STRATIGRAPHY OF CHI-LI AND SHAN-SI. 107 
At the surface the rock is a mottled greenish and brownish granitoid gneiss, 
composed of quartz and feldspar with muscovite and chlorite. Most of the 
rock has suffered recrystallization during metamorphism, and the micas 
are apparently not the original femic* minerals. Furthermore, the rock 
of which a specimen was obtained was weathered and difficult to deter- 
mine. The Pei-t’ai gneiss is surrounded on the south and east by green 
chlorite-schists of the Si-t’ai group of the Algonkian Wu-t’ai system. 
Between the exposures of the typical gneiss on the one hand and the green 
schists on the other, there is an ill-defined belt of gray muscovite-schist, 
which appears to grade into the adjacent gneiss on the one side and into 
the schist on the other. The gentle slopes of the peak, however, are 
covered with soil through which only scattered outcrops of rock appear. 
The relation between gneiss and chlorite-schist was not, therefore, closely 
observed. 
We can not correlate the Pei-t’ai gneiss definitely. It may belong to 
the T’ai-shan complex or the Wu-t’ai system, or be intruded in the latter. 
If the Pei-t’ai gneiss belongs to the T’ai-shan complex, it underlies the 
Si-t’ai group and may appear in its present position, either on the axis of an 
anticline or above an overthrust fault. We at first entertained the view 
that the larger structure of the Wu-t’ai range was anticlinal, and that the 
Pei-t’ai gneiss was of the T’ai-shan complex, but more thorough study of our 
notes and specimens indicates that the structure is that of a closed syncline, 
or of a monocline in which the strata are repeated by overthrust. 
If the Pei-t’ai gneiss is a member of the Wu-t’ai system, it probably 
overlies the Si-t’ai group and occurs as the highest stratum in a syncline. 
The fold may be limited on the northwest by an overthrust. The consti- 
tution of the gneiss is consistent with a sedimentary origin, since it is prob- 
able that the conditions which have converted the uniform shale of the 
Si-t’ai group into chlorite-schist would change an arkose or graywacke 
into such a gneiss as the Pei-t’ai. The presence of much calcite and of mus- 
covite and chlorite, rather than hornblende or biotite, is suggestive. The 
assignment of the Pei-t’ai gneiss to the Wu-t’ai system, and to the superior 
position in that system, is thus qualified only by our lack of definite knowl- 
edge of the structure of the range. 
Equal consideration should be given to the possibility that the Pei-tai 
is intruded into the Wu-t’ai schists. The petrographic evidence does not 
exclude an igneous origin, and the apparently rounded mass is not unlike 
that of a batholite. The encircling belt of muscovite-schist may represent 
the zone of contact metamorphism with the Wu-t’ai rocks. 
* For use of this term to designate the ferromagnesian constituents of rocks see ‘‘Quantitative Classi- 
fication of Igneous Rocks” by Cross, Iddings, Pirsson, and Washington. 
