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Abraham Creek Body

General Description. An irregular body of extreme structural and
lithological complexity outcrops over an area of about 20 square miles west
of the headwaters of Abraham Creek. At its northeast side this body is in
contact with andesites and andesitic tuffs of the Takla group. The rest of
the body lies within, and is intruded by, the various acidic phases of the
Hogem batholith.

Like several other bodies of this phase of the Omineca intrusions, the
Abraham Creek body is composed almost entirely of hornblende, plagio-
clase, and magnetite, with accessory ‘apatite and much epidote-clinozoisite
of secondary origin. The body is unique in the map-area, however, in its
extreme degree of differentiation into rocks of widely different textures and
mineralogical proportions, and in the remarkably complicated structural
relations between these rock types. The rocks range in composition from
hornblendite to hornblende diorite containing only about 30 per cent
hornblende (a few dyke-like bodies are composed entirely of feldspathic
material), and in grain size from pegmatitic rocks in which most crystals
are more than 3 inches long to material in which none of the grains can be
distinguished with a hand lens.

About half the rock of this body contains more than 60 per cent dark
minerals. One of the most common rock varieties is an appinite composed
of stout, prismatic crystals of black hornblende +4 inch to 1 inch long, in a
groundmass of light grey to pale pink, saussuritized andesine feldspar. The
feldspar, which comprises about 10 per cent of most of these rocks, is com-
pletely interstitial to the ewhedral hornblende. With decrease in the amount
of feldspathic material, this type of appinite grades into feldspathic horn-
blendite and hornblendite, in which the hornblende crystals lose the highly
developed euhedral habit seen in the appinite. Most of the hornblendite
shows no preferred orientation of its constituent crystals, but in a few
outcrops the elongated crystals are arranged more or less parallel with
each other, forming bands or sheets 1 inch to 5 inches wide lying perpen-
dicular to the long axes of the crystals. The sheets, which were only
observed to be vertical or steeply dipping, lie side by side, with occasional
bands of non-oriented crystals, across distances of as much as 6 feet, and
can be traced 50 feet up a steep slope. A few of these sheets are composed
of crystals not quite perpendicular to the general plane of layering, so that
the resulting rock has a ‘herringbone’ appearance. Adjacent sheets differ
slightly in feldspar (or epidote) content, and in texture. Some of the
hornblendite contains isolated, straight bands up to 1 foot thick that are
coarser grained, and richer in feldspathic material.

Another widespread type of appinite and feldspathic hornblendite is
characterized by clusters and knots of feldspathic material in otherwise
feldspar-free hornblendite. This texture, called synneusis (swimming
together) texture by Vogt (1921, p. 321), has developed to a remarkable
degree in some of the appinites, which, consequently, have a conspicuous
‘starred’ appearance, with clusters 4 to 4 inch in diameter of white or pale
grey feldspathic material evenly distributed at intervals of 4 inch to 4
inches in black hornblende rock containing almost no interstitial feldspar
(See Plate IX A). In thin section the ‘stars’ are seen to be irregular
aggregates of rounded grains of twinned andesine, generally smaller than