296 CLIMATE OF THE 



as throwing so much light on the transport of the blocks 

 now in question, is actually in a lower latitude than the Seine 

 , or the Somme. Moreover, geologists are agreed that at the 

 period of the boulder clay, a period immediately preceding 

 that now under consideration, the cold in Western Europe 

 must have been far more intense than it is at present. The 

 subject has been discussed in an excellent paper by Mr. 

 Hopkins* (then President of the Geological Society), and it 

 is admitted (p. 61) that many of our rivers have probably 

 followed their present directions "ever since the glacial 

 period." Mr. Prestwich's hypothesis involves therefore in 

 reality no change in our views as to the climate of Western 

 Europe. He only supposes that, in this early period of our 

 rivers, the temperature resembled that which had preceded, 

 more than that which now prevails ; or, rather, perhaps, 

 that, in this intermediate period, the climate had neither 

 the extreme severity of the glacial era, nor the exceptional 

 mildness of modern times, 



But though thus explaining the manner in which the 

 sandstone blocks may have been transported, these considera- 

 tions throw no light on the change of conditions which must 

 have taken place to produce an alteration of climate so great 

 as that which is supposed to have taken place. 



In Mr. Hopkins' memoir on the subject, the principal 

 causes which have been suggested for this change of climate 

 are the following : 



Firstly. A variation in the intensity of solar radiation. 



To this theory Mr. Hopkins sees no a priori objection ; but 

 he does not feel disposed to attach much weight to it, because 

 it is " 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 phenomena." 



It is, however, open to the objections stated with great 

 * Geol. Journal, 1852, p. 56. 



