GEOLOGICAL HISTORY OF THE LA URENTIAN BASIN. 40 1 



ancient lakes and neighboring glaciers. The possibility, how- 

 ever, of the bowlders having been brought into the ancient water 

 bodies by rivers, or floated outwards from the shore by lake ice, 

 should also be considered. Huge angular masses of limestone 

 have been reported as occurring in southern Michigan especially, 

 which rest on superficial deposits and are thought to have been 

 carried northward by lake ice. The relations of these masses to 

 well defined shore lines have never been determined. If it 

 should be found that they are above all former shores, it is evi- 

 dent that they must have been carried by some other agency 

 than the one mentioned. ' 



A chemical examination of the clays, or of their contained 

 water, may indicate whether or not the basin was formerly in 

 direct communication with the ocean. Analyses of the clays of 

 the Champlain valley and of the similar clays in the Ontario 

 and Erie basins might indicate whether or not they were 

 deposited under similar conditions. 



4. Shore records. Beaches and terraces have been studied at 

 many localities about the borders of the present lakes, sometimes 

 at a distance of more than twenty miles from their margins and 

 at various elevations up to several hundred feet above their sur- 

 faces. In some instances these ancient shore records have been 

 followed continuously for scores of miles. The tracing and map- 

 ping of individual beaches is one of the most important parts of 

 the study here outlined, and is already well advanced. Confusion 

 has unfortunately arisen, however, for the reason that topographic 

 features, due to shore action, have, in some instances, been con- 

 founded with somewhat similar features due to other causes. 

 Moraines and gravel ridges, formed by glacial streams, have been 

 mistaken for beach ridges, and terraces of various origin have 

 not been clearly discriminated. 



In order not to be led astray by topographic forms that simu- 

 late shore phenomena, the student should examine the shores of 

 existing lakes and learn what records are there being made. 

 In the study of topography, "the present is the key to the 

 past," just as definitely as in anv other branch of geology. The 



y 



