Whitesail Lake Map-Area

Lake, east of Sand Cabin Bay, are four conical shaped hills composed entirely
of basalt. Two of the small islands near the north end of Pondosy Lake are
composed of rocks that, on the basis of their composition and fresh appearance,
are included in this group though no other similar rocks are in the vicinity.
The flat-lying basalts occur as caps or remnants of caps on most of the
mountains east of the Coast Mountains and it is probable that they covered
a much wider area later denuded by erosion. Such rocks are very prevalent
in Nechako River area to the east and Anahim Lake map-area to the
southeast. A gabbro dyke, exposed in the northeast corner of the area near
Tatalrose, may be an intrusive phase of the basalts.

Lithology

Flows of the group are commonly fine-grained, dark grey to black
basalts and andesites. Where they are porphyritic they may exhibit feldspar
phenocrysts up to 1 inch in size. They may be massive, vesicular, or amygda-
loidal with the amygdules most commonly of chalcedonic quartz although
calcite and stilbite also occur. Olivine is present in some of the flows on
Chef Ridge but is not a common constituent.

Under the microscope a typical basalt from the Mosquito Hills shows a
few small broken and resorbed plagioclase phenocrysts in a very fine-grained
groundmass of feldspar microlites, magnetite, and interstitial glass. Limonite
is present as a secondary product. Hematite may replace magnetite as an
accessory mineral.

Flows from the height of land between Francois and Ootsa Lakes are
commonly very fine grained. They show few phenocrysts of feldspar, the whole
being a mass of feldspar microlites and magnetite in a glassy groundmass.
Flows from Chef Ridge are medium grained and commonly not porphyritic.
Augite is abundant and olivine is present in minor amounts in some flows.

Tuff and breccia comprise a small proportion of the group and on the
east end of Whitesail Range a soft, white ash bed occurs between two
basalt flows.

Near the foot of Pondosy Lake is a group of small islands, two of
which are composed of rocks included with this group. One of the islands is
composed entirely of volcanic agglomerate. The rock is composed of frag-
ments and blocks of Hazelton group rocks and minor amounts of intrusive
rocks embedded in a matrix of red volcanic tuff. The fragments are rounded
to angular and vary from pea size up to 10 inches in diameter, with no
indication of sorting. Commonly a cluster of small fragments is wedged
between the larger ones. The matrix consists of angular fragments of feldspar,
quartz, and basaltic material.

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