INTrxODUCTORY : THE OCCUERENCE OF LAKES. 17 



would be only a logical sequence of ideas to declare the ocean itself to have 

 been washed out from the land, and the continental masses built up from the 

 material thus eroded. 



The orIi>in of the smaller lakes with which certain areas of the earth's sur- 

 face are profusely dotted is a problem much more difficult than the one just 

 discussed. As already mentioned, there are two regions in which small 

 lakes occur in the greatest abundance. One is the country north of the St. 

 Lawrence, a district of both large and small bodies of water ; the other, Fin- 

 land. Such small lakes are almost invariably quite shallow ; it can only be 

 a very peculiar and highly exceptional condition of things which can produce 

 a very deep lake of small area. The essential facts influencing the forma- 

 tion of small lakes arc, a nearly level surface of the region in which they 

 occur, and an underlying rock formation impervious to water. 



Both Finland and tiie Canada lake region are underlain by crystalline 

 rocks, which are almost impermeable. If we follow on the map the outlines 

 of the district in Northeastern America where lakes abound, we find that 

 these are left behind as soon as we pass out of the domain of the crystalline 

 or metamorphic rocks into that of stratified and permeable formations. For 

 instance, the contrast between the frequency of lakes in Northern Wisconsin, 

 a region of Azoic rocks, and the central and southern portions of the same 

 State, where stratified sandstones are the predominating formation, is most 

 striking. The peninsula of Southern Michigan, and that triangular portion 

 of Canada lying between Lakes Erie and Huron, offer similar contrasts when 

 compared with adjacent regions of impermeable rocks. Minnesota and the 

 adjacent Territory of Dakota also afford an excellent illustration of the limi- 

 tation of lakes to regions of rocks whose te.xture is impervious tc water. 



That level regions of country should be more favorable to the development 

 of small lake basins than the flanks of mountains, is easily understood. A rapid 

 descent gives the streams great erosive power, and they are enabled to wear 

 away their beds so as to drain depressions which would othei'wise be partly 

 or wholly filled with water. Given a level region underlain by impervious 

 rocks, the precipitation being moderately in excess of the evaporation, and 

 it is manifest that the water must stand upon the surface over areas of 

 greater or less magnitude, unless there can be some reason assigned why 

 that surface should be absolutely level. Deposits laid down under the ocean 

 are more likely to be evenly distributed than those which result from fluvial 

 action, because the former are the result of agencies working on a grand 



