CHAP. XIII.] PLEISTOCENE EPOCH. 293 



tions of the time when the countries were united it would 

 only be necessary to imagine the replacement of material 

 cut away by the sea. But if the whole displacement were 

 due to marine erosion, we should expect the minimum 

 depth of the two channels to be approximately the same, 

 whereas the actual difference is very great, being no less 

 than 120 feet. Again, we know that subsidence has actu- 

 ally taken place, and it is more reasonable to suppose that 

 it was this movement which carried the sea over the greater 

 part of the submerged area, only the final breach being 

 directly accomplished by the waves. The vertical displace- 

 ment so effected would be small, and we may, I think, 

 fairly regard the present depth of the channels as indica- 

 ting the relative levels of the connecting isthmuses, unless 

 it can be shown that the submergence was not of uniform 

 extent. 



To account, therefore, for the dispersion of these large 

 extinct quadrupeds we must suppose that at one time the 

 whole of Britain stood at least 300 feet higher than it 

 does now, so that they could make their way westwards 

 along the bed of the Bristol Channel to the south of 

 Ireland. 



From the river- gravels we cannot learn much as to the 

 absolute elevation of the land, but their abrupt termina- 

 tions prove that the rivers had far longer courses than they 

 have now. Thus in the case of the Thames the height of 

 the old gravels above the present estuary (over 100 feet at 

 Dartford Heath), and the entire absence of estuarine shells 

 in them, prove that the course of the ancient river was 

 continued far beyond its present mouth. 



The Dogger Bank affords more precise evidence. This 

 bank may be described as a submerged island in the middle 

 of the southern part of the North Sea ; its border is about 

 75 miles E.N.E. of Flamborough Head, and that part of 

 its surface which rises to within a depth of 10 fathoms is 



