. ROCKS FROM NORTHERN AND CENTRAL CHINA. 463 
and interwoven rather than strictly parallel. The fact that the rock is 
cohesive rather than slaty is probably due to this interlacing of the fibers. 
There can be little doubt that the green hornblende is a secondary 
derivative from pyroxene, by the process commonly referred to as ‘‘uraliti- 
zation.”’ Although no traces of that mineral remain, sections of some of 
the less distorted hornblendes are nearly rectangular instead of rhombic, 
and thus suggest the form of augite. This hornblende occurs both as 
irregular compact bodies in the ‘‘augen,’’ and in the form of fine needles 
arranged with their axes parallel to the schistosity. The latter often indi- 
cate by their relation to the larger bodies that they have been derived from 
the latter by “‘slicing’’* or by recrystallization of the hornblendic material, 
or by both processes combined. 
The original feldspars of the rock were presumably lime-bearing 
plagioclases. They are now completely saussuritized, having been replaced 
by finely granular zoisite, quartz, and alkali feldspar. These materials are 
scattered through the ground-mass or occur in the lenticular areas in which 
phenocrysts formerly existed. In these bodies it is not uncommon to 
observe yellowish centers, such as Williams described with reference to 
some of the Michigan greenstones.} 
The minerals of this greenstone are almost all of secondary origin. 
The original rock was probably a basaltic porphyry, 7.e., one composed in 
large measure of pyroxene and a basic feldspar. It appears to have under- 
gone two distinct episodes of alteration: first, a mass-static change in which 
pyroxene was transformed to hornblende, and feldspar to saussurite; 
second, a mass-mechanical transformation which developed the schistose 
structure. In the latter process the materials of the ground-mass became 
arranged in rough parallelism, the amphiboles in long fibers and the other 
minerals in attenuated streaks of granules. At the same time the pseudo- 
morphs, after the phenocrysts were deformed into lenticular bodies, were 
fractured, faulted, and bent, and were reduced by marginal slicing and 
recrystallization. The mass-mechanical change was not permitted to pro- 
ceed very far toward the ultimate goal of such a process, which would be 
the formation of a coarse-grained amphibole schist. While undergoing 
the first of these alterations the rock was probably in the zone of kata- 
morphism;{ but during the second period it must have been subjected 
to greater pressure than obtains in that zone. The intense folding, which 
the rocks of that locality are known to have suffered, is probably ample to 
account for the moderate deformation of this greenstone. Since the close 
”) 

*C. K. Leith: Rock Cleavage, Bull. 239, U. S. Geol. Surv. p. 30. 
+ U.S. Geol. Surv. Bull. 62, p. 145. 
{Van Hise: A Treatise on Metamorphism, Monograph 47, U. S. Geol. Surv., pp. 260 and 279. 
