Schaw. —On the Last Glacial Epoch. 515 



successive waves of cold having been more carefully inter- 

 preted by comparison with existing glaciers, and having been 

 explained by the alternate advances and retirements of glacier 

 faces according as the seasons during the Glacial period were 

 more or less favourable to the deposition of snow or to its 

 melting away. 



In North America, the glacial condition extended on the 

 east coast as far south as New York, in lat. 41° ; it swept as 

 far south as lat. 38° in the valley of the Mississippi, whence 

 the boundary stretched north-west to about lat. 46° 30' on the 

 west coast, with great southward extensions on the mountain- 

 ranges of the Eockies, the Sierra Madre, and the Sierra 

 Nevada and Cascade Eanges, to lat. 42°, 38°, and 36° N. 



In Europe, ice and snow overwhelmed the whole of 

 Ireland, Scotland, Wales, and England as far south as the 

 valley of the Thames in lat. 51° 30', stretched across to Hol- 

 land and North Germany, trended south to the Carpathian 

 Mountains in lat. 48° N., stretched across Eussia to the 

 valley of the Volga in an irregular line, a little north of the 

 50th parallel of north latitude, and thence struck northwards 

 to the Ural Mountains in lat. 63°, and so back to the Arctic 

 Ocean. The Alpine glaciers formed an outlying glacial sea 

 overwhelming Switzerland and North Italy, and invading 

 France by the valley of the Ehone to lat. 46°. 



Two remarkable facts are evidenced by these boundaries 

 of the glaciated regions in America and Europe. First, we 

 notice that the Gulf Stream must have had the same sort of 

 influence during the glacial age that it has now ; for the 

 glacial conditions on the east coast of North America extended 

 as far south as lat. 41°, while on the west coast of Europe 

 they did not reach farther than lat. 51° 30'. It is probable 

 that this was the limit of the northward flow of the Gulf 

 Stream in the ice age, at least in winter, when the North Sea 

 must have been blocked with ice, and the Arctic Ocean must 

 have formed an icy barrier between Greenland, Iceland, and 

 the British Isles. 



Secondly, we notice how necessary moisture is for the 

 formatiou of glaciers, as there is no geological evidence of any 

 glacial action in north central Asia ; it is confined to the parts 

 of Europe subjected to the moist winds from the Atlantic and 

 the northern seas. In North America the great lake-system 

 was evidently much extended during the ice age, and thus the 

 needed moisture was afforded in the central parts of North 

 America where glacial conditions prevailed. 



In Siberia and Kamschatka and north - west Alaska the 

 climate is very dry, and, although the cold is intense, the effect 

 produced is not the formation of glaciers, but wdiat is known 

 as tundra, where the soil is perpetually frozen for hundreds of 



