academy of bcwkcbs] PROGLACIAL GREAT LAKES 153 



of the sun, and twice it was compelled to retreat; but when the sun finally surveyed its reconquered territory, 

 the land was no longer simply graven with a tracery of rivers; it sparkled with the sheen of innumerable lakes. 



The next passage to be cited is of special interest, as it contains the first statement, and 

 apparently the most definite statement that Gilbert made concerning the long-vexed question 

 as to the origin of the basins in which the Great Lakes are contained. It is explained that in 

 certain areas the ice sheet was — 



a powerful agent of erosion, scooping out great hollows from the solid rock. For some reason not clearly under- 

 stood the erosion was greatest along a zone parallel to the margin and a few hundred miles back from it, and 

 here were formed the basins not only of the Laurentian lakes from Ontario to Superior, but of Winnipeg, Atha- 

 basca, Great Slave, and Great Bear Lakes. Within this zone of greater erosion the points of greatest erosion 

 were determined chiefly by the pre-glacial shape of the surface. . . . How deep the original valleys were can- 

 not be told, for the details of the old topography have been ground away, but we may be sure they were shallow 

 compared with the existing troughs; 



for the depths of some of the lakes "reach from three hundred to five hundred feet below the 

 level of the ocean, and their origin cannot be referred to stream erosion without incredible 

 assumptions as to continental elevation. " 



The remainder of the article is concerned with the gently inclined shore lines of the extinct 

 proglacial lakes, and with the changes of those lakes as the ice melted back and the northeast- 

 ern land rose toward its present level. Then follows a remarkable inference: Had this rise 

 been continued to the amount of 3 inches in a mile more than the amount that is recorded in 

 the slanting shore lines, it "would send a great river from Chicago to the Mississippi, reverse 

 the current of the Detroit, stop Niagara Falls, and rob the upper St. Lawrence of seven-eighths 

 of its water." Gilbert's later work showed that such a change is within the possibilities of 

 the future, as will be told in a later section. 



