178 PHYSICAL GEOGRAPHY. 



its head lying within or to the eastward of the termina- 

 tion or elbow of the " intermediate primitive rocks." 

 The position of this head, or as it is well named from its 

 elevation above the other members of the basin, Superior 

 Lake, is midway between the Gulf of Mexico and the 

 Arctic Sea, and its water-surface is 641 feet above the 

 level of the tide.* The other great lakes descend suc- 

 cessively in the following order of their heights above the 

 sea: Lakes Huron and Michigan, 600 feet; Erie, 565 feet; 

 Ontario, 492 feet ; and Lake Champlain, 93 feet. 



At the west end of Lake Superior, and on its northern 

 shores, several promontories, having an altitude of from 

 800 to 1000 feet above the water, give a mountainous 

 character to the coast when seen from the surface of the 

 lake, but which it is far from possessing when viewed in 

 relation to the country lying behind it. It is such as 

 would result from the excavation of the basin by the re- 

 moval of the softer rocks which have covered the granites, 

 porphyries, and traps of these eminences. The silurian 

 beds, not having been so extensively broken up to the west- 

 ward of the Fon du lac, envelop the pyrogene nucleus so 

 as, in conjunction with recent arenaceous deposits and drift, 

 to cover it on that flank almost to the summit. From this 

 locality, which is rather a plateau than a mountainous 

 district, issue the feeding streams of the three several river 



* Some discrepancy exists between the heights assigned to this 

 lake by different authors. We have taken that deduced by Captain 

 Lefroy from barometrical measurements made in connection with the 

 observatory at Toronto. Dr. Houghton, the Michigan geological sur- 

 veyor, estimates its height at 641 feet; but he makes the descent 

 from it to Lake Huron 45 feet, while Professor Henry reckons this 

 descent at only 18 feet, which must be under-estimated. Mr. Logan, 

 in 1847, sets down the height of Lake Superior as 597 feet, having 

 adopted for the height of Lake Michigan 578 feet, from Professor 

 Henry. In the height of Erie and the inferior lakes authors are 

 generally agreed. 



