ROCKS FROM NORTHERN AND CENTRAL CHINA. 419 
specimen was obtained from the central part of a dike 4o feet, 12 meters, 
thick, which traverses gneiss 4 miles, 6.5 kilometers, west of Féu-p’ing. 
The aphanitic ground-mass is dull brown in color. In it are embedded 
medium-sized phenocrysts of pink orthoclase and plagioclase, glassy quartz 
and green hornblende. The feldspars are much the most prominent features 
of the rock. The ground-mass is a microcrystalline mass of quartz and 
feldspar granules, containing bits of hornblende, iron ores, and other minute 
accessories. 
With the exception of quartz all the phenocrysts are extensively 
decayed. Both feldspars are thus replaced by saussurite to such a degree 
that the plagioclase is difficult to identify; it seems to range from oligo- 
clase to andesine, in zonally built crystals. Scarcely any of the hornblende 
remains intact. It is now represented by perfect pseudomorphs, which 
consist largely of chlorite associated with epidote-zoisite, delessite, and 
calcite. The small grains of magnetite, titanite, and zircon are unaltered. 
Aside from such katamorphic alterations as these, no changes have 
been observed. ‘The rock has not been subjected to anamorphic processes 
nor even to effective stresses. Its undeformed condition, as well as its 
mineralogical contents, leads us to compare it with the porphyries along 
the T’ai-shan-ho and the similar dike which traverses the Ta-yang lime- 
stone near T’ang-hién. It is probably Post-Algonkian and not improbably 
Post-Carboniferous in age. 
White garnet aplite, No. 69.—Small dikes and sheets of felsite accom- 
pany the granite porphyries which intrude the gneisses near T’ang-hién. 
This specimen was taken from a sheet 2 feet, 0.6 meter, thick, near the 
summit of a hill 2 miles, 3 kilometers, west of Chuang-li village. 
Externally the rock is white, minutely flecked with brown, green, and 
red particles, which the microscope shows are dark epidote and garnet. 
The arrangement of these specks suggests a parallel structure and yet the 
rock is not in any sense schistose. The white aphanitic portion is composed 
of fine granules of quartz and alkalifeldspar. The other minerals are unim- 
portant in quantity and are arranged in discontinuous streaks. Small 
granules of garnet and epidote with parallel flakes of a pale pleochroic 
mica are the only common varieties present. 
The faint parallel structure of the mass is an evidence of effective 
mechanical deformation. The existence of the garnets and parallel micas 
is additional proof that the felsite has been subjected to anamorphic condi- 
tions. From the intimate association between garnet and epidote we 
infer that it also developed under mass-static conditions in the deeper 
zone. ‘These metamorphic features indicate that the rock is not younger 
than the gray granite porphyries of Algonkian age. 
