OVERLYING ROCKS. 481 



diate member being- claystone, and clinkstone. 

 In some cases, it forms the whole mass ; in 

 others, it is mixed with other minerals, in various 

 proportions and in various manners ; thus pro- 

 ducing great diversities of aspect, without any 

 material variations of the fundamental character. 

 As we are but very imperfectly acquainted with 

 the chemical composition of the simple rocks, it 

 is not at present possible to determine, whether 

 the gradations here supposed, be legitimate ; and 

 whether, under varying proportions, and under 

 various states of induration, there is a real transi- 

 tion between clay and compact felspar. The 

 present arrangement is founded merely on the 

 apparent transitions between the two, as they are 

 found in Nature ; all the several substances oc- 

 curring together in the same geological series, 

 often in the very same mass, and passing* into 

 each other without defirieable limits. 



It was formerly observed, that the term com- 

 pact felspar, used here, as well as to denote a 

 member of the primary strata, was injudiciously 

 chosen. Chemical analyses have shown, that 

 soda is an ingredient in many greenstones and 



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