SURFACE GEOLOGY. 77 



encountered by the glacier in its motion. These prove that the ice was 

 plastic, and folded itself around any opposing body. Near the south 

 point of Put-in-Bay Island is a nearly vertical wall — now partly quar- 

 ried away — which is distinctly beaded by the ice ; and in one place, 

 where a softer layer had worn faster than the others, a horizontal furrow 

 with a > shaped section was produced in this wall, and the upper sur- 

 face is as distinctly glaciated as the lower. These and many other exam- 

 ples of ice-carving, visible on these islands, demonstrate the truth of the 

 theory that the lake basins were excavated by glacial action, and not, as 

 has been urged by some geologists who have not seen these markings, by 

 icebergs. 



The sequence of events in the formation of the great lakes seems to 

 have been somewhat as follows : 



1st. The Laurentian belt, north of the great lakes, has been a land sur- 

 face since the beginning of the Palasozoic era, was formerly a high moun- 

 tain range, the degradation of which has supplied the mechanical materi- 

 als which compose the sheets of Palaeozoic rock that surround it. The 

 erosion of these highlands has continued uninterruptedly till the present 

 day, and was specially rapid during the ice period. The result has been 

 that this mountain range has been almost entirely worn away, the trunc- 

 ated bases of the various arches and uplifts which compose it alone re- 

 maining to testify to its existence. 



2d. The country lying between the Atlantic and Mississippi has been 

 above the sea since the close of the Carboniferous period, and during the 

 succeeding ages the general plan of its topography and its system of 

 drainage have remained the same. Since it emerged from the seas, this 

 area, too, has been constantly suffering erosion, and its lines of drainage 

 have been more and more deeply inscribed upon it. 



3d. Previous to the glacial period, the elevation of this portion of the 

 continent was considerably greater than now, and it was drained by a river 

 system which flowed at a much lower level than at present. At that 

 time our chain of lakes — Ontario, Erie, and Huron — apparently formed 

 portions of the valley of a river which subsequently became the St. Law- 

 rence, but which then flowed between the Adirondacks and Appalachians, 

 in the line of the deeply buried channel of the Mohawk, passing through 

 the trough of the Hudson and emptying into the ocean eighty miles 

 south-east of New York. Lake Michigan was apparently then a part of 

 a river course which drained Lake Superior and emptied into the Mis- 

 sissippi, the Straits of Mackinaw being not yet opened. 



4th. With the approach of the cold period, local glaciers formed on the 



