Shaler-J 130 [June 16, 



action would necessarily lead to the tlow of the tipper level of ice 

 over the part which was within the basin, leaving it locked within its 

 walls. I do not mean to deny the value of this sort of excavating 

 process, as shown in the theory so long and ably presented by Ram- 

 sey, Mortillet and others; I am inclined to think that it may have 

 done much at certain stages of the ice action to dig out basins, but 

 in many eases it is manifestly inapplicable. The lake-basins of cen- 

 tral New York, for instance, cannot possibly be explained on this 

 fcasis. We must bring in some agent tending to cause melting at the 

 base of the glacial mass, in order to effect the excavation of such 

 basins, it am inclined to think that the other class of excavations of 

 the Fjord Zone, the valleys which do not sink into the pit-like de- 

 pressions which form the lakes, may also be, in fact, accounted for by 

 the operations resulting from melting under pressure, for the coursing 

 of floods of water, released by pressure from its solid state, would 

 prove a powerful aid to the excavating action of the ice. 



By supposing that the principal transporting action of a ■conti- 

 nental glacier was accomplished by the water flowing beneath the 

 glaciers, we readily account for the water-worn look which is so prom- 

 inent a feature in the drift pebbles of the greater part of North 

 America; even when their position makes it clear that they have 

 never been worked over by water since they were left by the glacier. 



By this theory we can account for the excavation of such great 

 lake-basins as those occupied by the fresh water seas of North 

 America. These basins, by their trend, and by their distribution 

 .over a region where they cannot be explained by simple ice-erosion, 

 present an insuperable difficulty to any view which does not admit 

 that running water was largely concerned in their production. -On 

 the hypothesis here brought forward, we can, it seems to me, account 

 for their formation. The sheet of ice which had its southern border 

 on the Ohio, at Cincinnati, doubtiless leveled over the great trough 

 which separates the central part of that State from the Laurentian 

 Mountains. This valley of the great lakes has a depth of at least 

 six hundred feet below the table-land which separates the Ohio val- 

 ley from Lake Erie. In this great depression we may have had 

 melting occurring on a scale so vast as practically to arrest the south- 

 ward movement of the ice, the sheet only overlapping in an unknport- 

 .ant way, and for a short part of the glacial period, the southward 

 'boundary of the valley. The southern discharge of these waters may 

 Ihave been in part through the river beds of the 'State of Ohio, but 

 3 am inclined to think that the larger part of the waste went to the 



