118 ISLAND LIFE 



distinct formations of " till " separated from each other by 

 bed^ of sand from two to twenty feet in thickness.^ Facts 

 of a similar nature have been observed in other parts of our 

 islands. In the east of England, Mr. Skertchly (of the 

 Geological Survey) 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 inter- 

 vening warmer period, characterised by southern forms of 

 mollusca and insects, after which glacial conditions again 

 prevailed wdth northern types of mollusca.^ Elsewhere he 

 says : " Looking at the presence of such fluviatile mollusca 

 as Cyrena flitmincdis and Unio littoralis and of such 

 mammalia as the hippopotamus and other great pachy- 

 derms, 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 the earlier part of the 

 post-glacial period in England was possibly even warmer 

 than our present climate ; and that it was succeeded by a 

 refrigeration sufficiently severe to cause ice to form all 

 round our coasts, and glaciers to accumulate in the valleys 

 of the mountain districts ; and that this increased severity 

 of climate was preceded, and j^artially accompanied, by a 

 limited submergence, which nowhere apparently exceeded 

 300 feet, and reached that amount only in the northern 

 counties of England." * This decided admission of an 

 alternation of warm and cold climates since the height of 

 the glacial epoch by so cautious a geologist as Mr. Wood is 

 very important, as is his statement of an accompanying 

 dcjiression of the land, accompanying the increased cold, 

 because many geologists maintain that a greater elevation 

 of the land is the true and sufficient explanation of glacial 

 periods. 



^ The Great Ice Age, p. 177. 



2 These are named, in descending order, Hessle Boulder Clay, Purple 

 Boulder Clay, Chalky Boulder Clay, and Lower Boulder Clay — below which 

 is the Norwich Crag. 



3 "On the Climate of the Post-Glacial Period." Geological Magazine, 

 1872, pp. 158, 160. 



^ Geological Magazine, 1876, p. 396. 



