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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