Vol. 57*] CLIMATE OP THE PLEISTOCENE EPOCH. 429 



boreal character.^ This does not seem, however, to have been due 

 to the existence at that period of cold currents from the Polar seas. 

 Although, according to American geologists, the Behring Strait 

 region stood at a lower level in pre-Miocene times, causing then 

 considerable interchange of water between the Arctic Ocean and the 

 Pacific, this state of things appears to have ceased before the com- 

 mencement of the Miocene Epoch. The existence at that time of 

 an area of low pressure in the jN"orth Pacific would, however, 

 have caused southerly winds in winter, as it now does, with a warm 

 climate on the western coasts of America, and northerly winds and 

 colder conditions in Kamtchatka and Jap^. 



lY. The Formee Existence of Geeat Lakes in the Basin of 

 jS^evada and in Central Asia. 



Turning now to the J^ew World, we may remember that American 

 geologists have shown how enormous lakes existed during the 

 Glacial Period in the great basin lying between the Rocky Mountains 

 and the Sierra Nevada, a region at present without outfall, and of 

 extreme aridity. These, to the largest of which the names of 

 Bonneville and Lahontan have been given, have been described 

 by Prof. G. K. Gilbert' and Mr. I. C. Bussell,^^ of the United States 

 Geological Survey. 



Such facts, together with the evidence of more humid conditions 

 in Asia and elsewhere in later Pleistocene times, have given rise to 

 the hypothesis that generally the climate of that epoch was of a 

 pluvial character.^ While believing that such was the case, I think 

 that pluvial conditions may have been to some extent local. Exces- 

 sive rainfall occurs at present in many parts of the world, but always 

 locally, and it arises from one cause only, the prevalence of moist 

 and warm winds from the ocean.^ 



The summer rains in India, for example, do not now reach Persia, 

 the climate of which is consequently very dry. The pluvial condi- 

 tions of the former country at that season, and the arid conditions 

 of the latter, are equally due to the relative position of the areas of 

 high and low pressure of the Asiatic continent and the adjoining 



^ See also E. Kayser's * Text-book of Comparative Geology ' transl. Lake 

 (1893) p. 354. 



2 ' Lake Bonneville' 2nd Ann. Rep. U.S. Geol. Surv. (1881) p. 169. 



3 'Lake Lahontan' 3rd Ann. Eep. U.S. Geol. Surv. (1882) p. 195. 



'^ It naay have been during such a pluvial period that the excavation of gorges 

 like the chines of the South of England took place. These, with similar valleys 

 elsewhere, which are out of all proportion to the streams which now flow, or 

 ever could have flowed in them, seem to me to have been caused by sudden 

 and repeated floods of great violence, rather than by the steady and continuous 

 erosion of a time when the rainfall was excessive. Rainfall was no doubt 

 greater, however, in the South of Europe during the Pleistocene Epoch than in 

 the British Isles, as the diluvial deposits of the former region are on a grander 

 •scale. 



^ The term ' oceanic climate ' is frequently used, but this can only mean a 

 clitnate influenced by win^s blowing from the ocean. The winds could not all 

 have been oceanic at one time, however, during the Pleistocene Epoch. 



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