REVIEW OP GEOLOGICAL STEUCTUEE, 51 



Many difficulties stand in the way of the acceptance of this theory. 

 The rocks cut away to form the basin of Lake Michigan are the Upper 

 and Lower Silurian limestones, and are equal in hardness to the average 

 of sedimentary strata. We have no proof that any considerable portion 

 of the mass is " argillaceous shale," as reported by Prof. Winchell. The 

 same may be said of the rocks cut away by the Lake Huron glacier. At 

 the north end of the lake, and ija Georgian Bay, the excavation was 

 in Lower Silurian rocks ; at the south end in Devonian and Upper Silu- 

 rian limestones. In all this series there is no considerable mass of soft 

 material. At Goderich, near the south end of the Lake, borings show 

 that there are beds of rock-salt in th'e Salina Group below the lake bot- 

 tom, but the rocks out of which the basin is cut are chiefly the Cornifer- 

 ous limestone and the Waterlime. 



Lake Erie is not excavated, as Prof. Winchell says, mostly in the 

 Salina Group, for that was not reached except just at the summit of the 

 Cincinnati arch. All the eastern portion, and the deepest, of the lake 

 is formed by the removal of the Upper Devonian shales, soft rocks it is 

 true, but those which lie below and have resisted the action of the gla- 

 cier, are precisely those which have been renioved to form Lake Huron. 

 A better, explanation of the shallowness of Lake Erie is afforded by the 

 suggestion that the glacier which excavated it was the most southerly of 

 all the lake-producing local glaciers, and that it was the product of a 

 climatic condition which did not continue nearly as long as the next one, 

 when the ice sheet had retreated a step farther northward, and Lake 

 Superior, Lake Michigan, Lake Huron, and Lake Ontario, were formed. 



The basin of Lake Ontario, below the water-line four hundred and 

 fifty feet deep, is mostly excavated in the Utica and Hudson shales, but 

 the north shore of the lake is formed by the Trenton limestone, a hard 

 and tough rock, and much of the northern and eastern portions of the 

 basin are cut from this. 



The bearings of the glacial furrows as well as the drift of the trans- 

 ported materials prove that the basins of Lake Erie and Lake Ontario 

 were cut out by local glaciers moving from the north-west, and nearly at 

 right angles to the line of motion of the continental glacier. This latter 

 crossed the basins of the last mentioned lakes from north to south, and 

 the whole tendency of its action must have been to obliterate any such 

 troughs lying across its track. Besides this the local glaciers which 

 formed these basins came after the general one, for where their tracks 

 cross the lake, glaciers have obliterated, more or less completely, the 

 traces of the great ice-sheet. 



