CHAPTER XX 

 THE GROWTH OF BRITAIN 



THE preceding chapter has shown that the study of the 

 rock succession in any country reveals that that country 

 must have passed through numerous geographical changes. In 

 order, however, that our knowledge of each geographical phase 

 may be complete, we require to know two things : (i) The nature 

 and composition of each geological Formation throughout its entire 

 extent ; and (2) the changes which each one has undergone since 

 its formation. This has been summed up by Professor Lapworth 

 as " knowledge of the Formation and of its Deformation/' 



We can never hope to know the entire extent of a Formation, 

 because not only is a large part of it usually buried beneath 

 newer strata, but almost always much of it has been denuded 

 to provide material for the building of subsequent Formations. 

 Thus our knowledge is almost limited to the outcrop. Inside 

 this it is hidden ; outside, it has been destroyed. We can, how- 

 ever, make use of every exposure of a rock along its outcrop in 

 order to see whether any variation in character can be detected. 

 Outliers saved from the general denudation, beyond the main 

 outcrop, will give us an idea of its former extension in that direction, 

 and borings through newer strata will tell of its nature on its 

 underground continuation. 



All three sets of observations along and across the outcrop 

 give evidence in many cases of variations from the normal. 

 A limestone will become less pure and mingled with mud in 

 certain directions ; it may even pass completely into a shale or 

 sand. Sands are found to pass into clays in one direction, and 

 into conglomerates in the other. This is precisely the type of 

 variation we should expect from the conditions under which 

 we know the strata to have been laid down. No sea is world- 

 wide ; somewhere its clear waters and beds covered with organic 



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