258 OKIGINAL AETICLES. 



While, however, admitting these two possible objections, it is 

 still, I think, felt by most Palaeontologists, that though the presence 

 of one Arctic S2:)ecies would scarcely perhaps justify any very decided 

 inference as to climate, still that the co-existence of such a group 

 as this ; the musk ox, the reindeer, the lemming, the Mi/odes tor- 

 qitatus, the Siberian mammoth, and its faithful companion the woolly 

 haired rhinoceros, decidedly indicates, even though it may not prove, 

 the existence of a climate unlike that now prevailing in Western 

 Europe. But when, in addition, we get the physical evidence brought 

 forward by Mr. Prestwich, the disturbed condition of the beds, and 

 the presence of the large blocks, the inference is much strengthened. 

 The amount of diflereuce still remains to be ascertained. Taking the 

 present range of the Musk ox and Reindeer as his guides, Mr. Prest- 

 wich assumes a difterence in the mean winter temperature of 19° to 

 29°. While, however, admitting the probability of a somewhat greater 

 winter cold, we are not, I think, yet in a position to estimate the 

 amount of change. 



It must always be borne in mind that the temperature of Western 

 Europe is at present exceptionally mild ; if we go either to the east 

 or west, to Canada or Siberia, we find countries under the same 

 latitude as London and Paris suflering under a far more severe 

 climate. 



The river St. Lawrence, to which I have pointed 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 imme- 

 diately preceding that now vuider consideration, the cold in Western 

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

 subject is treated at length 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 direc- 

 tions " ever since the glacial period." Mr. Prestwich's hypothesis 

 involves therefore in reality no cliancje of climate. He only supposes 

 that, in this early period of our rivers, the temperature of Western 

 Europe agreed with that which had i)receded, rather than with that 

 which now prevails ; or rather, perhaps, that, in this intermediate 

 period, the temperature had neither the extreme severity of the glacial 

 era, nor the exceptional mildness of modern times. 



But though diminishing the imjjrobability of the suggestion, 

 these considerations throw no light on the alteration of the condi- 

 tions which must have taken place to produce an alteration of 

 climate so great as that inferred by IVIr. Prestwich. 



The principal causes which have been suggested are the fol- 

 lowing : — 



Istly. A possible variation in the intensity of solar radiation. 



* Geol, Journal, 1852, p. .56. 



