17

A fault in Marmot River valley offsets a bed of limestone at least 400
feet. Many straight, narrow valleys in the area follow crushed zones, but
in most cases the total amount of differential movement was not discovered.
The zone of dykes already referred to is apparently offset considerably
where it crosses Bear river. The contact of the Coast Range batholith is
offset similarly where it crosses Bear river. On the mountain side east of
Portland canal, in line with the above-mentioned points of offsetting, the
batholith contact is again offset, but the extent of the movement is not so
accurately known. Apparently a large fault strikes along Bear River val-
ley from American creek to Stewart. It may branch at Stewart, one branch
continuing as shown on the map and thence across Bulldog creek and along
Georgia river to Portland canal and the other branch, if there is another,
continuing along Portland canal. The fault probably branches at Ameri-
can creek, the main branch following the creek. It is not likely that any
branch of this fault runs east along upper Bear river. The apparent hori-
zontal offset along this fault in Bear River valley is approximately 14
miles. The east side has apparently moved southward or the west side
northward. In view of the fact that the rocks by which the apparent offset
is measured are intrusive, and as the contacts of intrusives may turn
sharply for no evident reason, it follows that the apparent offset along
Bear river does not necessarily prove the existence of a fault. It is very
likely, however, that a fault exists there. The fact that other faults in the
area with similar strike show horizontal movement lends weight to the
belief that movement along the big fault was chiefly horizontal. Evidence
on this point is also furnished by the mineral deposits on opposite sides of
the fault. As the fault offsets the Coast Range batholith, it is also later
than the mineral deposits of the area. .If the movement along the fault
plane had been essentially vertical, marked differences should exist between
the mineral deposits on opposite sides of the fault. Such differences do not
exist.

Narrow, straight valleys in the area commonly follow faults. The
extent of movement was not learned, but in places it was determined that
the fault planes are vertical and the presence of horizontal fault grooves
indicates horizontal movement. These faults strike north-northeast and
are probably thrust faults with large horizontal and little vertical
movement.

Several faults strike east-northeast and along one of these the hori-
zontal offset is over 400 feet. These faults may be normal faults.

Faulting in a north-northeast direction preceded and followed ore
deposition. Some at least of the faults striking east-northeast preceded
mineralization, but the relationship between the two groups of faults was
not ascertained.