73 



pre-glacial land, somewhat higher than the existing continents, 

 began to receive the ice sheet; the second, during which the 

 ice-bound land subsided to the extent of 1800 or 2000 feet; and 

 the third, during which the land was re-elevated step by step, 

 and the ice gradually disappeared. 



After speaking of the difficulty in reading aright the 

 phenomena of the Glacial Epoch — the second stage having 

 obliterated so much of the first, and the third so much of 

 both — he adds, " this much, however, is certain, that the 

 Pre-Glacial or Pliocene land surfaces, wherever they are found, 

 contain fossils ; that the first stage of the Ice Epoch is 

 characterized by boulders, little removed from their parent 

 rocks, by finely glacialized rock-surfaces, and by the true 

 boulder clay or till of Scottish Geologists, and is always 

 un-fossiliferous ; that the second stage is characterized by 

 re-assorted clays, by more rounded and widely-dispersed 

 bovilders, and is also un-fossiliferous ; while the third stage 

 has more moraines, ridges of sand and gravel, terraces with 

 occasional shells, and finally, in the lower levels, the silty clay 

 or brick clay, containing boreal shells, star fishes, bones of 

 seals, whales, northern ducks, and other kindred remains." 



By way of illustration, and to enable you the better to follow 

 me, I have reproduced Mr. Prestwich's section of the Valley of 

 the Somme in France ; and my explanations will be mainly based 

 upon his admirable paper, read before the Eoyal Society.* 



Theoretical Section across the Valley oftlie Somme. 



L 



a Recent alluvium. ft b' b" Loess on different levels and of different ages, 

 c Low level Gravels. d ffigh level Gravels. C The Valley excavated. 



* I have, however, where necessary, drawn largely from the works of Sir C. 

 Lyell, Professor Ramsay, and other competent authorities. 



G 2 



