STRATIGRAPHY OF CHI-LI AND SHAN-SI. 121 
erosion must have been prolonged to expose the T’ai-shan. The deforma- 
tion may also have involved folding, for our knowledge of the structure is 
not sufficient to exclude the possibility of unconformity by dip between 
the Nan-t’ai and Si-t’ai groups. 
An overlap of the Si-t’ai terrane on the T’ai-shan complex is suggested 
by Willis’s observations in the western Wu-t’ai-shan. He saw no con- 
siderable thickness of quartzites, such as we should expect in the position 
of the Nan-t’ai or Shi-tsui groups, between the Si-t’ai schists and the basal 
complex near the northern base of the range, and it is not probable that 
they could have been overlooked, even in a rapid ride, if they were present 
in characteristic form. Either they are replaced by mica-schists or are 
wanting. ‘The latter seems more probable, and in that case the Si-t’ai 
group is, in this section, in contact with the T’ai-shan, in unconformable 
overlap. 
Our inferences regarding the correlation of the groups of the Wu-t’ai 
system may be summarized as follows: 
(a) The oldest rocks of the Wu-t’ai seem to be the Shi-tsui quartzites 
and schists. 
(b) The Nan-t’ai group is probably younger, although it perhaps 
belongs to a conformable sequence. 
(c) The Si-t’ai green schists are probably unconformable upon the 
Nan-t’ai strata and are regarded as the youngest group of the Wu-t’ai series. 
IGNEOUS INTRUSIVES. 
Igneous rocks do not play a conspicuous part in the Wu-t’ai district. 
With the exception of one or more batholitic masses of granite, the intru- 
sions observed were all dikes, usually of moderate size. No superficial 
volcanic accumulations of any kind have been seen. 
As these igneous rocks are of different ages, some of them profoundly 
metamorphosed, some moderately altered, and others essentially unchanged, 
we shall divide them, for purposes of description, into metamorphosed 
intrusives and unaltered dikes. 
Metamorphosed intrusives.—Prominent among the metamorphosed 
intrusives are gneissoid granites, which occur associated with the green 
schists. A little more than a mile southeast of the village of Wu-t’ai-shan 
the wall of the canyon exposes a gneissoid granite, in which biotite is the 
principal dark mineral. The extent of this body northward was not deter- 
mined, but at its southwestern edge it lies in contact with green chlorite- 
schist. The intrusive nature of the granite is proved by the dikes and veins 
of acid granite and pegmatite, which cut across the chlorite-schists abun- 
dantly in the neighborhood of the contact. 
