xv The Continental Area 287 



Mackenzie River flows parallel to the Rocky Mountains to 

 the Arctic Sea, receiving the outflow from Great Bear Lake 

 on the Arctic circle. 



369. St. Lawrence System. The gentle transverse 

 ridge separating the northern and southern slopes of North 

 America is nowhere higher than 2000 feet, and it only 

 attains this elevation in the east. Its surface is slightly 

 concave, the northern eidge, called the Height of Land, 

 being a continuation westward of the Archaean plateau of 

 the Laurentides ; while the southern edge, known as the 

 Great Divide, is a prolongation toward the east of the 

 moraine heaps of the Cot'eau des Prairies. The central 

 hollow contains a remarkable group of lake basins, which 

 are claimed, with some probability, to contain half of the 

 fresh water in the world. Before the Ice Age they were 

 probably in connection with the Mississippi river system, 

 and from ancient raised beaches surrounding them they 

 were evidently at one time much more extensive than now. 

 The western group Lakes Superior, Michigan, and Huron 

 are closely connected, and their surface stands about 600 

 feet above sea-level. From the south of Lake Huron they 

 discharge into Lake Erie, whence the Niagara River 

 ( 330) leads northward into Lake Ontario, from which the 

 broad St. Lawrence sweeps, on to the Atlantic. 



370. Australia, the onlj^knoww continent entirely in the 

 southern hemisphere, is 2300 miles in extreme length along 

 the parallel of 26 S. (see section Fig. 60). The greatest 



FIG. 60. Section across Australia in 26 S. Vertical scale 300 times 

 the horizontal. Sea-level marked O. 



breadth is 2000 miles along the meridian of 143 E. from 

 Cape York in 11 S., which is the most northerly point, to 

 Cape Otway in 39 S. Incurves of the north and south 



