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out when the mountains are replaced by the elevated wéstern plateau. The
horizontal altitude is maintained for some miles, but before reaching the western
edge of the plateau the beds bend down and dip gently to the west.

“No limestones were observed along this section, and the rocks consist
of sandstones, quartzites, and shales, all of which are probably referable to the
Cretaceous.. At the starting point on Peel river the banks are formed of shales,
interbedded with some hard sandstones holding carbonized fragments of wood
and leaves. In the first fourteen miles the beds are concealed, but shales and
sandstones are again exposed on the banks of the valley in which we made our
first camp. Some fossils were collected here, among which is a Discina like D.
prleolus, Whiteaves, a Mactra and a Yoldia, both of which are probably new, but
the specimens are too imperfectly preserved to admit of specific determination.
Six miles farther west, at the entrance to the “Gap,” the trail passes over beds of
a bluish, rather compact calcareous sandstone. The beds are coated in places
with cale-spar, are highly ferruginous, and weather to rusty yellow. A Cardium
and some other poorly preserved fossils were obtained here. In the valley of the
river the sandstones are underlain by dark shales. After entering the mountains,
only alternating sandstones and quartzites were seen. The beds of this series
are greyish in colour, are evenly stratified, and are very uniform in appearance
all across the range. They have an estimated minimum thickness of 5,000 feet,
and may possibly greatly exceed this. The western plateau is built of compact,
greyish sandstones, passing in places into quartzites, similar to those found in
the mountains and evidently belonging to the same formation.”

STRUCTURAL GEOLOGY

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In the Precambrian rocks of the Laurentian Plateau portion of the Mackenzie
basin our knowledge of structure and structural relations is very imperfect and is
limited to the region about Athabaska lake and the territory along the Tazin and
Taltson rivers. Beyond this, little mention is made of structure in the reports
of explorers.

Throughout the whole Precambrian portion of the basin, however, the same
structural relations seem to hold in a general way. Great granite batholiths
have been intruded into an older series of stratified rocks, of which there are now
only remnants left. These two series have together suffered from severe orogenic
disturbances, so that the stratified rocks are now tilted at high angles and the
granites have in part become gneissoid. The remnants of the stratified rocks
seem to be synclinal in structure and the strike ranges from northeast on
Athabaska lake to north and even northwest on Taltson river. The trend of the
gneiss conforms in a general way to the strike of the stratified rocks, and in
certain localities there is much local contortion of the beds or foliation planes.

The younger rocks of the Precambrian, as illustrated by the Athabaska
sandstone on the south shore of Athabaska lake and others on Great Slave lake,
show little evidence of disturbance. The beds are horizontal or dip at very low
angles. They are cut by later dykes which, however, have created little distur-
bance and effected only shght local metamorphism. There is massive jointing
of the beds, but apparently little faulting.

In the Great Central plain there has been no igneous intrusion in the rocks
outcropping at the surface, and little orogenic disturbance, so that the structure

1 McConnell, R. G., Geol. Surv., Can., vol. IV, pp. 119 D-120 D.

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