59 
ORDOVICIAN AND SILURIAN FOSSILS FROM GREAT SLAVE 
LAKE 
By G. S. Hume 
CONTENTS 
Page 
Introduction 59 
Description of species 62 
Illustrations 
Hates XII and XIII. Illustrations of fossils 
123-125 
INTRODUCTION 
The rocks of North arm, Great Slave lake, have been described by 
A. E. Cameron 1 and by the writer 2 in the Summary Reports of the Geo- 
logical Survey for 1917, 1920, and 1921. In the report for 1920 the two 
horizons recognized by Cameron were discussed and a provisional list 
of fossils given from each. The age of both horizons was considered to be 
Silurian, but more detailed studies have shown that the Halysites on which 
the age of one horizon was based is in reality Halysites gracilis and the 
age is Ordovician. There is thus on the North arm at least one horizon 
of Silurian and one of Ordovician, the latter being now recognized for the 
first time. It is possible other Silurian beds occur above the Ordovician 
and below the Silurian, from which fossils were obtained. 
ORDOVICIAN BEDS 
At no place were the field relationships of the Ordovician and Silurian 
rocks seen, so that the position and age as here deduced are entirely based 
on fossil evidence. The character of the sedimentation has already been 
somewhat described. It varies for the different localities on the North 
arm. At Gypsum point at the south end of the North arm, the rocks 
are mostly red, arenaceous limestones and shales with gypsum. The 
gypsum occurs both bedded and in vein, the veins cutting across the strata 
and obviously being later than the sedimentation. Where the gypsum 
beds occur there are no fossils. To the north of Gypsum point a section 
has been described from Red Rock point. 3 The section shows a lower part 
of 35 feet of variegated coloured sandstones, but at no place was the base 
exposed. Observations elsewhere, however, led to the conclusion that 
the basal part of the formation is sandstone which rests unconformably 
on the Precambrian below. It is quite evident that the sandstone varies 
greatly in thickness as well as character from place to place, owing to the 
1 GeoI. Surv., Canada, Sum. Repta. 1917, pt. C; 1921, pt. B. 
*Geol. Surv., Canada, Sum. Rapt. 1920, pt. B. 
*Geol. Surv., Canada, Sum. Rept. 1920, pt. B. 
