188 THE POPULAR SCIENCE MONTHLY. 



have so influenced the ocean-currents as to cause great vicissitudes of 

 climate. 



Without entering on any detailed discussion of that last and great- 

 est glacial period which is best known to us, and is more immediately 

 connected with the early history of man and the modern animals, it 

 may be proper to make a few general statements bearing on the rela- 

 tive importance of sea-borne and land ice in producing those remark- 

 able phenomena attributable to ice-action in this period. In consider- 

 ing this question it must be borne in mind that the greater masses of 

 floating ice are produced at the seaward extremities of land glaciers, 

 and that the heavy field-ice of the Arctic regions is not so much a 

 result of the direct freezing of the surface of the sea as of the accumu- 

 lation of snow precipitated on the frozen surface. 



In reasoning on the extent of ice-action, and especially of glaciers 

 in the Pleistocene age, it is necessary to keep this fully in view. Now, 

 in the formation of glaciers at present — and it would seem also in any 

 conceivable former state of the earth — it is necessary that extensive 

 evaporation should conspire with great condensation of water in the 

 solid form. Such conditions exist in mountainous regions sufficiently 

 near to the sea, as in Greenland, Norway, the Alps, and the Hima- 

 layas ; but they do not exist in low Arctic lands like Siberia or Grin- 

 nell-Iand, nor in inland mountains. It follows that land glaciation has 

 narrow limits, and that we can not assume the possibility of great con- 

 fluent or continental glaciers covering the interior of wide tracts of 

 land. No imaginable increase of cold could render this possible, inas- 

 much as there could not be a sufficient influx of vapor to produce the 

 necessary condensation ; and the greater the cold, the less would be 

 the evaporation. On the other hand, any increase of heat would be 

 felt more rapidly in the thawing and evaporation of land ice and snow 

 than on the surface of the sea. 



Applying these very simple geographical truths to the North At- 

 lantic continents, it is easy to perceive that no amount of refrigeration 

 could produce a continental glacier, because there could not be suffi- 

 cient evaporation and precipitation to afford the necessary snow in the 

 interior. The case of Greenland is often referred to, but this is the 

 case of a high mass of cold land with sea, mostly open, on both sides 

 of it, giving, therefore, the conditions most favorable to precipitation 

 of snow. If Greenland were less elevated, or if there were dry plains 

 around it, the case would be quite different, as Nares has well shown 

 by his observations on the summer verdure of Grinncll-land, which, in 

 the immediate vicinity of North Greenland, presents very different 

 conditions as to glaciation and climate. If the plains were submerged 

 and the Arctic currents allowed free access to the interior of the Con- 

 tinent of America, it is conceivable that the mountainous regions re- 

 maining out of water would be covered with snow and ice, and there 

 is the best evidence that this actually occurred in the glacial period ; 



