INTRODUCTION. 7 



appears to have been covered with an ice-sheet siir.i'ar to that 

 now enveloping Greenland, occurred subsequently to the 

 deposition of the Norfolk forest-bed, but attained its maximum 

 before that of the brick-earths of the Thames Valley and the 

 loam filling our caves. The relations of these later deposits 

 are, however, far from being fully understood ; and it is quite 

 probable that some of the beds indicating a comparatively 

 mild cUmate were laid down during warm interludes in the 

 Glacial period, which would account for the curious oscillations 

 of southern and northern forms of Mammals met with in our 

 later deposits. 



With regard to the date when the last union of Britain with 

 the Continent took place, Dr. Wallace, in his " Island Life," 

 writes that this "was comparatively recent, as shown by the 

 identity of the shells [found in the later deposits] with living 

 species, and the fact that the buried river-channels are all 

 covered with clays and gravels of the Glacial period, of such a 

 character as to indicate that most of them were deposited 

 above the sea-level. From these and various other indications 

 geologists are all agreed that the last continental period, as it 

 is called, was subsequent to the greatest development of the 

 ice, but probably before the cold period had wholly passed 

 away." 



In referring to the poverty of Britain as compared with the 

 Continent in species, the same author observes that " the for- 

 mer union of our islands with the Continent is not, however, 

 the only recent change they have undergone. There is equally 

 good evidence that a considerable portion, if not the entire 

 area, has been submerged to a depth of nearly two thousand 

 feet, at which time only what are now the highest mountains 

 would remain as groups of rocky islets. This submersion must 

 have destroyed the greater part of the life of our country ; and 

 as it certainly occurred during the latter part of the Glacial 



