General Geology

grained, and massive, finely or coarsely porphyritic, or even coarse grained and
gabbro-like. Although certain rock types predominate in several small areas, most
types are common to all localities. A very coarse grained porphyritic basalt is
present as flows and dykes in the northwest quarter of the area, near Takysie Lake
and Danskin. The rock consists mainly of coarse, platy crystals of labradorite up
to 12 inches diameter in a fine-grained black groundmass. These platy phenocrysts
are sometimes parallel with one another and parallel with the flow layer or the
dyke wall. Fine-grained, grey or black massive basalt is probably the most common
rock type in all parts of the area. Black obsidian was observed infrequently.

Under the microscope Armstrong found that rocks of this group were mainly
andesites (1949, p. 75) but, although this may be true for the type area, it is not
true everywhere within Nechako River area. The plagioclase feldspars commonly
are close to the andesine-labradorite boundary—the criteria used to distinguish
between basalt and andesite—and a small difference in their composition would
throw the rock from one class to the other. It is, moreover, unlikely that the slides
examined by the writer or by Armstrong are truly representative of the group, and
at present it is best to say only that most of the flows are basaltic or andesitic. The
porphyritic rocks consist of laths of plagioclase feldspar, one eighth to one quarter
inch long embedded in a fine groundmass of feldspar, augite, magnetite, and glass.
As previously mentioned the plagioclase feldspar is labradorite or andesine and in
more basic flows, bytownite. Olivine basalt is not common.

Sedimentary rocks are found associated with the volcanic rocks, particularly
where streams have cut through lavas that filled pre-Miocene valleys. The sediments
seen so far occur, without exception, at the base of the group. The stratigraphic
position of these sediments is ample justification for mapping them as a separate
group or formation, but as they are poorly exposed and nowhere extensive it is best
to map them with the group with which they are most closely associated, particularly
in reconnaissance mapping. They consist of poorly consolidated, coarse sandstone
and conglomerate, minor siltstone, and lignite. All are soft and very friable and as a
result are rarely exposed; possibly this gives an erroneous impression of their
extent. Most of the material of the conglomerates has been derived from older
Tertiary and Mesozoic rocks and only rarely are there pebbles of Miocene basalt.

Mode of Extrusion

These plateau lavas were, in the writer’s opinion, at one time much more
widespread than now. Erosion has dissected them into several separate areas which
if united would resemble the lava plateaus of central Washington. Such a comparison
logically leads to a comparison of the modes of extrusion for the two areas.

The Columbia River lavas are considered to have erupted quietly from fissures
and spread out laterally, coalescing to form the great lava sheets of central Wash-
ington. The evidence for this is (1) the abundance of dyke swarms in older rocks
near the margins of the lavas, and (2) the absence of any known central vents
or cones from which the flows may have come (Campbell, 1950, p. 80).

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