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In this anfwer feveral particulars deferve confideration. In the 

 firft place water (to which felfpar is here affimilated) is never 

 regularly cryftallized when frozen by exceffive refrigeration, though 

 indeed vapour may ; confequently fince in the prefent cafe the 

 felfpar is faid to be regularly cryftallized, the parity does not 

 hold. 



Again, to juftify the comparifon of felfpar ading on quartz 

 as a menftruum, as water does upon fait, the felfpar fhould al- 

 ways be in the larger, and quartz in the fmaller proportion to each 

 other, as water always is to fait, and this is indeed the commoneft 

 cafe even where the felfpar is not regularly cryftallized ; yet in 

 Switzerland this does not happen, as Mr. Hoepfnerattefts, 4 Helvetic 

 Magaz. p. 266 j of which fpecimens may be met in 2 Lofke Catai, 

 Englifti edition, p. 375, 376, No. 37, 38, 40, 41, nor in Silefia, 

 as Gerhard remarks, i Grundrifs Min. Syftem, p; 404 and 405. 

 How then could the felfpar have ferved as a menftruum or flux 

 to the quartz in thefe cafes ? 



It is allowed by all obfervers that the cafes in which felfpar 

 in granite is regularly cryftallized are exceeding few, fee Lentz, 

 Emerling, Widenman, &c. Granites in which fuch cryftals are ob- 

 ferved are called porphyraceous granites, arid from that very 

 circumftance judged by many obfervers not to be ancient granites, 

 but of modern formation, fee 2 Widenman, p. 1005 '" the note. 

 An obfervation fimilar to that of Sir James Hall has alfo been 



made 



