Conditions favorable to Glaciation. 107 



There is a difference however of considerable importance. In 

 either case the maximum mean rate of receipt of sunshine, or 

 the maximum temperature during the northern winter, is at 

 the Tropic of Capricorn and it is from the maximum that the 

 winds laden with moisture will blow toward either pole.* 

 Now when the obliquity was 1° 9' greater than it now is, the 

 Tropic of Capricorn was so much further south, and the area 

 to the north of this tropic was about 1,800,000 square miles 

 greater than that north of the corresponding parallel to-day. 

 This area is somewhat greater than the sum of the areas of the 

 Mediterranean Sea and the Gulf of Mexico. The gradient 

 northward in winter was almost exactly what it is to-day, the 

 temperature was lower throughout, though not more than a 

 few degrees ; but this was compensated for, more or less com- 

 pletely, by the increased area of evaporation supplying precipi- 

 tation to high northern latitudes. It would appear therefore 

 that the precipitation may have been as great as it now is, but 

 that a larger part of the precipitation must have been snow. 



In summer at high obliquity the zone of evaporation was 

 1,800,000 square miles less than it now is, and the temperature 

 gradient toward the pole was smaller than any other, smaller 

 even than that of the summer in the genial hemisphere of the 

 period of highest eccentricity. Hence it was perhaps the dry- 

 est possible summer. Its temperature was somewhat below 

 chat of the present time in the southern hemisphere from the 

 equator to latitude 45°. Beyond this point it was a little 

 higher but, as has been pointed out, dry summer heat in very 

 high latitudes cannot greatly diminish the accumulation of 

 snow. 



I began this enquiry without the remotest idea as to what 

 conclusion would be reached. At the end of it I feel com- 

 pelled to assert that the combination of low eccentricity and 

 high obliquity will promote the accumulation of glacial ice in 

 high latitudes more than any other set of circumstances per- 

 taining to the earth's orbit. It seems to me that the glacial 

 age may be due to these conditions in combination with a 

 favorable disposition of land and water. This theory implies, 

 or rather does not exclude simultaneous glaciation in both 

 hemispheres. It does not imply that the ice age should last 

 only ten or twelve thousand years. If the conditions here 

 suggested are correct, variations in the disposition of land and 

 water may have determined intervals of glaciation, not neces- 

 sarily the same ones in New England and the basin of the 

 Mississippi ; and there may have been considerable time dif- 



* It is well known that the July and January winds blow across the equator. 

 This tendency is strongest in July because of the greater land area of the north- 

 ern hemisphere. 



