220 Dature'6 



second class erosion undoubtedly entered into 

 their formation as well as the piling up of 

 glacial drift. 



In order, however, that we may understand 

 more fully the formation of these greater lakes 

 it will be necessary for us to go back and ex- 

 amine the conditions that seem to have existed 

 before the glacial period. 



It is a fact well known that continents have 

 periods of elevation and depression. There is 

 abundant evidence that the northern portion 

 of the North American continent was elevated 

 to a much higher level in preglacial times than 

 it occupies now. This is evidenced in very 

 many ways by sounding the depths of old river 

 beds now filled with glacial debris. The old 

 beds show unmistakable evidences of having 

 been worn down to their present level by the 

 action of running water. They also prove to 

 be many feet below the present sea-level. This 

 fact seems to be sufficient to prove the theory 

 of a higher elevation of the North American 

 continent in preglacial times. It should be 

 said here that undoubtedly the constant filling 

 up of the ocean with the drift carried down by 

 the rivers has somewhat raised its level, but 

 hardly to the extent indicated by the old river 

 beds. The question naturally arises, Where 

 did all the dirt come from to fill up these great 

 river beds and change the whole topography 

 of the northern half of the continent? Dr. 



