IX 

 THE KEY TO TERRESTRIAL HI8TOET 



history of the earth, so far as it is known to us 

 JL since the ocean first came into existence, has been 

 obtained from a study of sedimentary strata, which are 

 arranged in a definite series. The series differs in com- 

 pleteness in different parts of the earth, but its various 

 terms always succeed each other in the same order. This 

 order of superposition in space corresponds to the order 

 of deposition in time, or the place of any stratum in the 

 stratified series represents its geological age. 



This principle, the foundation of our inquiry into ter- 

 restrial history, we owe to Steno (v. p. 224). 



But it is only very limited portions of the series 

 which are exposed to observation at any single locality ; 

 sections, revealing the edges of the strata and their 

 superposition, are provided for us by sea-cliffs, river- 

 banks, or artificial excavations, but these rarely enable 

 us to follow the succession for any great distance. The 

 beds themselves are frequently interrupted, partly as a 

 consequence of the conditions under which they were 

 originally formed, partly as the result of changes which 

 they have subsequently undergone ; faults of greater or 

 less magnitude traverse them, bringing different beds into 

 the same horizontal line ; undulations affect them, which 



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