CAUSES OF THE GLACIAL PERIOD 573 



seems to be well established and we have discussed its probable mechan- 

 ism. According to our assumption, the temperature would merely be 

 lowered still more than is now the case at times of sun-spot maxima. In 

 the storm belts the temperature would be lower than at present, just as 

 is now the case at such times, but the amount of lowering would not be 

 so great as within the tropics. In polar regions the temperature might 

 remain about the same as now. The facts are not yet well enough known 

 to give any certainty. 



> Having indicated the conditions that would prevail according to our 

 assumption, let us now set the mechanism in motion. In America, and 

 to a less extent in Europe, the more equatorial of the two belts of storms 

 would keep the air of the torrid zone in active motion. Tropical hurri- 

 canes would be more numerous than now, and storms of the eastward- 

 moving type, characteristic of the temperate zone, would abound some- 

 what to the north of the region of hurricanes. The active upward move- 

 ment of the air in the storm centers would produce an abundance of rain 

 and would carry away an abundance of heat. New air would be con- 

 tinually brought from the lands to the oceans and back again, so that 

 evaporation would increase, even though the temperature were lower than 

 now. Thus two conditions would tend to promote the accumulation of 

 snow and the formation of glaciers among the mountains. In the first 

 place there would be more precipitation than now, and in the second 

 place there would be less melting. Such conditions would prevail as far 

 north as the center of the subtropical storm belt. Beyond this would lie 

 the median belt of decreased storminess. The temperature there would 

 apparently be lower than now, but the degree of lowering would presu- 

 mably not be so great as . within the tropics. Storms would occur in 

 summer when the subtropical storm belt moved north, and in winter when 

 the boreal belt moved south. Yet the actual amount of precipitation 

 would probably, and indeed almost certainly, be less than at present. 



North of the subarid zone would lie the great boreal storm belt. Far- 

 ther north than now and more intense it would whirl its storms around 

 the edge of the highlands of Labrador and Scandinavia. It would not 

 only cause precipitation, but also constant cloudiness. Thus the snows 

 of winter would have scant chance to melt. In the colder districts they 

 would gradually accumulate, and as the storms grew more numerous 

 great areas of permanent snow would appear, and continental glaciers 

 would at length begin to creep forth. In their cold centers areas of high 

 pressure would doubtless exist like those which now prevail in Antarctica 

 and Greenland. The presence of these centers would in itself increase 



XLI— Bull. Gkol. Soc. Am., Vol. 25, ini.'^ 



