4 THE POPULAR SCIENCE MONTHLY. 



Hind ; but there are good reasons for believing that it reaches north- 

 ward to the Arctic Ocean, and that the great lakes of the north, like 

 those of the St. Lawrence chain, Superior, Huron, Michigan, etc., are 

 pre-glacial river-valleys scooped out and modified by ice.* 



From the facts already gathered, it is a justifiable inference that 

 fully half the Continent of North America and nearly all north of the 

 fortieth parallel was at one time covered with ice or perpetual snow, 

 and, so far as we can now judge, the glaciation of all the North Ameri- 

 can localities enumerated was synchronous. 



Some writers have attempted to prove that a large part of the 

 glacial phenomena described above is really the work of icebergs and 

 shore-ice, and one of the consequences of a great continental subsid- 

 ence ; but no man who has studied the inscriptions made by glaciers 

 will hold to this theory when he has traversed much of the glaciated 

 areas east or west of the Mississippi. To all the mountainous region 

 of the "West it is evident that the iceberg theory is inapplicable, and, 

 when the enormous glaciers of the West are conceded, it is difficult to 

 see why they should be denied to the East. Since, however, the ice- 

 berg theory is insisted upon for this section, it may be well enough to 

 say that it is demonstrated untrue by four unanswerable arguments : 



1. The inequalities of level in the fancied water-line formed by 

 the margin of the drift area are irreconcilable. Mount Washington, 

 Mount Marcy, Mount Mansfield, must have been totally submerged — 

 because their tops are worn and striated — while the shore-line was at 

 New York at the sea-level, in Pennsylvania twenty-one hundred feet, 

 and at Cincinnati three hundred feet higher. South of the drift-line, 

 high lands and low were alike beyond the reach of the flood, while in 

 Wisconsin it spared a special district not above the general level, and 

 all around it the rocks are scored and strewed with debris. 



2. The direction of the ice-scratches and the derivation of the 

 bowlders would require the submergence of all the northern portion 

 of the continent, so that icebergs (which had no land to start from, 

 and therefore could not have existed) could float southward over all the 

 Canadian highlands ; and the local variations of direction (southwest 

 by south, in the basin of Lake Erie, south in that of Lake Huron, 

 south-southwest in Lake Michigan, southwest in Lake Superior, and 

 southeast in New England) show an incomprehensible tangle of ocean- 

 currents. 



3. The complete absence of marine shells from the great drift area 

 of the interior, while they are abundant in the Champlain and bowlder 

 • lays on the coast, is incompatible with this theory. 



4. The inscription left by the eroding agent is altogether sui generis, 

 and characteristic of glacial action, and not at all that which could be 

 effected by dragging masses of ice over the sea-bottom. This in itself 

 is a conclusive refutation of the theory. The record made by a glacier 



* Sec position of northern lakes on map. 



