chap, via] 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 occurred 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. 
But 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 
