188 SPARKS FROM A GEOLOGIST'S HAMMER. 



not the mean cold or aggregate heat of the season. It is 

 the extreme of humidity or dryness which determines 

 whether crops shall fail, not the aggregate rain-fall for 

 the year or for the season. 



So, in this case, Mr. Croll perceived that the diminished 

 intensity of the sun's rays during winter would increase 

 the tendency to snowy precipitation. Five degrees of 

 temperature often decide whether precipitation shall be 

 in the form of rain or snow. The wintry days in a time 

 of extreme eccentricity would therefore be more abun- 

 dantly characterized by snow-falls. In this case the very 

 prolongation of the winter season, while bringing the ag- 

 gregate of solar heat up to the average, would only pro- 

 long the period of snowy accumulations. Both causes, 

 then, would contribute to an increased amount of snow. 

 The conclusion is that the direct effect of a coincidence 

 of the winter solstice with aphelion, during a period of 

 high eccentricity, would be the accumulation during each 

 winter of a vast amount of snotv, stretching many degrees 

 farther from the pole than the snow-cap of our present 

 winters. 



But when the short, hot summer should succeed, would 

 not the snow-cap be removed to as great an extent as 

 under the actual circumstances? This is a critical ques- 

 tion. Mr. Croll maintains that it would not; and the 

 second chief characteristic of his investigations consists in 

 his elucidation of the reasons for affirming that the short, 

 hot summer, at time of perihelion in extreme eccentricity, 

 would not undo the wintry work of a long, cold aphelion. 



Let it be the northern hemisphere which has its winter 

 solstice in aphelion during a period of high eccentricity. 

 The intense summer rays falling upon continents clothed 



