24 On the Causes of Change in the 



of its summit to be favourable, glaciers will be formed upon it, of 

 which the magnitude will depend on circumstances. If large, we 

 might expect them to descend 4000 or 5000 feet below the snow- 

 line, and to a distance proportionally less when the glaciers should 

 be small. 



As an example, we may take Snowdon, in the case in which the 

 Gulf Stream is supposed to be absent from the shores of western 

 Europe, and a large portion of that continent submerged beneath the 

 ocean. I have shewn that the temperature of Snowdon would pro- 

 bably not exceed 39° or 40° F. Assume it 39°. Taking a decrease 

 of temperature of 1° in ascending 320 feet, the height of the line 

 of 32° would be 2240 feet. The climate w r ould be entirely an in- 

 sular one, and therefore the height of the snow-line would probably 

 not exceed that of the line of 32° by more than 200 or 300 feet. If 

 we suppose this additional height to be 260 feet, the absolute height 

 of the snow-line would be 2500 feet, or 1000 feet less than that of 

 the present summit of the mountain. If we assume the mountain 

 to subside 400 or 500 feet with the surrounding region, it would 

 still rise 500 or 600 feet above the snow -line, a height sufficient to 

 admit of the formation of glaciers, which might descend to the level 

 of the sea. If, in addition to the hypothesis of the absence of the 

 Gulf Stream, we adopt also that of a cold current from the north, the 

 mean annual temperature might be reduced 3° or 4° F. lower than 

 above supposed, which would reduce the height of the snow-line to 

 1200 or 1 500 feet. This would admit of the formation of glaciers, 

 not only on Snowdon, but also on the lower mountains of Ireland. 

 And I may here remark, that if we can thus account satisfactorily for 

 climatal conditions consistent with the existence of glaciers in the 

 south-west of Ireland, the subject presents no difficulty with refer- 

 ence to any other part of western Europe, in which we observe the 

 traces of glacial action. 



For the application of the same method of investigation to the 

 other hypothetical cases of the distribution of land and sea, I must 

 refer to the memoir of which I am speaking. I have selected the 

 above case, not only because it seemed best calculated to elucidate the 

 subject, but also because I consider it that which has far the highest 

 claim to our acceptance as the real one of the glacial epoch. It 

 involves, as we have seen, the absence of the Gulf Stream as an 

 essential condition, the explanation which it affords of the existence 

 of ancient glaciers being rendered more complete by the supposition 

 of a cold current from the north. On these points it remains for me 

 to say a few words. 



I need scarcely remind you that the evidence of the submergence 

 of a very large portion of the North American continent during the 

 Drift period is similar to that for the submergence of Europe. A 

 subsidence of less than 2000 feet would render the ocean continuous 

 from the Apalachian chain, on the east, to the Rocky Mountains on 



