434 RESEARCH IN CHINA. 
quartz and feldspar. The abundant epidote associated with the biotite is 
probably an alteration product of that mineral, as is the chlorite. 
Two hypotheses concerning the origin of this rock appear tenable: (a) 
that it is a metamorphosed graywacke, and (b) that it was a porphyritic 
granite. If derived from the graywacke the existence of calcite as an 
integral part of the gneiss may be explained. The mineral occurs in com- 
pact allotriomorphic crystals which seem to have been formed at the same 
time with the quartz and feldspar. The explanation does not agree well, 
however, with the field occurrence of the rock. It is difficult to conceive 
of an arkose formation so thick and so uniform in constitution that, although 
it lies highly inclined and has an exposure 4 miles, 6.5 kilometers, broad, no 
considerable variations either in texture or in composition could be detected. 
We should confidently expect to find local beds of quartzite, conglomerate, 
or shale in an arkose of such enormous thickness. The second explanation 
fails to account satisfactorily for the calcite, but as it agrees with the other 
facts it is regarded as more probable than the other. If we suppose that 
the augen-gneiss is an ancient granite either intruded into the Wu-t’ai rocks 
or overthrust upon them from below, the uniform character and great 
extent of the rock may be explained. The twinned feldspars in the augen 
may be regarded as porphyritic crystals, as yet not destroyed by metamor- 
phism, which has given the mass its gneissic structure. 
Chlorite-muscovite gneiss, No. 103.—This specimen was taken from the 
top of Pei-t’ai, the highest peak of the Wu-t’ai range. The summit of the 
mountain consists of the gneiss, but in the adjacent slopes mica schists 
appear. ‘The relations between the two are unknown. 
Our weathered specimen is a mottled reddish-gray rock indistinctly 
streaked with dull green. It is neither banded nor readily cleavable, and 
yet a trace of parallel structure is observable especially in larger masses. 
The texture is granitic and moderately fine. Under the microscope the 
rock is separable into two kinds of material: (a) rounded or subangular 
bodies of feldspar and less quartz, and (6) the schistose material which 
binds them together and incloses the small accessory crystals of other 
minerals. 
The feldspars are much obscured by inclusions of muscovite, zoisite, 
etc., but the range of extinction angles indicates that orthoclase, albite, 
and labradorite are present. Most of the feldspar grains are enveloped in 
wavy seams of the schistose portion and thus form eye-spots. Some of 
them have been broken and the fragments dragged apart only to be rece- 
mented by secondary feldspar and muscovite. The less numerous quartz 
bodies are granular lentils which have doubtless been produced through the 
crushing of single quartz grains. The occurrence coincides with that of the 
feldspar. 
