ROCKS FROM NORTHERN AND CENTRAL CHINA. 433 
The aphanitic ground-mass is probably feldspathic, for it is now altered 
completely to grayish kaolinitic products sprinkled with specks of magnetite. 
Indefinite round blotches are suggestive of spherulites, but there are no 
other structures characteristic of glassy rhyolites. 
Although greatly altered by weathering this porphyry shows no marks 
of strain or distortion. It has evidently been intruded after the last epoch 
of compressive deformation of the crust in the Wu-t’ai region. ‘The folding 
of the Sinian and Carboniferous strata probably occurred in the Mesozoic, 
and the porphyries, which do not record the folding, may be regarded as 
even younger. 
Rocks OF DOUBTFUL ORIGIN. 
Two gneisses of uncertain origin are included in this division. They 
are closely associated with the Wu-t’ai rocks, but are not known to be of 
the same age. 
GNEISSES. 
Biotitic augen-gneiss, No. 83.—The gray augen-gneiss was observed at 
two points along the T’ai-shan-ho, and in both situations forms a homoge- 
neous body of broad extent. The specimen was obtained from the tributary 
valley, 4 miles, 6.5 kilometers, south of Shi-tsui (Fig. 20, g), but identical 
fragments might easily have been found above Shi-tsui (Plate XVIII, 
stratum 21). The available evidence indicates that the gneiss is a metamor- 
phosed granitic intrusive. 
A black-and-white gneiss dotted with lenses of pink feldspar. The 
seams of biotite are notably wavy and reticulate, and the cleavage is 
imperfect. 
The augen-spots are occupied by alkali feldspars (orthoclase and oligo- 
clase) and more rarely by quartz. The quartz bodies are granulated and 
have been deformed into lenticular shapes. The feldspars, some of which 
are Carlsbad twins, show strains, but have preserved their integrity to a 
much greater degree. About their edges they have been granulated, and 
the resulting debris, mingled with the quartz of the matrix, has been largely 
recrystallized. Internally microcline grating has been developed in shape- 
less blotches. ‘The microcline is always free from the inclusions of epidote, 
zoisite, and mica, which the orthoclase contains. The fact indicates that 
the microcline is the recrystallized product of the other feldspar. 
The mass in which the numerous augen are inclosed is a typical 
biotite gneiss. Quartz, orthoclase, and albite form ill-defined granular 
seams separated by short wisps of biotite flakes. These wisps bend out 
around the eye-spots and envelop them. ‘Titanite and calcite occur spar- 
ingly as constituents of this gneiss, forming irregular crystals like the 
