404 THE JOURNAL OF GEOLOGY. 
phosing them near the contact into mica and andalusite schists. 
In Yaqui Gulch some of the latter contain impressions of shells 
of Aucella erringtom, Gabb, proving their upper Jurassic age. No. 
399 is the fine-grained granite from Agua Fria Creek and not 
far from its contact with the coarse granite. Its mineral compo- 
sition is not greatly different from that of the coarse granite as 
represented by No. 369. It is microscopically a micropegmatite 
with minute foils of brown mica gathered in groups. No. 413 is 
a white medium-grained rock, and a fair sample of the larger part 
of the area. It is composed of feldspar and some quartz plainly 
discernible as such, with a large amount of micropegmatite. In 
Cathey Valley and to the southeast of it, the diabase before 
described and the fine-grained soda-granite come in contact, 
forming a zone of contact-breccia in places more than a mile wide. 
The fragments of diabase are cemented together by white felds- 
pathic material presumably from the granitic magma, indicating 
the later age of the white granite. 
The augitic porphyrites of Mt. Bullion, north of Mariposa, 
are cut by a dike of serpentine, doubtless originally a peridotite. 
Near Mormon Bar, south of Mariposa, is a dike of hornblende- 
porphyrite in the coarse granite. 
The succession of the old igneous rocks in the Mariposa 
region is then, so far as made out, as follows: 
Augite-porphyrite, chiefly tuff, effusive. 
Diabase,_ - - - - | 
Serpentine, - | 
Coarse granite (quartz-mica-diorite),  - - - } Intrusive. 
Fine-grained granite (aplite),  - | 
Hornblende-porphyrite. - - - = - J 
As bearing on the age of the hornblende-porphyrite, it might 
be stated that at many points the granite and other rocks are cut 
by dikes of a dark rock containing original hornblende, some 
occurrences of which are granular in structure and allied to 
diorite, and others have a distinct groundmass and are more 
correctly called hornblende-porphyrite. An analysis of one of 
these (No. 250 Butte county collection) is given in the table. 
This is apparently holocrystalline, but the feldspar between the 
