196 PRINCIPLES OF STRATIGRAPHY 



which would otherwise go to heat the water, this reflection being in 

 some instances with the sun near the horizon as high as 68% of 

 the total radiation received (Dufour-5). The heat taken up by the 

 water is chiefly absorbed by the upper layers ; and absorption is 

 greatest in water containing much sediment in suspense. Contact 

 with warm air further warms the upper layers of the water. Trans- 

 portation of the superficial heated layers to cooler depths occurs by 

 convection as well as by wind or by currents. Loss of heat from the 

 upper layers occurs through radiation, and through conduction or 

 contact of the surface with cold air. Fresh water has its maximum 

 density at +4° C. (-}-39-2° F.) and this may serve as a dividing 

 line between zvarm and cold fresh water, the former being above, the 

 latter below this temperature (Forel-8 :/ 05). The densest layers 

 sink, of course, to the bottom, and so warm water will show an ar- 

 rangement of strata progressively cooler downward, i. e., an ano- 

 thermal arrangement. This has been called direct stratification. 

 Cold water, on the other hand, i. e., that below 4° C, will have a 

 reversed stratification, the warmer but denser layers, i. e., those at 

 or near 4° C, lying at the bottom, and the colder above. The tem- 

 perature arrangement will be katothermal. For saline lakes, the 

 temperature corresponding to their maximum density must be 

 chosen as the dividing point. 



Classification of Lakes According to Temperature. From the 

 viewpoint of temperature three types of lakes may be recognized : 

 a. tropical, where the water is always above the maximum density 

 temperature (4° C. for fresh water) ; b. temperate, where it alter- 

 nately rises above and falls below this temperature ; and, c. polar, 

 where it is always below the maximum density temperature. These 

 lakes are, however, not restricted to the corresponding geographic 

 zones of the earth. Tropical lakes always have a direct thermal 

 stratification, or an anothermal arrangement. In spring and sum- 

 mer the stratification becomes increasingly marked, while in fall 

 and winter it decreases until homothermic conditions are approached 

 in winter. Examples of such lakes are the great lakes of upper 

 Italy, i. e., Lake Geneva (Germ. Genfer See, Fr. Lac Leman), etc. 



Polar lakes always have a reversed thermal stratification, or 

 katothermal arrangement, the warmest water (not above 4° C, how- 

 ever) lying at the bottom. Here the stratification becomes most pro- 

 nounced in fall and winter, decreasing in the spring and approaching 

 homothermal conditions in summer. Such lakes occur in the polar 

 regions and in the mountains of the temperate regions. 



Temperate lakes assume a tropical habitus in summer and au- 

 tumn, the stratification being pronounced in the former and tending 



