g8 On the Huttonian Theory eflhe Earth, 



among which the moft confiderable appeared to him to be this, that in fome cafes the 

 felfpar is feen in this ftone with Its cryftals regularly defined, whereas the quartz forms a 

 confufed and irregular mafs, being moulded on the cryftals of felfpar, whereas if the 

 granite were formed by fufion, the very contrary, he fays, fhould, it would feem, be ex- 

 pefted ; felfpar being very fufible, and quartz, on the contrary, highly infufible. In 

 anfwer to which, he fays, " that when quartz and felfpar are mixed and pounded to- 

 " gether, it is well known they may be melted without difficulty into a kind of glafs, the 

 •' felfpar ferving as a flux to the quartz, or the felfpar may be confidered as a menftruum 

 " in which the quartz is diflblved ; and in this view we may expe£b by analogy phasnomena 

 " fimilar to thofe of the folution of fait in water. Now it is certain that when exceffive 

 " cold is applied to fait water, the water is frozen to the exclufion of the fait ; why {hould 

 " not the fame thing happen in the folution of quartz in the liquid. Felfpar, when the 

 " mafs is allowed to cool beneath the point of congelation of the menftruum ? The felfpar 

 " may cryftallize feparately from the quartz, as we have feen pure ice formed feparately 

 « from the fait." 



In this anfwer feveral particulars deferve confideration. In the firft place, water (to 

 which felfpar is here affimilated) is never regularly cryftaljzed 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 a£ting on quartz as a menftruum, as water does upon fait, the felfpar 

 fhould always 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 cryftalized, yet in Swiflerland this does not happen, as Mr. Hoepfner atfefts, 

 4 Helvetic Magaz. p. 266, of which fpecimens may be met in 2 Lcfke Catal. Englifti 

 edition, p. 375, 376, No. 37, 38, 40, 41 ; nor in Silefia, as Gerhard remarks, 1 Grundrifs 

 Min. Syftem, p. 404 and 405. How then could the felfpar have ferved as a menftruum 

 or flux to the quartz in thefc cafes ? 



3dly, It is allowed by all obfervers, that the cafes in which felfpar in granite is regularly 

 cryftalized, are exceeding few ; fee Lentz, Emerling, Widenmaii, &c. Granites,, in 

 which fuch cryftals are obferved, are called forphyraceous granites^ and from that very cir* 

 cumftance judged by many obfervers not to be ancient granites, but of modern formation; 

 fee 2 Widenman, p. IC05, in the note. An obfervation fimilar to that of Sir James Hall 

 has alfo been made by Mr. Beflbn, in Limoges, 29 Roz. Jour. p. 89, for he difcovered 

 veins of granite in an argillite, though this fchift did not border upon any granitic mafs, 

 and hence he judged it of modern formation. Citizen Dolomieu alfo tells us, that fuch 

 inftances had occurred to him in his travels, but he thinks them perfeiSly diftinft from 

 the granite which forms granitic mountains, 16 Journ. 9es Mines, p. 22.. Neither was 

 Sauffurc a ftranger to fuch granitic veins ; but he accounts for their origin very differently 

 from Sir James. §. 600. 



4thly, 



