38 LAKES OF NORTH AMERICA. 



entire water-bod}' to become cooled to the degree of maximum density. 

 Until this happens the water cooled at the surface from contact with the 

 air, has its density increased and sinks, and is replaced by warmer and 

 consequently lighter water, rising from below, and ice cannot form. 



The surface waters of deep lakes are thus above the mean temperature 

 of the adjacent atmosphere in winter ; but in summer they are cooler than 

 the air, as 4he warmed surface layer loses heat by conduction downward. 

 The winds that blow over them are thus tempered in a manner congenial 

 to the growth of vegetation both in warm and in cold Aveather. 



The influence of the Laurentian lakes on the climate of their shores 

 is well marked, as was clearly shown many years since by Alexander 

 Winchell.^ On charts that have appeared showing the winter and sum- 

 mer isobar, that is, lines drawn through the various localities having the 

 same mean temperature, the lines showing the mean summer temperature 

 curve northward in the vicinity of Lake Michigan especially, Avhile the 

 lines indicating mean winter temperature present an equally marked 

 southern curvature, showing that the lakes cool the air that passes over 

 them in summer and warm it in winter. The genial influence of the 

 lakes is also plainly to be seen in the distribution of the fruit-belts of 

 Michigan, Ohio, and New York. 



If we should construct a map showing the mean humidity of the air, 

 by drawing lines through the localities having the same " relative humid- 

 ity," the influence of the lakes would be quite as apparent as in the case 

 of the isothermal lines, but the curvature in both winter and summer 

 would be southward. 



The amelioration of climate produced by large inland water-bodies 

 has an important influence on the flora and fauna of their borders, and 

 therefore on the character of the fossils entombed in their sediments. 

 Another fact of geological interest in this connection, is that rocks decay 

 more rapidly under Avarm, moist climates than in arid or in Arctic regions, 

 and deeper and richer soils are produced. This, again, influences the 

 hfe of lake regions, and is, perhaps, of sufficient imjjortance to be con- 

 sidered in interpreting the records of ancient inland water-bodies. 



Influence of lakes on the flow of streams. — Lakes act as storage 

 reservoirs and regulate the floAv of the streams, of Avhich they are enlarge- 

 ments. In the case of a river subject to sudden freshets, the disastrous 



i"The isothermals of the Lake Region," Am. Assoc. Adv. Sci., Proc., vol. 16, Troy 

 Meeting, 1870, pp. 106-117. 



