15

are present. Apatite, titanite, and iron oxides are the usual accessory
minerals. The rock in most places is quite fresh and the plagioclase feld-
spar in thin section is clear. Six thin sections from different parts of the
batholith showed but little variation in mineral composition.

A stock of granodiorite outcrops in Bear River valley near Glacier
ereek and another smaller stock occurs near the head of Bear river. The
mineral composition of the rock of these stocks is the same as that of
the batholith.

A small body of white intrusive rock outcrops below the Silverado
mine. The rock consists almost entirely of sodic plagioclase and quartz;
it contains no biotite nor hornblende. The exact size and shape of this
body, and its relationship to the batholith are unknown. The mass is at
least 200 yards wide and may be much longer. It lies at the contact
between the batholith and volcanic rocks of the Bear River formation.

The various stocks are probably associated in origin with the main
batholith. On Salmon river, in Alaska, there is evidence of at least the
former existence of granodiorite older than the Coast Range batholith,?
but no evidence was obtained by the writer to show that any of the
stocks in Bear River valley are older than the Coast Range batholith.

The exact age of the Coast Range intrusives is not determinable in
this district. The rocks intruded are probably not younger than Jurassic.
The intrusive may, therefore, be late Jurassic or younger.

DYKES

Commencing in Salmon River valley,? a zone of closely spaced dykes
of quartz porphyry and quartz diorite extends southeastward across Bear
River ridge and Bear River valley and up the east slopes of the valley.
This zone has been indicated on the accompanying map as having a width
of about 14 miles. Its boundaries have been arbitrarily drawn, for in
some places very many dykes occur in adjacent strips of territory up to a
quarter of a mile wide. But outside the zone country rock predominates,
whereas within the zone, dykes form 75 per cent or more of the whole
assemblage. On the upper part of Bear River ridge the individual dykes
are distinct bodies separate from one another. On lower slopes of the
ridge, in Bear River valley, the dykes are closer spaced and, due to
coalescence, are larger. Still lower down the slopes the dykes so coalesce
that the assemblage has the aspect of a stock. Farther southeast, in the
vicinity of mount Dickie, the dykes become more and more widely
spaced, narrower, and fewer, until the zone loses its identity.

Most of the dykes are vertical, strike parallel to the trend of the
zone, and are between 50 and 150 feet wide, but narrower ones are fairly
numerous and wider ones exist. Most of the dykes have approximately
the mineral composition of the Coast Range batholithic rocks. They con-
tain less orthoclase, however, and are mostly quartz diorites. Some are
quartz porphyries and a few resemble gabbro.

1Buddington, A. F.: “Coast Range Intrusives of Southeastern Alaska”; Jour. Geol., vol. XX XV, No. 3, p. 228

(1927).
38chofield, S. J., and Hanson, G.: Geol. Surv., Canada, Mem. 132, p. 27 (1922).

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