116 STRUCTURE With the exception of small faulted blocks east of Osilinka River and west of Uslika Lake, all of these late Paleozoic rocks strike northwest and dip to the southwest at angles ranging from 30 to 90 degrees and averaging about 60 degrees. The tops of beds, as shown by grain gradation in almost all exposures of banded tuffs, consistently face southwest. At the west edge of the map-area, a few banded tuffs strike north to north 15 degrees east, with a vertical dip. Local folds and drag-folds are rare, and are best developed in beds of argillite such as those exposed on Tutizika River (See Plate VIII A). Some of the tuffs show small, local slump structures. The minor variations of strike and dip are due principally to the lenticular nature of the beds. A series of northwest trending, steeply dipping faults has divided the rocks of this group, in their southern mountain area, into slices about 2 miles wide; the slice northeast of each fault appears to have been lowered relative to its neighbour to the southwest. The Uslika formation has apparently been emplaced along two of these faults, which in places nearly coincide with the bedding planes (See page 188). All of these rocks and structures have been offset by a transverse fault along Osilinka River Valley. The west side of this fault moved northward about 14 miles relative to the east side. As a result of these fault movements, a block about 2 square miles in area northeast of Uslika Lake has been tilted to the east, with dips ranging from 25 to 70 degrees; and another, consider- ably shattered part of the map-unit, exposed on Thane Creek, has a westerly strike with dips of 10 to 60 degrees south. The upper part of the map-unit exposed on Tutizika River is highly sheared parallel with the bedding, and some shears may represent south- west dipping faults of considerable displacement (See Plate VIII A). These faults all indicate that the rocks on the southwest side have dropped and moved to the south with respect to those on the northeast side, and they may be subsidiary breaks of a large compound fault zone forming the contact between this map-unit and the overlying Takla group rocks. Apparently, few faults occur in these rocks in the Lay Range, and no appreciable displacement was observed on any that cross the bedding. About forty carbonatized shear zones were noted in the entire range. Most of these are less than 5 feet wide, and parallel the bedding. The largest carbonatized zone is about 100 feet wide and at least 3 miles long, and cuts obliquely across the beds, striking about north 10 degrees east, near the west border of the map-area. Beds are, apparently, not offset along this zone. The hematite-impregnated shatter zone above the large conglo- merate body between the headwaters of Swannell River and Lay Creek does not seem to have been the locus of significant net movement. Shear zones up to 100 feet wide, generally parallel with the bedding and heavily impregnated with pyrite and pyrrhotite, were observed at the northwest end of Lay Range and in the canyon on Polaris Creek. The rocks of this late Paleozoic map-unit are adjoined to the south- west by volcanic flows and intercalated sedimentary rocks of the Takla group, which is Upper Triassic and Jurassic in age. The division between the two map-units is based mainly on structural considerations, for