INTRODUCTION. 9 



the clean sweep of the fauna altogether, and thinks it may have 

 survived in the southern part of England, whence it again 

 spread northwards with the return of more favourable con- 

 ditions ; although with the loss of such forms as were unable 

 to withstand a considerable amount of cold. On this latter 

 view it is considered that Britain was never connected with 

 the Continent after the passing away of the Glacial period. 



To make matters more intelligible, we may give a summary 

 (taken from Mr. Horace Woodward's " Geology of England ") 

 of what Prof. James Geikie considers to have been the 

 sequence of physical changes in Britain subsequent to the 

 deposition of the Norfolk forest-bed, arranged in chronological 

 order. 



1. Deposition of Norfolk forest-bed, and indications of ap- 

 proaching cold. 



2. "Till" deposits of Cromer, and the lowest Boulder Clay 

 (an ice-formation) of other parts ; this being a period of eleva- 

 tion of land, accompanied by severe glacial conditions. 



3. Period of considerable submergence, during which marine 

 sands and gravels were deposited in many parts, reaching nearly 

 to the summit of Moel Tryfaen ; this epoch being apparently 

 equivalent to Dr. Wallace's great period of submergence. 



4. Period of elevation of land, during which a large portion 

 of Britain was covered with sheet-ice, and the greater part of 

 the Boulder Clay was deposited ; this being the period when, 

 according to the more general view, the country was unin- 

 habited by the greater part of the Mammals with which it is 

 now populated. 



5. A period of less severe climatic conditions, during which 

 the brick-earths and cavern deposits were laid down, and the 

 climate gradually changed from intense cold to temperate and 

 genial; Arctic and Southern animals visiting Britain, according 



