CHAP. VIII.] THE CAUSES OF GLACIAL EPOCHS. 



137 



because we have no case at all parallel to it from which we can 

 draw direct conclusions. It is, however, clear from the various 

 considerations we have already adduced, that the increased 

 cold of winter when the exeentricity was great and the sun in 

 aphelion during that season, would not of itself produce a glacial 

 epoch unless the amount of vapour supplied for condensation 

 was also exceptionally great. The greatest quantity of snow 

 falls in the Arctic regions in summer and autumn, and with us 

 the greatest quantity of rain falls in the autumnal months. It 

 seems probable, then, that in all northern lands glaciation would 

 commence when autumn occurred in aphelion. All the rain 

 which falls on our mountains at that season would then fall as 

 snow, and, being further increased by the snow of winter, would 

 form accumulations which the summer might not be able to 

 melt. As time went on, and the aphelion ocQmiQdi in winter, 

 the perennial snow on the mountains would have accumulated 

 to such an extent as to chill the spring and summer vapours, so 

 that they too would fall as snow, and thus increase the amount 

 of deposition ; but it is probable that this would never in our 

 latitudes have been sufficient to produce glaciation, were it not 

 for a series of climatal reactions which tend still further to 

 increase the production of snow. 



Action of Meteorological Causes in intensifying Glaciation. — The 

 trade-winds owe their existence to the great difference between 

 the temperature of the equator and the poles, which causes a 

 constant flow of air towards the equator. The strength of this 

 flow depends on the difference of temperature and the extent 

 of the cooled and heated masses of air, and this effect is now 

 greatest between the south pole and the equator, owing to the 

 much greater accumulation of ice in the Antarctic regions. The 

 consequence is, that the south-east trades are stronger than the 

 north-east, the neutral zone or belt of calms between them not 

 being on the equator but several degrees to the north of it. 

 Bat just in proportion to the strength of the trade-winds is the 

 strength of the anti-trades, that is, the upper return current 

 which carries the warm moisture-laden air of the tropics to- 

 wards the poles, descending in the- temperate zone as west and 

 south-west winds. These are now strongest in the southern 



