Sect, VI.] 



GEOLOGY. 



189 



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ttudied, diagrams made of tliem, and their height m.ea- 

 sured at manv and distant parts of the coast. There is 

 reason to believe that in some instances such terraces 

 ran^e for surprisingly long distances at the same height. 



Where several occur on opposite sides )f a vallev a spirit 



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level is almost indispensable, in order to recogni' 



the 



corresponding stages. Where ranges of cliffs exist, the 

 marks of the erosion of the waves may sometimes be ex- 

 pected to occur, and as these generally present a defined 



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line, it is particularly desirable that their horizoiitality 

 should be ascertained by good levelling instruments, and 

 if not horizontal, that their inclination should be mea- 

 sured. Where more than one zone of erosion can be 

 detected all should be levelled, for it does not necessarily 

 follow that the several lines are parallel. Along extensive 

 coasts, and round islands which have been uplifted to a 

 considerable height, and w^here w^e now walk over what 

 was, within a late geological period, the bed of the 

 it would be well to observe whether extensive sedimentary 

 deposits have been upraised ; for it has often been tacitly 

 assumed that sedimentary deposits are in process of form" 



ation on all coasts. 



Subsidence of the Land.-'-Tlns movement is more dif- 

 ficult to detect than elevation, for it tends to hide under 

 water the surface thus affected. Evidence, therefore, of 

 subsidence is very valuable ; and this movement, more- 

 over, has probably played a more important part in the 

 history of the world than elevation, for there is reason to 

 believe that most great formations have been accumulated 



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st the bed of the s 



o^ was sinking. 



may 



sometimes be inferred from the form of the coast-land ; 



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