REVIEW OF GEOLOGICAL STRUCTURE. DL 
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 in 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 the 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 Hrie 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- 
cler, are precisely those which have been removed to form Lake Huron. 
A better explanation of the shallowness of Lake Hrie 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 draft 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. 
