IGNEOUS. ROCKS OF THE SIERRA NEVADA, — 393 
does not appear to have had the specimens examined microscop- 
ically. Neither Mr. Diller nor the writer found any peb- 
bles of the Spanish Peak rock (a quartz-mica-diorite) in the 
conglomerate. The southern extension of the Red Hill serpen- 
tine area is shown on the geologic map a little west of Quincy. 
The long belts of porphyrite and amphibolite indicated on 
the map on either side of the narrow belt of Paleozoic (in this 
case certainly Carboniferous) rocks are very probably Jura-Trias 
in age. In fact, certain portions are without doubt of Jurassic 
age. A section across the Bear Mountains, which lie east of 
Stockton, shows the relation of the igneous and sedimentary for- 
mations. 
Fic. 1. Section across the Bear Mountains, extending vertically to sea-level. 
C.—Carboniferous slates. J.—Jurassic slates. ap.—Porphyrite, porphyrite tuffs, and 
amphibolite. Vertical and horizontal scale two miles to the inch. 
We have here a central belt of Carboniferous slates flanked 
on either side by augitic porphyrites and tuffs, and at both the 
east and west base of the Bear Mountains are Jurassic slates, 
which have been designated the Mariposa formation. The por- 
phyrites and their tuffs being surface formations, there seems 
good ground for regarding them as products of volcanoes that 
existed after the Carboniferous and before the close of the Juras- 
sic, It may also be assumed that the portions of each porphyrite 
belt lying nearest the Carboniferous slates are the oldest. The 
tuffs along the east borders of the eastern area merge into the 
Jurassic slates, the latter containing frequent layers of the augite- 
tuff, so that it is difficult on the east slope of the Bear Mountains 
to draw an exact line between the tuff and the slates. There can 
be no doubt as to the age of these tuff layers, for in some of 
them ammonites have been found. The one found nearest the 
line of the section came from a branch of Cherokee Creek at the 
