CHAP. Yiii THE CAUSES OF GLACIAL EPOCHS 



157 



summers being proportionately shorter and hotter, without 

 any other change whatever. The long cold winter would 

 certainly bring down the snow-line considerably, covering 

 large areas of high land with snow during the winter 

 months, and causing all glaciers and ice-fields to become 

 larger. This would chill the superincumbent atmosphere 

 to such an extent that the warm sun and winds of spring 

 and early summer would bring clouds and fog, so that the 

 sun-heat would be cut off and much vapour be condensed 

 as snow. The greater sun-heat of summer would no doubt 

 considerably reduce the snow and ice ; but it is, I think, 

 quite certain that the extra accumulation would not be all 

 melted, and that therefore the snow-line would be per- 

 manently lowered. This would be a necessary result, 

 because the greater part of the increased cold of winter 

 would be stored up in snow and ice, while the increased 

 heat of summer could not be in any way stored up, but 

 would be largely prevented from producing any effect, by 

 reflection from the surface of the snow and by the inter- 

 vention of clouds and fog which would carry much of the 

 heat they received to other regions. It follows that 10,000 

 years hence, when our winter will occur in a'plidion (instead 

 of, as now, in perihelion), there will be produced a colder 

 climate, independently of any change of land and sea, of 

 heights of mountains, or in the force of oceanic currents. 



But if this is true, then the reverse change, bringing the 

 svm back into exactly the same position with regard to us 

 as it is in now (all geographical and physical conditions 

 remaining unchanged), would certainly bring back again 

 our present milder climate. The change either way would 

 not probably be very great, but it might be sufficient to 

 bring the snow-line down to 3,000 feet in Scotland, so that 

 all the higher mountains would have their tops covered 

 with perpetual snow. This perpetual snow, down to a 

 fixed line, would be kept up by the needful supply of snow 

 falling during autumn, winter, and spring, and this would, 

 as we have seen, depend mainly on the increased length 

 and greatly increased cold of the winter. As both the dura- 

 tion and the cold of winter decreased the amount of snow 

 would certainly decrease, and of this lesser quantity of snow 



