THE ICE AGE IN NORTH AMERICA. 



CHAPTER I. 



WHAT IS A GLACIER? 



To the ordinary man of science, water is a mineral and 

 ice a rock ; but to the glacialist both are fluids. The appar- 

 ent solidity of ice is an illusion due to the dullness of our 

 senses. The reason why its viscous or semi-fluid character 

 remained unsuspected until a comparatively recent period is 

 due to the fact that the ordinary movement of accessible 

 glaciers was so slow that we could not by observation readily 

 note their rate of progress. 



The difference between water and other substances is 

 most noticeable in the phenomena connected with solidifi- 

 cation and fusing. Lead melts at 612° Fahr. above zero ; 

 sulphur, at 226° ; water, at 32° ; while mercury becomes 

 liquid at 39° below zero, and some other substances at even 

 lower temperatures. Thus, with reference to its fusing- 

 point, water appears toward the middle of the scale. If, 

 like the fabled salamander, man were able to endure intense 

 degrees of heat, he might, very likely, sustain relations to 

 iron similar to those he now sustains to water. He might 

 then bathe with pleasure in a molten flood, and venture on 

 the thin crust of a glowing mass of metal. 



The suddenness with which water passes from the solid 

 to the liquid state, and the amount of heat absorbed in the 

 process of fusion, involve many important consequences, 

 Down to the freezing-point water may be made to part with 

 its heat by gradual stages, but in the act of freezing it sud- 



