ROCKS FROM NORTHERN AND CENTRAL CHINA. 431 
Excluding the porphyritic hornblendes, the rock is a biotite gneiss in 
which the microscope reveals a granular mosaic of quartz and alkali feldspars. 
The small ragged flakes of biotite are parallel, but are not sufficiently abun- 
dant to form continuous seams. Accessory ilmenite, apatite, and zircon 
complete the list of minerals present. All of these constituents are probably 
secondary products of recrystallization in the anamorphic zone. 
The large hornblende crystals traverse this matrix at random. Many 
are roughly parallel to the banding, but not a few cross it at high angles and 
include portions of the schistose bands (Plate LVII, Fig. B). Obviously 
the hornblendes have grown in the deep-seated zone since the gneissic 
structure was produced. 
More recent katamorphic changes in this specimen have resulted in 
kaolinized feldspars, the local alteration of biotite to chlorite and epidote, 
and of hornblende to chlorite with a little calcite. 
The origin of this amphibolite can not be declared with confidence, 
although its association with quartzite and pelite schist in the field leaves 
little doubt that it is a sedimentary rock. Amphibolites of closely similar 
character which have been derived from impure limestones are described by 
Emerson* from the metamorphosed Paleozoic sediments of New England. 
The change in such cases involves the loss of CO, and the silication of the 
bases. In order to produce the high percentage of quartz, feldspar, and 
hornblende it would seem necessary to have either a rock which was 
originally very impure or one from which a large part of the CaCo, has 
been removed, thus increasing the relative importance of the impurities. 
In either case a considerable decrease in the volume of the limestone would 
be expected. 
Rocks OF IGNEOUS ORIGIN. 
The Wu-t’ai rocks are traversed by a few intrusions, of which some have 
been subsequently metamorphosed, while others are unaltered. The first 
class includes granitic batholiths and basic dikes; the second consists of 
quartz-and-feldspar porphyries. The granites and greenstones are prob- 
ably of Algonkian age, but the porphyries are so little altered or deformed 
that we may well suppose them to be much younger. 
GREENSTONES. 
Under this convenient field name may be grouped the dark hornblendic 
derivatives of diabases, basalts, gabbros, diorites, etc. Our specimens are 
both recrystallized, but one is massive and the other is schistose. In the 
Wu-t’ai district they occur as thick black dikes traversing the folded Algon- 
kian rocks. 

* Emerson, B. K., Geol. of Old Hampshire Co., Mass., U.S.G.S. Mono. xxrx, pp. 300-306, pls. v and VI. 
