62

Great Slave lakes and of the western part of the basin. The rocks in these areas
doubtless consist mainly of Silurian and Devonian sediments but Cambrian and
Ordovician horizons are known to exist in the mountains at the west.

Silurian

Rocks of Silurian age form the base of the Paleozoic section over a consider-
able portion of the eastern part. of the Mackenzie River basin. They consist of
dolomitic and non-dolomitic limestones, the former generally predominating ;
beds of gypsum and anhydrite are conspicuous elements of many of the sections.

The formation is exposed in Bear mountain near Norman and in mount
Charles on Great Bear river. At the latter point the Silurian formations have a
thickness of not less than 3,000 feet. The discovery by E. M. Kindle of Halysites
catenulatus and other well-known Silurian corals, together with a brachiopod
fauna which includes Conchidium sp., leaves no doubt of the Silurian age of the
Mount Charles section. A Silurian fauna was also found in the eastern front
of Franklin raountains 150 miles south of mount Charles. On the west side of
the Mackenzie near the mouth of North Nahanni river 1,500 feet or more of
dark dolomitic limestones which have furnished only a few poorly preserved
fossils are referred to the Silurian. :

Sediments carrying Receptaculites and several species of Silurian brachio-
pods and cephalopods are exposed on the west side of the north arm of Great
Slave lake. These form an escarpment which probably continues northward
towards Great Bear lake. According to G. S. Hume the rocks consist of a grey
dolomite overlain by beds of red arenaceous limestone and gypsum.

_ Limestone, gypsum, and anhydrite of Silurian age are exposed in cliffs along
Peace river between Peace point and the mouth of Jackfish river. They are
overlain unconformably by beds of upper Devonian age. A fossil closely allied
to Spirifer corrallensis has been found in limestone outcropping on the trail to
Salt river at a point 2 miles west of Fitzgerald. This limestone and the gypsi-
ferous dolomite exposed along Salt river near Salt Springs west of Fort Smith
are thought to represent a late horizon of the Silurian. Sediments of Silurian
age are also exposed on Caribou island in Slave river, and the only two outcrops
of the Paleozoic known on the lower Slave have furnished fossils that appear to
correlate them with late Silurian.

Devoniant

The Devonian system.as it is developed in the Mackenzie basin includes a
large variety of rocks which refined studies will divide into a number of litho-
logie units. For the purpose of a report having the broad scope of the present
one it appears best to group the various formations which in the Great Slave
Lake district have received separate names” and the terranes of the northern
part of the basin into two subdivisions. Shales predominate in the uppermost
of these divisions and limestones in the lower one. The fossils show these two
divisions to belong respectively to the upper and middle Devonian and these
terms will be used in referring to them.

An unconformity marks the base of the Devonian, and both the rocks on
which it rests and the basal beds of the Devonian system vary widely in age
in different parts of the Mackenzie basin. In the eastern part of the Clearwater

1The description of the Devonian formations is contributed by E. M. Kindle.
2 Cameron, A. E., Geol. Surv., Can., Sum. Rept., 1917, pt. C, p. 25.