8

a small amount of intrusive matter. The flows and fragmental rocks are
not easily distinguished, as some breccias appear to have a flow matrix
and some flows contain numerous fragments. Contacts between different
rock types, too, are rather difficult to follow, as individual flows or beds do
not continue far but lose their identity by merging with adjoining yol-
canic rocks. The usual types of fragmental rocks are breccia and tuff,
The tuffs are usually of intermediate grain, but some fine-grained types
resembling chert occur. The breccias contain many fragments of augite
andesite. The lava flows range in composition from augite andesite to
rhyolite, but the common type is andesite. Rhyolite occurs locally and is
fairly plentiful in the vicinity of the Porter Idaho mine. The predomi-
nant colour of the rocks is green and this accounts for the local name
“ greenstone” for rocks of this formation. The coarser fragmental rocks
are in many places purple, red, or brown. The green colour is due to the
presence of chlorite, and the purple, red, and brown to iron oxides.

The beds of argillite in the formation rarely exceed 100 feet in thick-
ness, but continue for long distances. Individual beds were not traced far
with certainty, but this was due to difficulty encountered through forest
and drift cover. A bed of argillite would be an excellent horizon marker
if it could be traced and it would serve to show clearly the structure of
the area. The few limestone beds are thin except in one place where a
faulted bed is 100 feet thick.

The base of the formation consists of interbanded tufis and sediments,
the whole being somewhat calcareous. Tufts predominate in the middle
part of the formation and breccias and flows seem to be more plentiful
near the top.

The thickness of the formation was not measured, but it is probably
at least 5,000 feet on Upper Bear river and on Marmot river.

The formation is exposed on Marmot river in two northwesterly
trending bands. The two bands join at the southeast corner of the area,
The northeastern band does not continue northward past Marmot river,
as it has been eroded. The southwestern band continues north to form
Bear River ridge and in the northern part of the area it swings eastward
and extends southward along the east side of the area and may join the
northeastern band outside the confines of the area.

The Bear River formation overlies the Bitter Creek formation con-
formably. The change from argillite to voleanic rock is transitional
through interbedding of sedimentary and tuffaceous rocks. The Bear
River formation underlies the Nass formation. This contact was not
studied, but is conformable.1 In Salmon river a conglomerate is present
locally at the base of the Nass formation,2 and its presence indicates a
possible though brief local break in the continuity of rock deposition.

Both bands of the formation on Marmot river are synclinal, Far-
ther north the southwestern band becomes the eastern limb of a syncline
and the western limb of an anticline. The axis of the syneline is repre-
sented approximately by Bear River ridge, and that of the anticline lies

1McConnell, R. G.: Geol. Surv., Canada, Mem. 32, p. 17 (1913).
*Schofield, 8. J., and Hanson, G.: Geol. Surv., Canada, Mem. 132, Pp. 13 (1922),