THE NORTH AMERICAN LAKES. 335 



necticut flow each through a bead-roll of small lake-basins, walled 

 around by solid rock. Through their margins the rivers for untold 

 ages have been deepening their channels until the lake-bottoms have 

 become dry land, and the homes of men. The Great Lakes themselves 

 have, from a like cause, been much reduced from their former dimen- 

 sions. The evidences are abundant that Lakes Michigan, Erie, and 

 Huron are but the relics of what was once a large body of water, cov- 

 ering all the intervening and much of the adjacent lands. The work 

 of depletion is still going on. Not only is Niagara deepening its 

 channel and sinking thereby the surface of Lake Erie, but by the 

 gradual recession of the falls a much greater work is prophesied. 

 It is only a question of time when Erie will be robbed of its waters, 

 and the other Great Lakes reduced to insignificant parts of their pres- 

 ent dimensions. Lake Pepin, now but an expansion of the Mississippi, 

 was once a much larger body ; and Peoria, a similar widening of the 

 Illinois, once spread over the adjacent level lands equaling in area 

 that of Lake Champlain. 



Doubtless, in many of these lake-outlets, natural fractures and 

 marginal depressions have not only given direction to the effluent 

 streams, but greatly aided in the work of abrasion. The evidence, 

 however, of the former greater extent of these lakes is abundant and 

 apparent. The railroad from Lafayette, Indiana, northward cuts 

 through several low lake-margins, marking the gradual retreat of the 

 waters ; and runs within sight of several sand-hills similar to those on 

 the lake-shore near Michigan City. There are evidences that the Illi- 

 nois was once the outlet of Lake Michigan and the "Wabash that of 

 Erie, carrying the waters of these lakes to the Gulf of Mexico. 



It is now generally conceded that the whole northern part of the 

 continent, reaching southward in some places to the thirty-eighth par- 

 allel, once wore an ice-cap of immense thickness, through which only 

 the mountain-peaks projected. I have already alluded to the work of 

 the ice-plow in the excavation of lake-basins. I am now about to give 

 to the glacier the credit of their preservation when formed. 



Although it has been found that the glacier flows like the water of 

 a river, only more slowly, the ice, except when wedged in between 

 two walls, as the Mer de Glace, could not have been confined in nar- 

 row channels, and can not, therefore, have grooved out long, tortuous 

 river-beds. The abrasion and drainage were indeed going on, but by 

 a slow process, as compared to the work of the released and active 

 waters. 



When the ice-field began to disappear it gradually receded north- 

 ward, first uncovering that part of the drift-region in which the lakes 

 have been wholly drained. The southern half of the continent has 

 had even larger rivers than now fed by the ice and snow of the gradu- 

 ally disappearing glacier. The length of time during which these 

 rivers were doing their work of excavation, while the North was still 



