ROCKS FROM NORTHERN AND CENTRAL CHINA. 395 
appears to consist of feldspars in minute irregular grains, together with 
some quartz and a ferro-magnesian constituent. 
The plainly visible crystals consist of plagioclase, hornblende, quartz, 
and decomposition products probably derived from biotite. Iron ores are 
rather abundant in small particles. 
The composition of the plagioclase appears to be intermediate and 
sodic rather than calcic. ‘The crystals are idiomorphic, but are now so 
completely altered to brownish-gray saussurite and kaolin as to make iden- 
tification impracticable. 
The hornblende crystallized earlier than the feldspar, as is shown by 
the fact that it is occasionally included in phenocrysts of the latter. It 
occurs, however, in much smaller and less regular crystals. Where least 
decayed the hornblende is bright green and moderately pleochroic, but 
the katamorphic alterations into chlorite, serpentine (?), etc., have reached 
an advanced stage. 
The least abundant of the three minerals which occur as phenocrysts 
are fibrous yellowish-brown plates, consisting of chloritic or serpentinous 
products. The shapes and cleavage of these bodies suggest that they are 
altered scales of biotite, but none of the original substance remains to 
verify the inference. 
Quartz occurs sporadically in small irregular bodies scattered through 
the ground-mass. Magnetite appears to be the only iron ore present which 
is primary in origin. 
Hornblende-dacite porphyry, No. 42.—This porphyry was found as an 
intrusive sheet in the shales of the Man-t’o formation north of Sin-t’ai-hién. 
The lava seems to have had but little effect upon adjacent sediments. 
Within a few inches of the porphyry the shales are somewhat indurated 
and in that narrow zone they have taken on slaty cleavage, but otherwise 
the effect seems to have been slight. Specimen from an intrusive sheet 
12 feet, 3.5 meters, thick and roo feet, 30 meters, in breadth, 1.6 miles, 
2.5 kilometers, east-southeast of Kau-kia-p’u. 
In this variety we find a close approach to the monzonite porphyries 
of Brégger. It is much like No. 44 and is doubtless a result of the rise 
of the same magma. The hand-specimen shows a dark-gray rock through 
which are scattered abundant moderate-sized phenocrysts of pale feldspar. 
The microscope reveals a finely granular ground-mass and altered 
phenocrysts of feldspar, hornblende, and mica, together with small amounts 
of iron ores, apatite, and zircon. ‘The constituents of the ground-mass are 
not easily distinguished from each other. The minute clear granules are in 
part, at least, quartz; while another large component is a limpid feldspar 
whose low index of refraction indicates that it is dominantly alkaline, 
