of the Glacial Epoch. 331 



depression of the snow-line itself from the attendant circumstances 

 just enumerated : viz., 1st, from increased precipitation, which 

 would cause such an accumulation of snow during the winter 

 months, as would defy the heat of the succeeding summer to 

 melt ; for it must be borne in mind that the amount melted is 

 proportional, not to the mass, but to the surface exposed to the 

 thawing influence ; and 2nd, from the reduction of summer 

 temperature, owing to the interception of the sun's rays by an 

 overcast sky. But it is evident that this lowering of the snow- 

 line by increased oceanic temperature could only occur within 

 certain limits ; for although the mean temperature of the snow- 

 line might rise from 21° (its present position in Norway) to 35°, 

 its height under the equator, and perhaps even still higher, 

 without any elevation of the snow-line itself, yet a further rise 

 of mean temperature, which would result from a continued aug- 

 mentation of oceanic heat, could not fail to elevate the snow-line 

 itself, and eventually to chase the last portions of snow even 

 from the loftiest mountain peaks. A process the inverse of this 

 I believe to have gone on in nature, leading gradually to the so- 

 called glacial epoch, and eventually to the present meteorological 

 condition of our globe. The ocean once possessed a tempera- 

 ture so high that the snow-line floated above the summits, pos- 

 sibly even of the most lofty mountains ; but with the reduction 

 of oceanic warmth it gradually descended, enveloping peak 

 after peak in a perennial mantle, until during the most severe 

 portion of the glacial epoch it attained its lowest depression, 

 whence it again rose to its present position, owing to diminished 

 evaporation, the effect of which, in elevating the line of perpetual 

 snow, has already been explained. 



Having thus endeavoured to prove that all the phenomena of 

 the glacial epoch would be normally evolved by the gradual 

 cooling of the ocean from a higher degree of heat down to its 

 present temperature, it remains for me to suggest a cause of 

 such a higher oceanic temperature, and to remove certain geolo- 

 gical and palseontological objections which may be urged against 

 this hypothesis. 



First, as regards the cause of the assumed higher tempera- 

 ture, it may be stated as an exhaustive proposition that the 

 accession of heat must either have been from without, from a 

 cosmical source, or from within, that is, of secular origin. 

 Now only two cosmical causes of the variation of terrestrial tem- 

 perature have yet been suggested ; and to these I have already 

 alluded. The first of them, viz. the hypothesis which assumes 

 that our solar system may have passed through portions of space 

 in which it would receive more heat-radiations from the stars, 

 has been elaborately reviewed by Mr. Hopkins in the memoir 



