398 THE JOURNAL- OF GEOLOGY. 



by streams tributary to the main drainage line to the north. If 

 this conclusion is well founded, a study of the hard- rock 

 topography should reveal other similar channels and finally indi- 

 cate a well matured drainage system. If even the broader and 

 stronger features of the pre -glacial surface can be determined, 

 then the modifications due to glacial abrasion will become con- 

 spicuous, and the amount that glaciers have broadened and 

 deepened the basin be determinable. 



A study of the lithological character of the drift south of the 

 present lakes should show, at least in a rough way, what portion 

 of it was derived from the waste of rocks within the Laurentian 

 basin. This inquiry has already been undertaken by at least 

 two geologists, and estimates of the quantity of material re- 

 moved from the basins of lakes Michigan and Erie respectively, 

 have been made. This method may be extended so as to embrace 

 a larger area, or some special portion of the great depression best 

 suited for the trial may be selected. If the material removed from 

 the basin or re - distributed within it by glacial action can be shown 

 to be approximately equivalent in volume to the amount of rock 

 excavated in order to form the depression, it would evidently 

 tend to support the hypothesis of glacial erosion. If, on the con- 

 trary, the amount of debris derived from the basin should fall 

 far short of what would be requisite to refill it, no very definite 

 conclusion would seem to be indicated unless account could also 

 be taken of the fine material carried away by glacial streams. 



As the case stands at present it appears that there is evidence 

 of a pre -glacial valley or series of valleys as has been claimed 

 by several geologists, and that all but the boldest features of the 

 old topography have been obliterated or greatly modified by 

 glacial erosion followed by glacial and other sedimentation. 

 Additional observations should show somewhat definitely the 

 amount of work assignable to particular portions of the history. 

 How far the results of subaerial and of glacial erosion have been 

 modified by other agencies, more especially by orographic move- 

 ments, has also to be considered. 



If the St. Lawrence basin shall be shown to be largely the 



