SECTION IV, 1916 [125] TRANS. R.S.C. 
Geological Structure of the Basin of Lake St. John, Quebec. 
By Joun A. Dresser, M.A., F.R.S.C. 
(Read May Meeting, 1916). 
GENERAL. 
Lake St. John is situated 120 miles north of the city of Quebec, 
and about the same distance from the mouth of the Saguenay river, 
its outlet, which enters the St. Lawrence at the historic little town 
of Tadousac. It is a beautiful sheet of water, rudely circular, 
rather more than 20 miles in breadth, and has an area of 350 square 
miles. It lies in a basin of roughly triangular shape whose area is 
not yet entirely defined but is approximately seven or eight times 
as great as that of the lake. The average altitude of the basin may 
be taken as 500 feet, that of the surrounding country, as 1,000 feet 
above sea level. The boundary of the basin in most places is a well 
defined escarpement varying in height from 100 to 600 feet, followed 
outward from the basin by a more gradual rise in the surface until 
the general level of the highland is reached. 
GENERAL GEOLOGY OF THE REGION.! 
The region is a part of the great Pre-Cambrian protaxis of eastern 
North America and considered physiographically it is within the 
Laurentian plateau or peneplain. The underlying rocks are princi- 
pally granites, gneisses and anorthosites, of Pre-Cambrian age. With- 
in the basin, however, there are outliers of Ordovician sediments. 
These consist chiefly of limestones and shales belonging respectively 
to the Trenton and Utica formations, with a small exposure of 
Richmond age. They are, evidently remmants of sedimentary 
deposits of greater thickness and extent, which at one time probably 
occupied the entire basin, and perhaps a much greater area. Their 
preservation from complete erosion and removal is apparently due 
to their protected position in the basin. 

1 References to previous geological work in the district :— 
Jas. Richardson, Geological Survey of Canada, Annual Report, 1857. 
Rey. J. C. K. Laflamme, Geological Survey of Canada, Annual Report, 1883-4, 
Dr. F. D. Adams, Geological Survey of Canada, Summary Report, 1883-4. 
Dr. G. A. Young, Geological Survey of Canada, Summary Report, 1900. 
(Published by permission of the Director of the Geological Survey of Canada). 
