1851.] HOPKINS ON CHANGES OF CLIMATE. 87 



favourable to a lower position of the snow-line with reference to the 

 line of 32°, than at present, and the latter also to the production of 

 snow. Under such circumstances glaciers might descend to the sea- 

 level, where the configuration of the mountains should be sufficiently 

 favourable to their descent, and supposing the sea to stand at such a 

 relative height as to reach the bases of the mountains. That this 

 was the case, I have little doubt ; for with the conviction that an 

 enormous erratic block like the pierre a bot, above Neufchatel, must 

 have been transported across the valley of Switzerland by floating 

 ice, I think it most probable that the whole Alpine region was, at 

 the glacial period, 2000 or 3000 feet at least lower than its present 

 level ; so that the sea might not only extend to the base of the x\lpine 

 range, but might also penetrate into many of its lower valleys. 



Thus it appears from this investigation, that the same conditions 

 which would produce glaciers on our Welsh and Irish mountains, 

 descending to the level of the sea from a snow-line from 1000 to 1500 

 feet above that level, might also produce similar phsenomena in the 

 Alps with a snow-line 5000 or 6000 feet above the sea. In more 

 northerly regions there would, of course, be no difficulty in accounting 

 for the existence of similar glaciers. 



§ III. Discussion of the Relative Claims of the preceding 

 Hypotheses. 



I have already stated that I considered the hypotheses discussed 

 in the first part of this memoir entirely insufficient to account for any 

 sensible changes of terrestrial temperature in the later geological 

 periods, as they obviously are to render account for a change from 

 a lower to a higher temperature. In the earlier periods of the earth's 

 history, supposing our globe to have been originally, as there are 

 many reasons for believing it to have been, in a state of fusion, its 

 superficial temperature must have been greatly affected by its internal 

 heat long after the solidification of its surface had commenced. Un- 

 doubtedly this cause may be appealed to as sufficient for the produc- 

 tion of almost any amount of terrestrial temperature ; but, if the tem- 

 peratures thus to be accounted for be many degrees above the existing 

 temperatures, we can only account for them by this theory with 

 reference to periods of very remote geological antiquity (art. 6, p. 59). 

 I have also shown that any sensible effects of a difference of intensity 

 in stellar radiation can only be referred to similarly remote epochs, 

 and even for those periods the theory founded upon this notion 

 appears to me but vague and unsatisfactory. 



Another theory of the changes of terrestrial temperature has been 

 founded on the notion of a variation in the intensity of solar radiation. 

 This cause, once admitted, might undoubtedly be deemed adequate to 

 account for all the changes in question, nor does there appear to be 

 any well-defined a-priori objection to it. No theory, however, can 

 be satisfactory which presents itself as a mere hypothesis framed to 

 account for a single and limited class of facts, and unsupported by 

 the testimony of any other class of allied, but independent phse- 



