OF LAKE AREA OF WESTERN CANADA. 227 
the deposition of the lower clays. These latter were evidently derived 
from the limestones and other Silurian and Devonian strata lying 
beneath and around them. Hence their generally calcareous nature. 
Their derivation from this source is proved, moreover, by the pebbles 
of Trenton limestone and other fossiliferous rocks which they fre- 
quently contain. Extensive denudation must thus have occurred 
both immediately prior to, and during, the deposition of these clays; 
but it may be questioned whether the bolder contours offered by the 
denuded rocks, such as the escarpment that sweeps from the Niagara 
river to Cabot’s Head on Lake Huron, were not produced during 
the first uprise of the palzozoic strata from the earlier seas in which 
their materials were accumulated, ages before the period now under 
discussion. It appears, at least, to be a well-admitted point, that 
these rocks had been elevated into dry Jand before the deposition of 
the higher formations in the south and west. 
2. After the deposition of the lower Drift clays, a sudden and 
abrupt change in the character of the sediments took place. A 
striking example of this may be seen in the natural sections about 
Hogg’s Hollow, a few miles north of Toronto. The change in ques- 
tion must have been effected by a still further depression of the 
country, bringing the higher lands and gneissoid strata of the north 
within the influence of the waves, and yielding the sands, gravels, 
and boulders of the upper Drift accumulations. This depression 
permitted an invasion and broad extension southwards of the ice- 
covered Arctic seas, the true cause, in all probability, of the cold of 
this epoch. The depression must have exceeded 1,500 feet, since 
northern boulders are found at that height above the sea, on the 
Collingwood escarpment. The gneissoid boulders there met with, 
must at least have traversed the basin of Georgian Bay ; but the 
glacial striz which also occur there, may have been produced by 
the action of ice, originating at the spot itself. The three or four 
distinct sets of striz observed at this locality, however, do not ra- 
diate from any fixed point, but run in the usual north and south 
direction, some being a little east and others a little west of north.* 
3. At the close of this second series of phenomena, a gradual up- 
rise of the land appears to have taken place, and a vast area, extending 
* On a visit to this spot, since the publication of the “ Note on the Geology of the Blue 
Mountain Escarpment,” in the Canadian Journal, Vol, V. p. 304, some additional sets of 
striz were observed. 
