ROCKS FROM NORTHERN AND CENTRAL CHINA. 459 
All of the minerals in this rock, with the possible exception of the iron 
ores, appear to be of secondary origin. There is no trace of mechanical 
deformation in the specimen, but the entire substance of the rock seems to 
have recrystallized into new minerals. Originally the rock was probably 
a basic gabbro. 
Potkilitic saussurite gabbro, No. 137.—This gabbro is a fair sample of 
the basic intrusives* which are common along the Nan-kiang in southern 
Shen-si. The rocks usually occur in the form of large dikes, scores of feet 
in thickness. All of these intrusions break through folded Paleozoic rocks 
and are almost certainly of Post-Triassic age. Specimen collected 0.5 mile, 
0.8 kilometer, northeast of Pai-kiu-hia. 
The rock is holocrystalline, but the different constituents are not 
readily distinguished in the hand-specimen. The mass has a uniform 
dark-gray color somewhat lighter in indefinite patches. Owing to the 
intergrowth of large pyroxenes and small feldspars, fresh surfaces exhibit 
the feature which has been variously called “luster mottling’’ or poi- 
kilitic texture. Grains of ilmenite and pyrite are easily seen with the 
unaided eye. 
The rock in its present condition consists of augite and alkali feldspars, 
together with such accessory minerals as brown hornblende, apatite, 
ilmenite, pyrite, titanite, and zircon. 
The augite has a very pale violet-red color, but is not pleochroic. 
The outlines of the large augite crystals are controlled by the numerous 
feldspar laths and are therefore quite irregular. Along the various cracks 
and edges the pyroxene has been altered, producing a fibrous green chlorite, 
calcite, and minute needles of a colorless amphibole. These needles extend 
out from the cracks into the chlorite, and they always preserve a parallel 
orientation with reference to the principal cleavage of the original pyroxene. 
From the absence of color the needles are believed to be tremolite. It is 
inferred that the augite is not rich in iron, for magnetite and other ferru- 
ginous minerals have not been produced in the process of alteration. 
The feldspars which are now present are albite with a little orthoclase. 
It is strongly suspected, however, that these are not primary minerals. 
This inference is supported by the fact that the alkali feldspar occurs in 
clear, fresh-looking crystals associated with an abundance of zoisite and 
epidote. In order to produce these lime-bearing minerals in such large 
quantities, there must have been present some primary mineral which was 
rich in lime. Augite contains a small amount of calcium, but the altera- 
tion of the pyroxene has not proceeded far, and much of the lime is 

*A brief description of this rock, under the name of diabase, has been published by Dr Karl Vogelsang 
(Peterm. Mitth, 1904, p. 18). 
