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In the third volume of the Edinburgh tranfadions hift. p. 9, 

 we are informed that Sir James Hall, though convinced from 

 various obfervations that granite had once flowed in a ftate of 

 fufion, yet acknowledged that fome difficulties accompanied this opi- 

 nion; among which the moft confiderable appeared to him to be this, 

 that in fome cafes the felfpar is feen in this flone with its cryftals 

 regularly defined, whereas the quartz forms a confufed and irre- 

 gular 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 expeded ; 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 

 " together, it is well known they may be melted without diffi- 

 " culty 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 diflTolved ; and in this view we may expedt 

 " by analogy pha:nomcna 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 frozeiT to the exclufion of the fait ; 

 " why fhould 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 

 " cryftalize feparately from the quartz, as we have feen pure ice 

 '•' formed feparately from the fait." 



In 



