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ILLUSTRATIONS OF 



Some of the fpecies of whinftone are the I ' 



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common matrices of agates and chalcedonies 

 which lie inclofed in them in the form of 

 round nodules. The original fluidity of thefe 

 nodules is evinced by their figured, and fome- 

 times cryflallized ftrudure, and indeed is fo ge- 

 nerally admitted, that the only queilion con- 

 cerning them is, whether this fluidity was the 

 efTed of heat or of folution. To anfwer this 

 quefl:ion, Dr Hutton obferves, that the forma- 

 tion of the concentric coats, of which the agate 

 is ufually compofed, has evidently proceeded 

 from the circumference toward the centre, the 

 exterior coats always imprefliing the interior, 

 but never the reverfe. The fame thing alfo fol- 

 lows from this other fad, that when there is 

 any vacuity within the agate, it is ufually at 

 the centre, and there too are found the regu- 

 lar cryftals, when any fuch have been form- 

 ed. It therefore appears certain, that the pro- 

 grefs of confolidation has been from the circum- 

 ference inwards, and that the outward coats of 

 ^the agate were the firfl: to acquire folidity and 

 hardnefs. 



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74. Now, it mufl: be confidered that thefe coats ] 

 are highly confolidated ; that^ they are of very 



pure fiiiceous matter, and are utterly impervious I 



to every fubftance which we know of, except f 



light and heat. It is plain, therefore, that what- 

 ever 



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