418 RESEARCH IN CHINA. 
At first glance the rock appears to be a fine-grained gray granite, but 
the microscope shows that it consists of a felsitic ground-mass in which 
numerous larger crystals of quartz, feldspar, and biotite are embedded. 
The largest and most conspicuous of these phenocrysts are biotite. 
The matrix is an irregular granular mass of quartz and limpid feldspar, 
with minute shreds of biotite and secondary minerals. 
Among the phenocrysts quartz and feldspar are about equally numer- 
ous, but the latter forms the larger bodies and is, therefore, present in 
greater volume. Both are imperfectly idiomorphic. The feldspars are 
chiefly orthoclase with a little microcline and oligoclase. 
This specimen is extensively altered. Abundant muscovite and epi- 
dote have been produced from feldspars and biotite. The alteration of 
biotite to muscovite is unusual and apparently has not been recorded 
previously. In this slide the colorless mica may be observed feathering 
into the biotite after the manner of chlorite. The change involves a loss 
of magnesium by the biotite, but as no secondary magnesium mineral 
is present we can only suppose that this element has been removed in 
solution. All of the alterations noted appear to be those of the belt of 
cementation. 
Buiotite-granite porphyry, No. 68.—Like the last the rock was found in 
small dikes cutting the T’ai-shan gneisses northeast of T’ang-hién. 
A clear-gray rock in which the biotite flakes are smaller and the ground- 
mass more aphanitic than in No. 67. It is also a much fresher specimen 
and therefore worthy of more detailed description. 
The felsitic ground-mass is minutely and uniformly granular. In 
composition it resembles that of No. 67. The phenocrysts are prevailingly 
idiomorphic, and biotite is subordinate to quartz and feldspar. 
Muscovite, epidote, and zoisite occur in small irregular bodies, fre- 
quently unassociated with the minerals from which they have been derived. 
In this slide, as well as in the last, partial pseudomorphs of muscovite after 
biotite may be observed. 
Marks of deformation are limited to strain-shadows in the larger 
crystals and local spots of microcline grating in the feldspars. The exist- 
ence of long unbroken prisms of apatite is further proof that the rock has 
not been severely deformed. 
These granite porphyries at T’ang-hién resemble the gray granite 
above Foéu-p’ing-hién in their mineral constitution, except that they are 
devoid of hornblende. It is probable that both belong to a single episode 
of igneous activity in Algonkian times. 
Brown hornblende-granite porphyry, No. 78.—Dikes of this porphyry 
were observed near Féu-p’ing-hién both east and west of the city. Our 
