338 Prof. Frankland on the Physical Cause 



logists, still I may as well here remark that such comparatively 

 remote glacial action is perfectly compatible with the views I 

 am here advocating. It is in fact a necessary consequence of 

 these views, that the so-called glacial epoch should not be a 

 sharply limited period, although its termination, for the reasons 

 just given, would probably be much more definite than its com- 

 mencement. I have already argued that perpetual snow would 

 first tip the mountain peaks, and then slowly and gradually 

 descend to the sea-level. But it must be borne in mind that 

 during the whole of the pre-glacial period the atmospheric pre- 

 cipitation was even greater than during that period, and conse- 

 quently wherever the land rose well above the snow-line, glaciers, 

 on a scale far surpassing any of the present time, would be the 

 inevitable consequence. It is, I believe, exceedingly difficult 

 for a geologist to reconstruct even an approximation to the con- 

 tour lines of land during the Permian period. Nevertheless in 

 suggesting a glacial episode during this period, Ramsay* 

 believes that he has considerable evidence of the existence of a 

 range of hills whence these glaciers descended. 



Such considerations respecting the pre-glacial period present 

 to us a vivid picture of the then terrestrial climate. The shores 

 of the warm seas would possess a genial and remarkably equable 

 temperature, the air always warm and moist, the earth screened 

 from the summer's sun by a clouded sky, and protected from the 

 cold of winter by a canopy of transparent aqueous vapour, imper- 

 vious to terrestrial radiation. Receding from the coast on level 

 ground, these peculiarities would become gradually fainter ; but 

 on a somewhat steep shore the approximation of warm and cold 

 climates must have been much closer than at present. The 

 rivers and lakes fed by mountain-snows would also present a 

 marked contrast to the surrounding lowlands — thus exhibiting 

 within a narrow region a wide range of temperature, adapted to 

 the thermal habits of widely different organisms. 



As the glacial epoch approached, this genial zone would be 

 gradually narrowed by the progressive descent of the snow-line ; 

 for although the actual amount of heat in activity at the surface 

 of the earth was greater during the glacial period than subse- 

 quently, yet the cold of winter became stored up in masses of 

 falling snow, which in melting absorbed the heat of the succeed- 

 ing summer, and thus reduced both the mean and summer tem- 

 perature of such tracts of land as were not situated greatly below 

 the snow-line. Nor was this downward march of a frigid cli- 

 mate arrested even by the ocean itself, but the vast flow of 

 glacial ice into the sea formed a belt of cold water along the 

 shores, where many marine organisms that have left their traces 

 * Proceedings of the Geological Society, August 1855. 



