Lake Ramparts. 



229 



effectually displaced by a natural theory which seems 

 reasonable and adequate. 



A second tentative explanation is that the boulders 

 were moved by waves. We know that the waves which 

 beat on the shores of lakes accumulate sand and gravel in 

 beaches, and we know that the greater ocean waves roll 

 boulders also. But the waves of lakes are smaller and 

 weaker than those of the ocean, and they are very weak 

 in comparison on many small lakes where the ramparts 

 are characteristically developed. There are other reasons 

 for rejecting the wave theory, but as the lack of power 

 seems sufficient, they need not be stated. 



The theory which geographers have accepted makes ice 

 the agent — the sheet of ice which constitutes the winter 

 armor of the lake. If this theory is true, then the ram- 

 parts should be found only in cold countries, and such 

 appears to be the fact. They have been described in 

 Canada, in the New England States, in various States 

 bordering the Lawrentian lakes, and in northern Europe. 

 So far as I am aware, they have not been described in 

 warmer regions, and I have myself noted their absence 

 in Virginia, where the winter coating of ice is thin, in 

 Florida, where the lakes never freeze, and in various 

 parts of Marin County, California. 



Stated in full the theory has the following form. In 

 countries where the winter is cold and long, ice forms on 

 all lakes of moderate size and depth, and acquires a thick- 

 ness of several feet. Ice differs from most solids in that 

 it is made to occupy less space by melting, but so long as 

 it remains solid it follows the general law of solids by 

 contracting when cooled and expending when warmed. 

 It is to the contraction and expansion of ice that the 

 theory appeals. The rate of change has been measured 

 in physical laboratories and found to be such that a plate 

 of ice one mile long would be shortened one foot by hav- 

 ing its temperature lowered a little more than seven 

 degrees of the Fahrenheit scale. 



