Nechako River Map-Area

flows form the valley walls. Elsewhere a few small outcrops rise above the glacial
mantle, and these, together with the characteristically flat topography and the large
fields of basaltic erratics, are the only evidence to indicate the presence of the
group in areas of low relief. Much of the drift-covered area, particularly in the
northeast quarter, is believed to be underlain by this group.

Lithology

Most of the Endako Group consists of red, brown, grey, and black, andesitic
and basaltic, massive, vesicular, and amygdaloidal lava flows. No dacitic, trachytic,
or rhyolitic flows are present although some are tentatively included in the type
area (Armstrong, 1949, pp. 74-76). Where such flows were encountered in Nechako
River area they were mapped with the Ootsa Lake Group. Interbedded with these
flows, and forming less than 15 per cent of the whole, are breccia, tuff, and
agglomerate of closely related composition.

Features characteristic of plateau basalts are present in these rocks (see Pl.
V). Columnar basalt is fairly common although the columns are rarely well
formed. In a few places, such as the hill at the east end of Natuza Lake and on the
north side of Tatalrose Lake, columnar basalt is exposed with well-formed, five-
sided, vertical, straight columns 200 to 300 feet long. Some necks or plugs of basalt,
such as Tyee Butte and The Devils Thumb, have exceptionally long columns but
are not as perfectly formed; those of Tyee Butte are over 700 feet long. Pillow
lavas on the other hand are a feature not commonly encountered and no well-
formed or obvious pillows were noted; some flows weather into ellipsoidal masses
but whether these are true pillow lavas is doubtful. Like most Tertiary plateau
lavas of British Columbia and Washington, most of these rocks are flat-topped and
when observed in section are horizontally layered. At a distance these layers appear
even and regular but on closer examination are seen to consist of several flows from
10 to 75 feet thick with interlayered beds, lenses, or pockets of fragmental material.
Although a cursory examination gives an impression of a great lateral extent to
these flows, closer inspection shows that, as a distinct unit, they can be traced for
only a few thousand feet and that flows 5 to 10 miles long are rare.

Some finer features are characteristic of the flows. Vesicular basalt is common
but with many varieties. Some flows have a few scattered vesicles whereas in others
the vesicles are so numerous that they form the greater part of the rock, which is
in effect a scoria. Such rocks are abundant northwest of Holy Cross Lake. Vesicles
are commonly small but have been noted up to 6 inches in diameter. Some are
elongated parallel with the length of a flow. Amygdules are not characteristic but
do occur; rarely are the cavities completely filled. The amygdules are commonly
composed of chalcedony, common opal, and calcite, as well as siderite and
aragonite.

These lavas are generally basic, basalts or andesites, but except for this broad
generalization, there is little uniformity of rock type. The rocks may be very fine

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