IXTRODUCTOKY : THE OCCUraiEXCE OF LAKES. 15 



tvveen two adjacent regions of country, and it will be desirable to inquire into 

 the causes which have brouo-ht about this curious condition of thino-s. 



The causes which have produced this extraordinary complex of lakes in 

 the northeastern region of North America are by no means simple. They 

 are partly orographic and geological, partly climatological. Ice has also 

 played a part in this work, although, as it appears to the writer, quite a sub- 

 ordinate one. 



The orographic element in the work will be easily recognized from an 

 inspection of a good map of North America. If two parallel lines be drawn, 

 about 500 miles apart, one touching Georgian Bay of Luke Huron, tlie other 

 the southwest corner of Lake Michigan, and prolonged in a northwesterly 

 direction to the Arctic Ocean, the great series of lakes belonging to the sys- 

 tem will be found lying within the belt of country thus limited. These lines, 

 however, are parallel with the general trend of the adjacent portion of the 

 Cordilleras, showing at once the orographic character of the belt of depres- 

 sions. Furthermore, the direction of the axes of the larger lakes within this 

 area is usually at right angles to the line of trend mentioned, which fact 

 is another important guide to the character of the forces by which these 

 depressions have been produced. Again, the connection of the orographic 

 character of the lake belt in question with its geological position is most 

 marked. This great chain of lakes, as will be apparent whenever a good 

 geological map of the region shall have been published, lies on and in the 

 neighborhood of the belt separating the modern geological formations which 

 belong to the Rocky Mountain uplift, from the Azoic or older crystalline 

 schists and granites, forming the mass of the Lauren tian Range. This belt is 

 made up of Palaeozoic rocks, and it is in these that the large lakes are chiefly 

 developed. The moment that we pass from the older rocks on to the more 

 recent, in going westward, Ave at once leave the lacustrine region behind. 

 A portion of this belt is depressed beneath the sea-level even, Lake Superior, 

 the surface of which is 609 feet above that line, being in places over 1,000 

 feet in depth. This lake occupies a synclinal depression of the Palaeozoic 

 rocks, just along the edge of the Azoic Series.* 



Lake Huron is similarly situated, its northern edge abutting against the 

 older crystalline rocks. In tact, all the Great Lakes, including those of 

 British America, from Ontario arouml the great sweep of 3,000 miles to the 

 Arctic Ocean, are in a similar geological position, occupying a depressed 



* As long since shown liy Fostui' and Wliitiiey. See Ri'iiort ou tile Geology of Lake .Superior, Part II. ji. 117. 



