*78 Canadian Record of Science. 



seas. Lakes Superior and Michigan may be regarded as 

 two distinct reservoirs — the former of cold and the latter of 

 warmer water — which constitute the largest sources of sup- 

 ply for the lower Great Lakes. Hind found the surface of 

 Lake Superior on 30th July, at noon, as low as 39.50° at 

 fifty miles from land. The outlets of these two lakes into 

 Lake Huron are close to each other, the Michigan waters 

 flowing directly into the main basin of Lake Huron, and 

 the colder waters from Superior, while joining them in part 

 through the detours between the Manitoulin Islands, appear- 

 ing in part also to find their way eventually to the Georgian 

 Bay by the channels north of the same islands. ISTow, Lake 

 Huron in its profound depths forms three great basins — 

 the Georgian Bay, the Central and the Southern Basins. 

 The Georgian Bay is separated from the CentralfBasin, not 

 only by the Bruce Peninsula,! but by a continuous sub- 

 aqueous ridge which comes to the surface in islands at dif- 

 ferent points, whilst under water it presents on the one side 

 bold precipitous cliffs facing the Georgian Bay, and on the 

 other, shelves somewhat gradually towards the deeper 

 waters of the Central Basin. This ridge prevents the free 

 interchange of water between the deeper portions of the 

 Georgian Bay and Lake Huron proper, and makes the for- 

 mer a somewhat isolated basin of cold water without any 

 considerable free current of warmer water flowing into and 

 through it. This isolation aids in retaining in the Bay the 

 colder waters which have accumulated there during the 

 winter months. Thus, whilst the surface in July and 

 August may be as high as 65° F., the bottom temperature 

 at 31 fathoms and upwards, varies between 39.5° and 3^.^15° F. 

 The Central and Southern basins of Lake Huron, on the 

 other hand, are separated by ^the^'sub-aqueous corniferous 

 escarpment which diagonally crosses the lake in a south- 

 eastern direction from the outlet of Lake Michigan, and 

 which also appears to have its^ effect on free circulation 

 between the deeper waters of these two basins. In the 

 Central basin, at the bottom in 65 fathoms the temperature in 

 July was 42° F., whilst in the Southern basin at the bottom 



