198 PRINCIPLES OF STRATIGRAPHY 



of tropical lakes, where a thermal barrier separates this from the 

 littoral zone, as in the above illustration from Lake Geneva. In 

 quiet lakes, where the surface water has reached 0°, a slight lower- 

 ing of the temperature, as during a still, clear night, will cause the 

 formation of a uniform, continuous, though thin sheet of ice over 

 the entire surface, which, if not melted during the succeeding day, 

 will thicken the following night by addition of ice on its under side, 

 until a thick crust is formed. The increase in thickness is, however, 

 at a diminishing rate, owing to the low conductivity of ice. 



In disturbed waters ice floes are formed, a method of ice forma- 

 tion characterizing the sea and rivers as well. These pancake 

 masses (Fr. glacons-gateaux, Ger. Kuchschollcn) result from the 

 union of the ice needles tossed about by the waves, these needles 

 often forming suddenly in the super-cooled mass, through agita- 



FiG. 30, Diagram representing thermal barrier of 4° between the littoral and 

 pelagic districts of Lake Geneva in winter. The barrier is in reality a 

 vertical sheet. (After Forel-8.) 



tion, or through the dropping in of snowflakes, analogous to the 

 formation of crystals in a supersaturated solution on agitation or 

 on dropping in of a crystal. The round form of the pancake floes is 

 due to the constant friction they undergo. The density of ice is 

 about 0.92, while that of pure water at 0° is 0.9998676 and that of 

 normal sea water is 1.028. (Pure water at 4° [or better 3.947°], 

 its maximum density, is taken as i.) It is, therefore, evident that 

 ice will float on fresh as well as on salt water of the temperature 

 permitting its formation. Owing to the addition of ice fragments 

 and crystals on the margins of the cakes where they are thrown by 

 the waves, the weight of the mass will increase until it sinks suffi- 

 ciently to be covered by a thin stratum of water, which will cause 

 further addition to the ice mass on its surface, while at the same 

 time it grows marginally as well as on its under side. The ice mass 

 eventually acquires the form of a plano-convex lens, with the con- 

 vex side downward and a thickness of a meter or more. The ice 

 floes will freeze together when heaped up by waves or currents or 

 when coming in contact after the water has quieted down. 



