ROCKS FROM NORTHERN AND CENTRAL CHINA. 449 
there is serious difficulty in the fact that other constituents of sands, such 
as quartz and feldspar, are not represented. 
Rosenbusch* and others have described rocks of a somewhat similar 
nature, under the name of ‘‘Knotenschiefer,’’ from the outer zones of 
contact-metamorphism near massive igneous intrusions. ‘The first writer 
considers them in some cases to be pseudomorphs after such contact- 
minerals as chiastolite and cordierite. In the described rocks, however, 
there is apparently no such definite arrangement of the particles parallel to 
planes of sedimentation as is characteristic of our specimen. Possibly 
the composition of certain laminze in the rock was more favorable to 
the formation of these knots, and thus they developed in layers instead of 
promiscuously, as in the German examples. 
Olive-gray clay slate, No. 128.—This is one phase of a prominent forma- 
tion of greenish slates, which appears to underlie the black slates and lime- 
stones of the anthracite coal-measures on the upper Han river and is 
regarded as equivalent to the Sin-t’an formation. Specimen from the right 
bank of the Han river, 4 miles, 6.5 kilometers, below the village of Han- 
wang-ch’6ng, Shen-si. 
A soft, clay slate of olive-gray color, with a suggestion of silky luster on 
the flat faces. Slaty cleavage is well developed and the original bedding 
is not distinguishable in the hand-specimen. In weathering the rock 
assumes a light ocherous brown color through the production of limonite. 
Under the microscope it appears to consist largely of a felty mass of 
exceedingly minute crystals. Quartz and sericite, with a few less common 
constituents, are mingled with a clay-like material of olive-green color. 
Magnetite and quartz occur as porphyritic grains, which were probably 
original constituents of the clay. As in many other slates, ‘‘ thonschiefer- 
nadeln’”’ (supposed to be rutile needles) are scattered through the rock. 
Slide No. 128 B, which is cut across the cleavage, shows the slaty 
character of the rock admirably (Plate LVII, Fig. D). At frequent inter- 
vals along the cleavage planes, there are lenticular bodies of quartz, 
which have been produced from more rounded grains by diagonal slicing 
and, less commonly, by granulation. Some of the smaller grains of quartz 
are not fractured, but show, by undulatory extinction, that they have 
been subjected to mechanical strain. With the exception of these grains 
the constituents of the rock are all arranged in thin lamine, which are so 
uniform and exactly parallel that the section has the appearance of a coarse 
woof of cloth before the warp is added. ‘These lines are partly composed of 
minute parallel sericitic flakes and a light-green micaceous mineral which 
is probably chlorite in exceedingly fine particles. These micas have their 
*Physiographie der Mineralien, 3d ed., vol. 11, p. 90. 
