10

The Baker member is a fairly homogeneous unit consisting mainly
of fissile calcareous quartzites containing relatively few veins. The Rain-
bow member consists of several rock units. Veins are very numerous but
do not cross the whole member and rarely cross more than one unit. The
B.C. member is a homogeneous, unsheared unit. It contains relatively
few veins, but those that do occur appear to be more persistent than those
in the Rainbow member. The Lowhee member is locally quite schistose.
It has many veins but few compared with the Rainbow member. The
veins have about the same persistence as those of the Rainbow member and
careful examination might show that they terminated in less favourable
rock bands. The Basal member is homogeneous and unsheared. Very few
veins were seen in this member but they appeared to be quite persistent.

The reason for the occurrence of veins in one rock type and not in an
adjacent rock of different type has not been established, but the probable
reason is that one rock failed under stress by fracturing whereas the other
failed by bending or flowing. It is assumed that when the fracture-
forming stresses acted on the district the rocks of the Baker member yielded
by bending and flowing rather than by fracturing, those of the Rainbow
and Lowhee yielded by fracturing rather than by bending, and those in the
B.C. and Basal members yielded only to a slight extent and only by
fracturing.

At the Island Mountain mine the transverse veins strike 30 to 45
degrees and dip approximately 70 degrees southeast. At the Cariboo Gold
Quartz mine they strike 30 to 55 degrees, but the dip is vertical, steep to the
southeast, or steep to the northwest. Near Barkerville the prevailing strike
of the transverse veins is 15 to 30 degrees and the dips appear to be mainly
steep to the northwest. Near Grouse creek the prevailing strikes appear to
be 25 to 35 degrees and the dip approximately vertical or steep to the
southeast. Transverse fractures are very numerous. Many appear to be
joints and in some places are so closely spaced as to resemble fracture
cleavage, but in general they are too far apart to be mistaken for fracture
cleavage cracks. Commonly fissile rocks traversed by these fractures show
an abrupt flexure parallel to the fracture and on both sides of it. In the
Cariboo Gold Quartz mine most of such flexures show a downward bend or
a convexity downward on the northwest side of the fractures and an upward
bend or a convexity upward on the southeast side. The flexures are
approximately at right angles to the fissility and strike of the rocks, their
lengths are in most cases less than 100 feet and perhaps less than 50 feet,
and their widths 8 inches or less. It would seem that the flexures were
formed by compressive stresses acting parallel to the main structure and
that either while they were being formed, or afterwards, the flexures broke
along a medial line forming the fractures. A peculiarity of some of the
transverse veins occupying a medial fracture along a flexure is that they
have short, narrow, wedge-shaped branches that leave the veins at angles
of about 30 degrees and penetrate the flexures from their convex sides.
These little wedges of quartz are in general less than 1 foot long and 2
inches wide and commonly are only 6 inches long and 1 inch or less wide
at the base. Exceptionally they are several feet long and have a more
uniform width of 1 inch or less. Such branches suggest strongly that the
flexure had been formed prior to the deposition of the quartz veins.