248 THE ORY of the EAR T H. 



per fubject on which to reafon, for afcertaining the order of 

 production in certain bodies. This body, which we are now 

 to examine, is of the agate fpecies. 



WE have now been confidering the means employed by na- 

 ture in confolidating ftrata which were originally of an open 

 ftructure ; but in perfectly folid ftrata, we find bodies of agate, 

 which have evidently been formed in that place where they 

 now are found. This fact, however, is not ft ill that of which 

 we are now particularly to enquire ; for this, of which we are 

 to treat, concerns only a cavity within this agate ; now, what- 

 ever may have been the origin of the agate itfelf, we are to 

 mew, from what appears within its cavity, that the cryftalliza- 

 tions which are found in this place had arifen from a {imply 

 fluid ftate, and not from that of any manner of folution. 



THE agates now in queftion are thofe of the coated kind, fo 

 frequent in this country, called pebbles. Many of thefe are 

 filled with a filiceous cryftallization, which evidently proceeds 

 from the circumference towards the centre. Many of them, 

 again, are hollow. Thofe cavities are variously lined with cry- 

 ftallized fubftances j and thefe are the object: of the prefent 

 examination. 







BUT before defcribing what is found within, it is neceflary 

 to attend to this particular circumftance, that the cavity is per- 

 fectly inclofed with many folid coats, impervious to air or wa- 

 ter, but particularly with the external cortical part, which is 

 extremely hard, takes the higheft polifli, and is of the moft 

 perfect folidity, admitting the paflage of nothing but light and 

 heat. 



WITHIN thefe cavities, we find, frft, The coat of cryftals 

 with which this cavity is always lined ; and this is general to 

 all fubftances concreting, in fimilar circumftances, from a ftate 

 of fufion ; for when thus at liberty they naturally cryftallize. 

 idly, We have frequently a fubfequent cryftallization, fet upon 

 the firft, and more or lefs immerfed in it. $dly, There is alfo 



ibmetimes 



