1898-99. | THE GEOLOGICAL HISTORY OF LAKE SUPERIOR. 51 
long interval, during which we can only surmise what was going on. 
Cambro-Silurian rocks, forming part of the northern margin of the 
great area of these strata lying to the southward, occupy the shore of 
the lake from Marquette to the outlet, a distance of 130 miles, and 
farther south they are succeeded by the Devonian and Carboniferous 
systems in their regular order. Patches of Cretaceous rocks, resting on 
the Archzean, occur in Minnesota at no great distance west of the lake. 
It is therefore probable that this part of the continent was submerged 
throughout much, if not all, of the time up to about the commencement 
of the Tertiary period. During this period there must have been a 
very long interval of erosion, in which the land surface was deeply 
sculptured and the present inequalities to a great extent produced. 
After this, when the glacial period came on, the deeply decomposed 
surface was ploughed up and its materials were transported to greater 
or less distances. Thus its removal from one part of the land and its 
deposition on another would add to the inequalities of the surface and 
might deepen and extend the larger existing, lake-basins very much, 
while it would be the means of creating innumerable smaller ones. On 
the glaciated surface of the crystalline rocks, great numbers of lakes now 
show at a glance what their history must have been. We see the glacial 
furrows and striz descending into the rock-basins on the one side and 
emerging on the other, while heaps of boulders and drift material are left 
wherever they could find rest or shelter from the moving ice or where 
they may have been deposited by the final melting away of the glacier. 
A few of the lakes have been formed by the damning up of valleys and 
partial basins by moraines of drift, but the great majority of them are 
complete rock-basins. There seems to be no limit to the size of the 
basins which may be formed in this way, and if we extend our 
conception of the power and volume of these old continental glaciers 
and imagine them to have acted upon a deeply softened surface, there 
is no reason why we may not believe that the greatest of our lake-basins 
might have been excavated in this manner. It is a question of what is 
most probable. If, in addition to these processes, we take into con- 
sideration the slow undulation, tilting or warping of the crust of the 
earth, which is known to have been going on, and which is still in 
progress all over this part of the continent, we shall have no trouble in 
accounting for the existence of our great lakes. When we remember 
how slight is their depth in comparison with their area, we see how 
easily they could be formed on this extensive plain of the continent. 
To give you an idea of the insignificance of the actual depth of these 
great sheets of water as compared with their extent, let us construct a 
vertical section through Lake Huron from north to south on a natural 
