ROCKS FROM NORTHERN AND CENTRAL CHINA. 401 
rocks are basalts and syenite porphyries. They occur for the most part 
in the form of dikes and sills, but some of the basalts form extrusive lava 
flows. In regard to age, some of them are contemporaneous with the Po- 
shan series, while others evidently appeared later than even the Sin-t’ai 
beds. The entire volcanic epoch, however, is very remote from the present. 
Since that time the region has been deeply eroded, so that all trace of the 
old volcanoes and lava flows as topographic features has long since disap- 
peared. 
SYENITE PORPHYRIES. 
Augite-syenite porphyry, No. 55.—This gray syenite porphyry occurs 
in the form of large dikes cutting across the red sandstones of the Sin- 
t’ai formation, southeast of Chéu-ts’un. Richthofen observed this rock 
during his journey from Chdéu-ts’un to Po-shan and described it as a “‘ basic 
eruptive rock of porphyritic character, with a dark ground-mass and crystals 
of light feldspar.’’* The rock differs from the porphyries in the vicinity 
of Yen-chuang chiefly in the fact that the ferro-magnesian mineral is augite 
rather than hornblende. The chemical composition is doubtless not notably 
different in the two cases. Specimen collected from a 10-foot, 3-meter, 
dike in red sandstones, 6 miles, 9.5 kilometers, southeast of Chou-ts’un, 
on the main road to Tzi-ch’uan-hién. 
A dark-gray porphyry in which the majority of the phenocrysts are 
gray feldspars and the rest are black augite. These phenocrysts are for 
the most part small, not exceeding 2 or 3 millimetersin breadth. Internally 
the rock consists of a matted ground-mass of alkali feldspars, associated 
with various other minerals in subordinate amount. Inasmuch as the 
ground-mass is visibly crystalline and the majority of the feldspars alkaline 
in composition, the name “‘syenite porphyry’’ seems most appropriate. 
The phenocrysts of plagioclase are decidedly idiomorphic and twinning 
according to the Carlsbad law iscommon. ‘They are now so much altered 
that it is difficult to identify them with certainty. The extinction angles, 
however, indicate that the composition is near that of labradorite. The 
alteration of this feldspar produces the usual grayish material of which 
kaolin appears to be the principal constituent. 
The augite phenocrysts are not numerous and seldom show their 
proper crystal forms. They also differ from the feldspars in the fact that 
they are but little decayed. Large primary crystals of magnetite are 
usually associated with these phenocrysts of pyroxene, but in the rest 
of the rock it is abundant only in small grains. 
The ground-mass consists largely of an intergrowth of irregular laths 
of feldspar which is unstriated and which possesses a very low index of 
* China, vol. 0, page 201. 
