer psammites to pelites. It does not 
contain any pebbles of older sedimentary rocks, so far as we observed. We 
therefore place the Shi-tsui group at the base of the stratigraphic column, 
in a relation to the T’ai-shan complex similar to that of the lowest Algon- 
kian on the Archean. In thus stating the position of the group, we would, 
however, avoid any implication of correlation closer than the most general 
parallelism of grand divisions. 
Between the Shi-tsui and Nan-t’ai groups lies a mass of augen-gneiss, 
which interrupts the sedimentary sequence, and the base of the Nan-t’ai 
group is unknown, the lower part being cut off by an overthrust. The 
relation of the two groups is therefore indeterminate. We may speculate 
as follows: Since the Shi-tsui strata were heterogeneous sediments, grading 
upward from psammites to pelites, and the Nan-t’ai sediments were largely 
