THE BUILDING OF THE EARTH 



to Limpsfield and Westerham a great ridge of chalk, at 

 some points 600 to 800 feet high. That ridge must at 

 one time have been at the sea bottom. And if we were 

 to examine the whole of England and sink borings in it, 

 we should at one point or another come to some remains 

 of rocks, or some " strata," as they are called, which are 

 of such make and material that we can only believe them 

 to have been laid down at the sea bottom. The only 

 conclusion we can come to, therefore, is that by some 

 means or other, and at some time or other, the islands of 

 England were slowly lifted above the sea, and that at 

 some other time the sea was slowly lifted above them. 

 What is true of England is true of nearly all the 

 regions of the world that have been closely examined by 

 geologists. Everywhere there is the evidence of different 

 stages of existence in the land's history stages when it 

 was covered by the sea ; stages when it was dry land 

 again ; perhaps stages when it was covered by lakes, 

 by vast forests ; stages when it may have been covered 

 by ice ; stages when it was desert. Some of these 

 stages show far vaster upheavals than others, and the 

 changes wrought were of far greater extent. Everybody 

 has heard that the great Saharan desert was perhaps 

 once the bed of an ocean. That is an assertion to which, 

 perhaps, we may be a little chary of committing ourselves ; 

 but there is excellent reason for believing that once some 

 of the great African lakes were connected with the sea ; 

 and we are quite certain that once Africa was an island. 

 So that in the case of that vast continent we know that 

 it must have seen periods of great depression and eleva- 



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