684 MR DAVID MILNE HOME ON THE BOULDER-CLAY OF EUROPE. 



difficult of explanation on the glacier theory, are very intelligible on the other. 

 A few of these difficulties will now be referred to. 



(1.) It has been mentioned, as a result of the examination of the sea-shells in 

 the pleistocene beds of Sweden, that the period of greatest cold was when the 

 land was most deeply submerged. If this be the case, which of the two theories 

 is most reconcilable with it ? 



When the land was most deeply submerged, the mountains would be elevated 

 above the sea less than at any other period ; and therefore circumstances would 

 not be favourable for the formation of glaciers. 



On the other hand, circumstances would be especially favourable for the 

 drifting of icebergs among the archipelago of the British Islands. 



(2.) The unequal distribution of boulder- clay over North- Western Europe 

 deserves a passing remark. The deposit is much more abundant in Scotland than 

 in any other country. Whilst it exists in both England and Ireland, it is chiefly 

 in the northern and midland counties. In the southern parts of both England 

 and Ireland it is hardly known. 



Then in Denmark, the beds which are there called boulder-clay appear not 

 to have been so disturbed as in Scotland. There are beds of clay which contain 

 boulders and pebbles, evidently transported, and which also contain Arctic shells; 

 but these are not described to be in a fragmentary or mutilated condition. These 

 shells are described as belonging to species which are known to inhabit shallow 

 water; and it is added by Forscitammer,* that there are extensive beds of sand, 

 containing boulders and pebbles, which seem to belong to the same epoch as the 

 boulder-clay. These beds of sand he also looks on as indications of a shallow 

 sea. 



Why should there be in the south of England and Ireland a less develop- 

 ment of boulder-clay ? May it not be that the icebergs melted before reaching 

 so far south ? 



Why should there be little or none of the true " till " in Denmark ? May it 

 not be that the icebergs, brought by a north-west Arctic current, were inter- 

 cepted by the Scotch archipelago ? and if any drifted towards Denmark, would 

 not the shallowness of the sea prevent them floating over and disturbing the 

 banks of mud and sand forming the sea-bottom there ? 



Shore ice alone probably floated over these Danish waters, carrying boulders 

 and pebbles, and spreading them on the submarine banks. 



Whilst some such explanation of the unequal distribution of boulder-clay is 

 suggested by the theory of water-borne ice, it is not easy to draw any explana- 

 tion from the glacier theory. 



(3.) Many persons have been perplexed at finding a ridge dividing two valleys, 

 forming what is sometimes called in the Highlands a col, consisting of rock smoothed 



* Journ. of London Geological Society for 1845, vol. i. p. 373. 



