1898-99. | THE GEOLOGICAL HISTORY OF LAKE SUPERIOR. = 
systems are all associated with the depression and subsequent elevation 
of the land, it will be in order for me here to say something of these 
movements. Geologists have not yet agreed as to what produced the 
glacial epoch—a unique event in the history of the earth—but they 
think the most probable cause of the depression of the northeastern 
- part of the continent during and just following that epoch was the 
great weight of the mass of ice which had accumulated to a depth of 
one or two miles over a vast area. The oscillations of the land, as 
compared with the sea-level, which have always been going on in one 
part of the earth or another are mainly due to the shifting of the surface- 
load by its partial removal from> one region to another through the 
agency of water, volcanic action, etc. The rocks, which appear to be 
quite rigid on the small scale are really not so, and on the large scale 
they yield slowly to pressure. The relatively small depression 
produced by this continental ice-sheet was less than might have been 
expected, and the rising of the land which is still going on, is the 
rebound, as it were, or the effort to regain its equilibrium after the load 
has been removed. Around any of our great lakes, one may easily 
observe abundant and distinct evidence of higher stages of the water in 
the form of terraces, old beaches, ridges, curving spits and other shore 
phenomena. They are found at many different levels around all the 
lakes. The principal ones at various heights may be connected so as to 
show that the water stood long enough at each of these heights to wear 
into the land and leave these permanent records. But a curious fact 
about the beaches and terraces is that in a northeasterly and south- 
westerly direction they are not horizontal but slope upwards in the 
former at a rate which is sufficient to be easily measured, amounting to 
from five inches to three feet or more per mile. This important 
circumstance was first noted by a Canadian writer in referring to the 
terraces around Georgian Bay about 50 years ago, but | have forgotten 
at this moment where I read about it. The changes in the elevation of 
numerous well-marked beaches around the various lakes have been 
determined throughout long distances by several well known geologists, 
among whom I may mention G. K. Gilbert, F. B. Taylor, Warren 
Upham, Frank Leverett and Professor W. C. Chamberlain of the United 
States and Drs. J. W. Spencer and A. C. Lawson of Canada, both 
formerly assistants of my own. The bearing of the line of maximum 
rise was easily found after the rate of increase in elevation had been 
ascertained along various lines forming greater or smaller angles with it 
and Professor Gilbert gives it as N. 270 E. for the lower lake region. 
If we look in the opposite direction along this line we would speak of 
the movement as a depression. We have seen, however, that it is really 
