127

_ The most striking feature about the chromite layers is their unsystem-
atic arrangement over any large area. Individual layers are almost always
curved, and festoon- or garland-like patterns are common. In the south-
east part of the stock many layers were seen to be arranged in concentric,
canoe-shaped or basin-shaped shells; in other places the layers appear to
form a series of small basins, a few inches to 5 feet or more deep, whose
upper rims, over an area 100 feet or more square, lie in a roughly horizontal
plane. In all such structures the lowest layers of a series are the most
irregular and have the sharpest curvature; the layers are farthest apart
and, in general, thickest in the bottom of the ‘basins’ or ‘festoons’, and
close together or in places missing at the rims or nodes (See Figure 6).

———  — Approximately 18 feet

7
h,
CON

ee
ee of chromite end abruptly ie \

N

Figure 6. Festoons of chromite-rich layers in serpentinized peridotite near south
end of ultramafic stock east of Polaris Creek, as seen on a vertical face.

Almost all the layers of chromite terminate abruptly. In several
places they stop at a serpentine-filled seam that probably represents a
small fault or slip joint, but in many other places no fracture can be
observed, and relatively large layers end abruptly, in some cases leaving
a faint ‘tail’ of grains to suggest that they have been truncated by shearing
(See Figure 7).

Some lenses and vein-like bodies of granular chromite change in
thickness abruptly upon a sharp change of direction, in such a way as to
suggest that they fill openings created by slight movement along irregular
fractures (See Figure 6). In a few places irregular masses occur at inter-
vals in a network of fine, lacy bands.

PETROGRAPHIC DESCRIPTION

The ultramafic rocks are for the most part coarse textured, and except
in the highly serpentinized varieties textures and mutual relations between
rocks can be seen to best advantage on outcrops and in hand specimens.
Thin section study is of value mainly in determining the composition,
range of composition, minor textural features, and relative proportions
of the individual minerals.

78609—10