1868.] 



147 



[Shaler. 



of accounting for the successive occurrence of ice sheets at many 

 stages in our earth's development, tlian we could otherwise have; 

 and the difficulties which it thereby removes renders it by far the 

 most valuable and best supported hypothesis which has yet been ap- 

 plied to the phenomena. There can be no doubt that it is reasonable 

 to make it the basis of any considerations we may apply to the phe- 

 nomena of the glacial period. 



This hypothesis requires us to sujopose that eacji hemisphere in 

 succession has been submitted to alternating periods of heat and cold, 

 each period extending over many tens of thousands of years. The 

 diminution in the duration of the summer heat prevented the melting 

 of the glacial accumulations of the winter season so tjiat the ice sheets 

 gradually grew wider while they enwrapped the fountain chains, 

 and finally buried much of the continents beneatl); continuous ice. 

 The reduction of temperature would be necessarily ^bout the same, 

 in all regions beneath the same parallels. The is[)thermals would 

 probably be pushed southward without any radical alteration of the 

 curves which now characterize them. The ocean and atmospheric 

 currents probably have been enlarged so that the relative heat of 

 different regions of either hemisphere would not pj-obably be much 

 'influenced by the change from the period of extreme jieat to extreme 

 cold. j 



It will be seen by the inspection of a map of thej northern hemi- 

 sphere, which gives the isothermal lines, that the i^thermal of 30° 

 Fahr., which passes through the valley of the Yuk(|n Kiver, passes 

 close to the southeastern coast of Greenland, north jof Iceland, and 

 touches the European continent at the northernmost point of the 

 Scandinavian peninsula. It is quite beyond question ihat, during the 

 last glacial period, the whole of Labrador, Greenland aild Scandinavia, 

 and the land for many degrees to the southward, weie subjected to 

 glacial conditions ; nearly every great valley being the seat of gigan- 

 tic glaciers, and much of the surface buried to the depth of many 

 thousands of feet. Now, it is impossible to understand aow conditions 

 affecting the whole northern hemisjahere equally, could have carried 

 the ice line so far south, that near Harrisburg, Penn.', in about 41° 

 of latitude, the ice sheet was over one thousand feet (leep, while at 

 the mouth of the Yukon River, in about 68°, there should have been 

 no glacial accumulation whatever. The question then arises, shall 

 we suppose that although the temperature of the region may have 

 been such as to admit of glaciation, there was a want of precipitation 

 of water so great as to prevent the formation of glaciers, or are we 



