174 EFFECTS of HEAT 



fubftance, which, in this fluggifh ftate, would be little difpofed 

 to move, being confined in its original fituation by contiguous 

 beds of more refractory matter, would cryflallize, without 

 undergoing any change of place, and conftitute one of thofe 

 beds of whinftone, which frequently occur interftratified with 

 fandftone and limeftone. 



In other cafes where the heat was more intenfe, the beds of 

 fand, approaching more nearly to a ftate of fufion, would ac- 

 quire fuch tenacity and toughnefs, as to allow themfelves to be 

 bent and contorted, without laceration or fracture, by the in- 

 fluence of local motions, and might alTume the fhape and' 

 character of primary fchiflus : the limeftone would be highly 

 cryftallized, and would become marble, or, entering into thin 

 fufion, would penetrate the minuteft rents in the form of cal- 

 careous fpar. Laftly, when the heat was higher ftill, the fand 

 itfelf would be entirely melted, and might be converted, by the 

 fubfequent effects of flow cooling, into granite, fienite, &c. > 

 in fome cafes, retaining traces of its original ftratification, 

 and conftituting gneifs and ftratified granite ; in others, flowing 

 into the crevices, and forming veins of perfect granite. 



In confequence of the action of heat, upon fo great a quan- 

 tity of matter, thus brought into a fluid or femifluid ftate, and 

 in which, notwithftanding the great preflure, fome fubftances 

 would be volatilized, a powerful heaving of the fuperincumbent 

 mafs muft have taken place ; which, by repeated efforts, fuc- 



ceeding 



cryftals of ice, like the blade of a knife, formed in a mafs of clay, of fuch ftiffnefs, 

 that it had juft been ufed to make cups for chemical purpofes. In many of 

 my former experiments, I found that a fragment of glafs made from whinftone or 

 lava, when placed in a muffle heated to the melting point of filver, affumed a 

 cryftalline arrangement, and underwent a complete change of character. During 

 this change, it became foft, fo as to yield to the touch of an iron rod ; yet retained 

 fuch ftiffnefs, that, lying untouched in the muffle, it preferved its fhape entirely ; 

 the fharp angles of its frafture not being in the leaft blunted. 



