2 INTRODUCTION. 



quently than others, and conversely every part of the area 

 which is now covered by the environing seas has more than 

 once been part of the dry land. Sometimes nearly the 

 whole area has formed part of continental land, and at 

 other times it has been almost entirely submerged beneath 

 the sea. 



But we can look back to an epoch in the geographical 

 history of Britain when the foundations of our islands had 

 been laid, and when the older and more mountainous parts 

 of the country had been brought into the relative positions 

 which they now occupy. We may, in fact, regard the 

 Palaeozoic districts of the British Islands as portions of 

 ancient lands which have been broken up and reduced to 

 their present dimensions during successive periods of 

 erosion and denudation ; these tracts and the vanished 

 lands of which they formed part have yielded the ma- 

 terials that compose the more recent (Neozoic) strata, 

 and there were at least three periods in these later times 

 when Neozoic strata filled up the gaps between the older 

 blocks of ground and welded the British Islands into a 

 continental whole. Erosion and submergence, however, 

 again began the work of destruction and separation, with 

 the ultimate result of reducing them once more to the state 

 of islands and of giving them the outlines which they 

 now present. 



Before proceeding to describe the successive phases of 

 this process of geographical evolution, some consideration 

 of the evidence on which we have to rely in trying to 

 restore the geography of any period seems to be desirable. 

 The greater part of the strata with which the geologist has 

 to deal are marine deposits, and it is only rarely that actual 

 proof of the former existence of land in any district is 

 found in the intercalation of purely freshwater deposits. 

 Occasionally we meet with estuarine beds which indicate 

 the close proximity of land ; and of more frequent occur- 



