REVIEWS 543 
colorless amphibole comprises probably both edenite and gedrite. 
The magnesian rocks above described are cut by soda-feldspar dikes, 
which appear to have a genetic connection with the peridotite-pyroxenite 
magma. 
3. Granular intrusive rocks, mostly acid in character, including 
granite, granodiorite, quartz-diorite and gabbro. All of the granitic 
rocks are cut by dikes of aplite, which is regarded as the residual acid 
material of the magma squeezed up into cracks after the consolidation 
of the granitic rock; and by dikes of a fine grained diorite, which 
usually contains idiomorphic needles of brown amphibole. The 
occurrence of these fine grained diorites or diorite-porphyries in small 
dikes at widely separated intervals in the Sierra Nevada, suggests that 
they are the differentiation product of some other magma. These 
dike rocks are very similar in mineral and chemical composition at 
widely separated localities, and appear to be among the latest of the 
pre-Cretaceous intrusives, inasmuch as the dikes cut nearly all the pre- 
Cretaceous rocks. It does not seem possible to ascribe the origin of 
these dikes to a batholithic magma, underlying the range, inasmuch as 
the rock occurs almost nowhere in masses sufficiently large to be 
represented as areas on the geological maps. A genetic connection 
with the great granodiorite batholith underlying the Sierra Nevada, is 
suggested by the field relations of this diorite-porphyry. 
Superjacent sertes.— The rocks of the Superjacent series in the Bid- 
well Bar quadrangle consist almost entirely of gravels and Tertiary 
lavas, and tuffs. During the Neocene period the Bidwell Bar quad- 
rangle was a country of low relief, as were other portions of the Gold 
Belt. The Auriferous river gravels formation represents deposits 
made by the rivers of the Neccene period. These are very largely 
covered at the present time by lavas. While volcanic rocks are very 
abundant, there appears to have been few volcanoes in the district. 
The lavas come chiefly from vents located in the Downieville quad- 
rangle, which lies just east. This is not true, however, of the lavas of 
the plateau west of the north fork of the Feather. These originated 
in the Lassen Peak volcanic area. 
West of Franklin Hill is the base of a former volcano. The 
Superjacent volcanic rocks are grouped under the following heads : 
Basalt : 
Older basalt, with litle olivine. 
Later basalt, dark colored and rich in olivine. 
