244 THE GLACIAL OR ICE EPOCH. 



ground the chief heat conveyed to the north being by- 

 atmospheric currents. During the third stage, when the 

 land began to rise, the elevation, they argue, was only 

 partial, and accompanied by depressions, which permitted 

 warm oceanic currents to penetrate further north, and thus 

 gradually to dissolve the ice -sheet to its present limits. 

 Others also, who seek for the solution in the peculiar dis- 

 tribution of sea and land, contend that the ice-mantle was 

 never general over the northern hemisphere, but that it 

 passed from area to area according to the set and direction 

 of the polar sea-currents. Were the existing arctic current, 

 say they, to pass down by the coasts of Norway instead of 

 by the coasts of Labrador, the glaciers of Scandinavia would 

 envelop the whole peninsula, and come down, as they do 

 in Spitzbergen, to the very sea-shore. It is only necessary, 

 these theorists contend, to arrange the polar currents and the 

 warm currents (like the Gulf Stream), or, in other words, the 

 northward and southward currents of the ocean, in such a 

 way as to produce over certain areas glaciers and icebergs, 

 and all the accompanying phenomena of the glacial epoch. 



Those, on the other hand, who appeal to extraneous causes, 

 maintain that the ice-sheet was general and contempor- 

 aneous, and could only be produced by forces affecting alike 

 the whole of the northern hemisphere. In their opinion, 

 there was only a gradual advance, and as gradual a departure, 

 of the ice- epoch. And this advance and departure may 

 have been brought about by our planetary system passing 

 through some colder region of space ; by some secular or 

 recurrent diminution of the sun's heat; by greater eccen- 

 tricity of the earth's orbit; or by some secular deviation 

 in the earth's inclination, depending upon change of centre 

 of gravity, or the precession of the equinoxes. In other 

 words, the causes they seek to establish are planetary 

 and not terraqueous, and such as have already occurred, 



