400 THE JOURNAL OF GEOLOGY. 



age system. The glacial hypothesis assumes that the basin was 

 excavated mainly by glacial abrasion and does not require that 

 the land should be either higher or lower than at present. The 

 study in this direction merges with that of the general glaciation 

 of the northeastern part of the continent, and cannot be treated 

 at this time. 



3. Sediments. — Regularly stratified deposits of clay and sand 

 occur along many portions of the borders of the present Lauren- 

 tian lakes. These were clearly formed in water bodies which 

 formerly existed within the Laurentian basin, and which in cer- 

 tain directions, at least, were of wider extent than the present 

 lakes. The areas occupied by these deposits have been partially 

 mapped, but much remains to be done in this direction. Fresh 

 sections, particularly of the stratified clays, are exposed from 

 time to time by artificial excavations, in which much of their 

 history may be learned. Not only should records be made of 

 the facts noted at special excavations, but the extent and charac- 

 ter of the stratified deposits in one area should be determined 

 and compared with similar data obtained in other areas. For 

 example : the clays covering large tracts on the west shore of 

 Lake Michigan and on the southern and western border of Lake 

 Superior are of a red color, while other areas bordering Lake 

 Erie are covered with blue clay. These two deposits have been 

 supposed to have been laid down at the same time and in the 

 same lake. The definite correlation of the clays of these two 

 areas by direct contact, however, does not seem to have been 

 made, and there are reasons for thinking that they may be quite 

 distinct and that they originated in separate lakes. 



The outer limits of the deposits of clay and sand here re- 

 ferred to are known in some instances to be determined by 

 ancient beaches and terraces. Such associations of deep and 

 of shallow water deposits require special attention, as the 

 study of one may assist in interpreting the significance of 

 the other. The fine, evenly stratified clays frequently contain 

 large angular bowlders, which appear to have been dropped from 

 floating ice and to show an intimate connection between the 



