402 THE JOURNAL OF GEOLOGY. 
The time relation of the peridotite and quartz-porphyry is 
not known to the writer. 
At the south end of the Gold Belt in Mariposa county there 
is a very interesting series of igneous rocks, the oldest of which 
appear to be augite-porphyrites and their tuffs, and their dyna- 
mometamorphic derivatives, some of which may be Paleozoic, 
and others Jura-Trias in age. They in no way differ from simi- 
lar rocks that have been described above from the Jackson sheet 
area and elsewhere. These surface augitic volcanics are inter- 
bedded with schists and slates, and the entire series is cut off 
and metamorphosed by the granite a little south of the town of 
Mariposa (see Plate VII.) About Cathey Valley and westward 
from there to Hornitos are considerable areas of a dark granular 
rock some of which is seen under the microscope to be typical 
diabase with ophitic structure. In portions of the area the 
augite is replaced by brown hornblende, and the rock is then an 
ophitic diorite. No. 446, Sierra Nevada collection, probably 
represents nearly the average composition of this diabase. 
This specimen is from a dike by the road northeast of Hornitos. 
It is composed of augite and hornblende apparently inter- 
grown, in which are imbedded feldspar laths. The area about 
Cathey Valley forms much the largest area of true diabase 
known to the writer in the Sierra Nevada. As the rock occurs 
in dikes in the greenstone tuffs and schists (augitic tuffs and 
their derivatives), it is evidently younger than at least a portion 
of the greenstone series. 
Forming the high ridge east of Cathey Valley, and abutting 
against the coarser hornblendic granite that cuts off the slate 
series south of Mariposa, is a fine-grained granite in places prac- 
tically an aplite. This fine-grained granite may be but a modi- 
fication of the coarser granite, but is thought to be later. _ Fer- 
ro-magnesian constituents are rare in the larger part of the area, 
much of which may be called a soda-aplite (see analysis No. 413). 
Analysis No. 369 is of the coarser granite, from the Chowchilla 
River, and from the west edge of the same granite mass that cuts 
off the Mariposa slates west of the town of Mariposa, metamor- 
