SURFACE GEOLOGY. a 
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 Paleozoic era, was formerly a high moun- 
tain range, the degradation of which has supplied the mechanical materi- 
als which compose the sheets of Palsozoic 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 1% 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 érosion, and its lines of drainage 
have been more and more deeply inscribed upon it. 
od. 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 
