LECTURE XI. 319 



beds of which are somewliat vertical. Upon the heads of these 

 beds rests slate clay of considerable thickness with boulders^ 

 and large angular blocks, so that this clay manifestly corre- 

 sponds to the glacial clay. The existence of these large angu- 

 lar erratics, hitherto unnoticed, in the lower clay beds, has 

 recently been clearly demonstrated by Messikomer's examin- 

 ation of pile buildings. Then come coals in horizontal strata 

 nearly twelve feet thick, and upon the coals, drift with clay and 

 rounded blocks, and upon them angular erratics, which, in our 

 opinion, have been floated and not directly deposited by gla- 

 ciers. The substratum had hitherto not been sufficiently 

 attended to, hence it was believed that the coal beds had been 

 formed before the glacial extension ; but Messikomer's re- 

 searches have shown that the coals overlie the glacial clay, and 

 had therefore been formed immediately after the retreat of the 

 glaciers, after which they were covered with the old alluvium 

 and floated erratics. 



On comparing the coal beds of England with those of 

 East Switzerland, they appear at first sight so strikingly iden- 

 tical, that it leads to the belief that they belong to the same 

 period, and were formed simultaneously either before or 

 after the glacial extension. But as such is not the case, and 

 as the two respective formations are, on the contrary, separated 

 from each other by the glacial period, it follows that this glacial 

 extension must have been an intermediate event, which even 

 in countries where it occurred produced no important change. 

 There is no doubt, as we shall presently see, that there have 

 been, specially in the North, great changes produced in the 

 level of various regions. It is probable, that at least before 

 the commencement of the glacial period, if not immediately 

 after it, England and the north of France, Denmark and Nor- 

 way were connected, whilst, on the other hand, large districts 

 towards the east, now dry land, were under water. With the 

 increase of cold in the north, the northern population retreated 

 southward, an immigration rendered evident by the present 

 composition of the Fauna of the Grerman Ocean and the Baltic. 

 Again, with the cold it retreated, as we have shown, towards 

 the north. Such immigrations and emigrations, like the phy- 



