370 Dr. J. Croll on the Physical Theory 



covered, almost entirely destitute of animal and vegetable life. 

 But in several places in Scotland fine layers of sand and 

 gravel, with beds of peaty matter, have been found resting on 

 'till' and again covered by 'till.' Sometimes these interca- 

 lated beds are very thin, but in other cases they are twenty or 

 thirty feet thick, and in them have been found remains of the 

 extinct ox, the Irish elk, the horse, reindeer, and mammoth. 

 Here we have evidence of two distinct periods of intense cold, 

 and an intervening milder period sufficiently prolonged for 

 the country to become covered with vegetation and stocked 

 with animal life." 



Let us now see to what all this leads. It has been proved 

 beyond the possibility of a doubt that, at the time the Till 

 was being formed which overlies the Scottish interglacial beds, 

 the whole of Scotland, Scandinavia, the bed of the North Sea, 

 and a great part of the North of England were covered with 

 one continuous sheet of ice upwards of 2000 feet in thickness. 

 This sheet overwhelmed the Hebrides, the Orkney and Shet- 

 land Islands, extended into Russia, filled the basin of the 

 Baltic, overflowed Denmark and Holstein, and advanced into 

 North Germany as far at least as Berlin. It has also been 

 demonstrated that, at the time the Lower Till was being formed 

 which underlies these interglacial beds, North-western Europe 

 was under a still more severe state of glaciation. The ice- 

 sheet at this time advanced further south into England, and 

 extended into North Germany as far as Saxony. It is per- 

 fectly obvious that this sheet must have destroj^ed all plant 

 and animal life in Scotland ; and before the country could 

 have become covered with vegetation and stocked with those 

 interglacial animals, to which Mr. Wallace refers, the ice must 

 have disappeared and the climate become mild. 



Equally conclusive are the facts adduced by Mr. Wallace in 

 reference to the interglacial beds of England. " In the east 

 of England Mr. Skertchly," he says, " enumerates four distinct 

 boulder clays with intervening deposits of gravels and sands. 

 Mr. Searles V. Wood, Jun., classes the most recent (Hessle) 

 boulder-clay as ' post-glacial/ but he admits an intervening 

 warmer period, characterized by southern forms of mollusca 

 and insects, after which glacial conditions again prevailed 

 with northern types of mollusca. Elsewhere Mr. Wood says: — 

 6 Looking at the presence of such fluviatile mollusca as Cyrena 

 fluminalis and JJnio littoralis, and of such mammalia as the 

 hippotamus and other great pachyderms, and of such a littoral 

 Lusitanian fauna as that of the Selsea bed, where it is mixed 

 up with the remains of some of those pachyderms, as well as 

 of some other features, it has seemed to me that the climate of 



