28 THE MIGRATIOX OF BRITISH BIRDS 



off tlic south coast of Ireland, and to about lat. 51^7' or 

 52 across the south of England and Holland ; thence 

 southwards more or less irregularly to lat. 50 as far east 

 as the northern Carpathians, whence it contracted north- 

 wards across Russia, here and there sending out furca- 

 tions or spurs to about lat. 61 near the Urals. South of 

 these limits were many local glaciers in Spain, in the 

 Pyrenees, the Alps, the Carpathians, and the Caucasus. 

 By far the most important of these vast ice-sheets were 

 those in the Alps. For not only did the}- coalesce in 

 the west with those on the mountains of Eastern France, 

 but in the north probably with the Great Ice-sheet itself, 

 through the glaciers on the mountains of Germany, 

 Austria, and Bohemia. There can be no doubt what- 

 ever that during the climax of the glacial periods Europe 

 was even more effectually divided into two portions by 

 this vast central glacial system and its attendant snow- 

 fields, tlian if a sea of equal breadth had stretched 

 between them. There is evidence of two distinct glacial 

 periods in the Black Forest, and, it is said, of three 

 such glaciations in the Alps, separated by inter-glacial 

 mild eras. During the maximum of glaciation or second 

 of these glacial periods Central Europe, as we have 

 already seen, was completely occupied by ice and snow- 

 fields, the grand Mcr de glace extending through the 

 Black Forest into the valle\' of the Rhine and elsewhere, 

 and coalescing with the glaciers of the Alps. During 

 the third and the fourth glacial periods the northern 

 ice-sheets appear not to have extended so far south- 

 wards, and did not deploy upon the low grounds 

 beyond say lat. 52 , or coalesce with those of the Alps ; 

 so that the Mcr dc trlacc during the latter part of the 



