LAKE ALGONQUIN. 
17 
readvance of the ice. The remarkable continuity and uniform- 
ity in strength of the highest Algonquin shore-line both north 
and south of the Trent Valley outlet isobase is also difficult 
of explanation unless it is all of the same age. 
DESCRIPTION AND INTERPRETATION OF OUTLINE MAP AND PROFILE, 
SHOWING CHARACTER OF THE DIFFERENTIAL UPLIFT. 
The accompanying map is an outline of the Lake Simcoe 
and Trent Valfey region of southern Ontario. On this outline 
is shown the highest Algonquin shore-line in Lake Simcoe dis- 
trict and its correlatives along the line of the Trent Valley chain 
of lakes. The Algonquin river and the shore-line of Lake 
Iroquois are also shown. The Iroquois shore-line is probably 
not a correlative of the highest Algonquin shore-line, in the 
southern portion of Lake Simcoe district. Altitudes of the 
highest Algonquin shore-line and its correlatives are shown in 
feet above sea-level at localities where measurements have been 
made. The altitudes are all instrumental determinations. In 
most cases the take levels were used as a datum plane, the 
altitudes of which were determined from bench-marks established 
by precise levelling by the Public Works Department of Canada. 
The shore-line of Lake Iroquois is taken from Coleman’s map of 
Lake Iroquois accompanying the Bureau of Mines report for 
Ontario, 1904, with slight modifications in Rice Lake area. The 
altitudes of the Iroquois shore-line at Silver lake and near Trenton 
are also instrumental determinations by Coleman. Isobases, 
or lines of equal deformation, of the Algonquin beach and its 
correlatives and the direction of tilt or maximum uplift are also 
shown on the map, the isobase interval being 10 feet. 
The fact that the isobases are closer together in the northern 
than in the southern portion of the district shows that the plane 
of the Algonquin shore-line is more steeply inclined in the former 
than in the latter. Near Beaverton in the central portion of 
the area there is an irregularity in the rate of uplift as shown by 
the closer spacing of the isobases. In the Kirkfield-Balsam 
Lake district there are remarkable irregularities in the trend 
of the isobases showing marked changes in the direction of tilt 
and in the character of the uplift. 
