CAUSES OF CLIMATAL CHANGE 389 



mists which have been invoked as agencies to shield the snow 

 from the sun. In North America the melting snow is ordinarily 

 carried off as liquid water, or as invisible vapour, and the sky is 

 usually clear when the snow is melting in spring. It is only 

 when warm and moist winds are exceptionally thrown upon the 

 snow- covered land that clouds are produced ; and when this is 

 the case, the warm rain that ensues promotes the melting of the 

 snow. Thus there is no possibility of continued accumulations 

 of snow on the lower parts of our continents, under any imagin- 

 able conditions of climate. It is only on elevated lands in high 

 latitudes and near the ocean, like Greenland and the Antarctic 

 continent, that such permanent snow-clad conditions can occur, 

 except on mountain tops. Wallace and Wceickoff^ very pro- 

 perly maintain, in connection with these facts, that permanent 

 ice and snow cannot under any ordinary circumstances exist in 

 low lands, and that high land and great precipitation are neces- 

 sary conditions of glaciers. The former, however, attaches 

 rather too much importance to snow and ice as cooling agents ; 

 for though it is true that they absorb a large amount of heat in 

 passing from the solid to the liquid state, yet the quantity of 

 snow or ice to be melted in spring is so small in comparison 

 with the vast and continuous pouring of solar heat on the sur- 

 face, that a very short time suffices for the liquefaction of a deep 

 covering of snow. The testimony of Siberian travellers proves 

 this, and the same fact is a matter of ordinary observation in 

 North America. 



Setting aside, then, these assumptions, which proceed from 

 incorrect or insufficient information, we may now refer to a con- 

 sideration of the utmost importance, and which Mr. Croll him- 

 self, though he adduces it only in aid of the astronomical theory 

 of glacial periods, has treated in so masterly a manner, as 



^ Von Woeickoff has very strongly put these principles in a Review of 

 Croll's recent book, ** Climate and Cosmology"; American Journal of 

 Science. March, 1886. 



