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ILLUSTRATIONS OF THJ 



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one of the principal fads on which that theory 

 is founded. It has indeed been argued by feme 

 mineralogifts, that bodies thus contiguous mud: 

 owe their origin to the fame element, and that 

 a mineral fubftance cannot be of more recent 



formation than that which lies above it. 



But 



the maxim, that a foffil mud have the fame 

 origin with thofe that furround it, does not 

 hold, unlefs they have a certain fimilarity of 

 llrudure. It is, for inftance, the want of this 

 iimilaritj, that authorifes us to affign different 

 periods of formation to mineral veins, and to 

 the rocks in which they are included. 



In a fucceffion of Itrata, no one can doubt, 

 that the lowed were the firft formed, and the 

 others in the order in which they lie ; but, when 

 between two ftrata of fandllone or of limeflone 

 we find an intermediate rock, fo different as to 



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refemble lava, and to have nothing fchiflofe or 

 flratified in its compofition, the fame inilrument 

 cannot be fuppofed to have been employed in 

 the formation of both ; nor is there any reafon 

 why we may not fuppofe, that the intermediate 

 body was interpofed between the other two, 

 fome a6lion fubfequent to their formation. It 

 was thus that Dolomieu concluded, when he 



faw a lava like flon 



reous ftrata 



pofed bet 



in the Val di Koto, that, though 



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contiguous, 



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