Nechako River Map-Area

(Armstrong, 1949, p. 92). The Topley Intrusions or similar rocks underlie much
of the southern part of Fort St. James area and extend into the northeast quarter
of Nechako River map-area, from Francois Lake to Lily Lake, Nulki Hills, and
Tatuk Lake. These rocks have been mapped as Topley Intrusions but, from evidence
within Nechako River map-area that does not conflict with evidence in the type
area, the writer has assigned to the group a different age (Tipper, 1959a). The
Topley Intrusions in Nechako River area, like those in Houston and Fort St.
James map-areas, form fairly large bodies of granitic rocks overlain by patches of
Tertiary rocks and intruded by necks and dykes of basalt. Most of the granitic
rocks occur in a single, large belt, with a few small stocks separated from the main
belt. This group extends for an unknown distance east and southeast from Nechako
River area.

Lithology

In Fort St. James map-area the Topley Intrusions were mapped as three
subunits (1) granite and granodiorite, (2) diorite, and (3) syenite, and these
were distributed in distinct mappable areas. Only the first two subunits have been
recognized in Nechako River area. The granite and granodiorite subunit enters
the area near the east end of Francois Lake, extends southeast as a unit 15 miles
wide to Mount Hobson, from where it narrows to a belt about 3 miles wide.
The diorite and quartz-diorite unit underlies the Nulki Hills and smaller hills
farther southeast.

Coarse-grained pink porphyritic or non-porphyritic granite comprises the
Topley Intrusions northwest of Nechako River. This is a characteristic rock that,
in places, is so coarse as to be described as a pegmatitic granite. It is composed of
pink potash feldspar, with smaller amounts of oligoclase and quartz, and a little
biotite and hornblende. The pink feldspar forms the phenocrysts of the porphyritic
types; plagioclase is restricted to the matrix, and never occurs as phenocrysts.
Although the grain size varies, most of the rocks are coarse and crystals of pink
feldspar up to 3 inches long have been noted. The quartz content may be as great
as 35 per cent. Mafic minerals in some places are less than 1 per cent of the
rock.

Grey and greenish granodiorite and granite with minor pink granite phases
form the Topley Intrusions of Mount Hobson, and Finger and Tatuk Lakes. These
are in part hybrid rocks, particularly those of Mount Hobson, with numerous
inclusions of Takla volcanic rocks. The intrusive contact on Mount Hobson is
difficult to delineate as inclusions, granitic dykes, part assimilated rocks, and
normal granites are intermixed over a zone about 3 miles wide. Plagioclase feldspar
is more abundant and the rocks grade from granites to granodiorites. Porphyritic
rock types are not common.

The diorite and quartz diorites are easily distinguished from the granites
because of the foliation almost all display. In some places this foliation is very

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