Revleim and Notices of Books, 221 



the north within the influence of the waves, and yielding the 

 sands, gravels, and boulders of the upper Diift accumulations. 

 This depression permitted an invasion and broad extension south- 

 wards of the ice-covered Arctic seas, the true cause, in all proba- 

 bility, 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 leas*, have travel sed 

 the basin of Georgian Bay ; but the glacial striae which also 

 occur there, may have been produced by the action of ice, origi- 

 nating at the spot itself. The three or four distinct sets of stiise 

 observed at this locality, however, do not radiate 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 

 uprise of the land appears to have taken place, and a vast area, 

 extending over and around our present lake basins, then became 

 converted into a fresh-water sea. This probably found its outlet 

 to the ocean through what is now the broad valley of the Missis- 

 sippi. Its waters stood at a great elevation above the waters of 

 our present lakes, and were gradually lowered to these levels by 

 physical changes in the surrounding country, and more especially 

 by the depression of a higher region lying to the east. Durino- 

 this gradual fall and retrocession of the great lake waters, the 

 upper layers of the Drift were re-sorted, mixed with newer sedi- 

 ments, and thrown up here and there into secondary ridges ; and 

 the remarkable terraces which form so salient a feature in the 

 general aspect of our lake shores and intervening districts, were 

 then in chief part produced. The escarped faces of these Diift 

 terraces, it should be observed, always front the present lake- 

 basins^ and thus look in some places towards the north, and in 

 others towards the south, &c., according to the direction of the 

 nearest shores. This would necessarily arise if they w^re pro- 

 duced, as here imagined, by a gradual lowering of the waters, 

 with intervening periods of repose. The shells of fresh-water 

 mollusca, buried in the modified Drift, at various levels above 

 the existing lake-waters, and in localities so far apart — for these 

 shells have been found throughout the region south of the lakes, 



♦ Oq 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 seta of striae were observed. 



