530 THE ICE AGE IN NORTH AMERICA. 



zling. The heat of the sun goes through the glass, but can 

 not readily get out again. It is well known, also, that a 

 slight increase of moisture in the atmosphere, or a slight 

 film of cloud over the sky, prevents a frost. The real prob- 

 lem lies, therefore, in the meteorological field. Now, during 

 Mr. Croll's " aphelion " winters, the summers are in " perihel- 

 ion," and the summer heat in this hemisphere while in peri- 

 helion is more intense than at other times. In fact, the 

 earth receives at all times the same absolute amount of heat 

 from year to year. Thus, we can not avoid the conclusion 

 that the predominant influence in climate may consist in the 

 power of moisture-laden atmosphere to retain and transport 

 the heat, thus determining its distribution. As a matter of 

 fact, we find that the equator is not so hot as theoretically it 

 should be, and the arctic regions are by no means so cold as, 

 on Croll's theory, they ought to be. The difference between 

 the mean temperature on the equator and that at the coldest 

 point on the sixty-seventh parallel is really only about 75° 

 Fahr. ; whereas, if the temperature at these points were pro- 

 portionate to the amount of heat received from the sun, the 

 difference would be 172°. Such facts as these lead meteor- 

 ologists to regard Mr. Croll's theory with much less favor 

 than formerly. 



But the glacialist is not so much concerned to know the 

 ultimate cause of the Glacial period. as he is to collect the 

 facts which characterize the period. The truth is, that the 

 meteorological forces of Nature are so powerful and complex 

 that there is an embarrassment of riches in the field of gla- 

 cial theory. It is easy to see that a slight increase of snow- 

 fall over the Alps would cause a permanent enlargement of 

 all the glaciers of Switzerland, and threaten every interest of 

 that republic, and perhaps of central Europe ; for the ulti- 

 mate effects of a climatic disturbance in one such center can 

 not well be estimated. 



Much light upon the condition of things during the Gla- 

 cial period in America must yet come from a careful study 

 of the lobate contour of the terminal moraines. The shapes 



