THE ORY of the EAR T H. 245 



the cryftalline fait is fuddenly expofed to a fufficient degree of 

 heat, that is, fomewhat more than boiling water, it enters into 

 the ftate of aqueous fufion, and it boils, in emitting the water 

 by means of which it had been cryftallized in the cold, and 

 rendered fluid in that heated ftate. It is not poffible to cryftal- 

 lize this alkaline fait from a diflblved ftate, without the combi- 

 nation of that quantity of water, nor to feparate that water 

 without deftroying its cryftalline ftate. 



BUT in this mineral fpecimen, we have a folid cryftalline 

 fait, with a ftrudure which, upon fracture, appears to be fpar- 

 ry and radiated, fomething refembling that of zeolite. It con- 

 tains no water in its cryftallization, but melts in a fufHcient 

 heat, without any aqueous fufion. Therefore, this fait muft 

 have been in a fluid ftate of fufion, immediately before its con- 

 gelation and cryftallization. 



IT would be endlefs to give examples of particular facts, 

 fo many are the different natural appearances that occur, at- 

 tended with a variety of different circumftances. 



THERE is one, however, which is peculiarly diftinct, admits 

 of fufHciently accurate defcription, and contains circumftances 

 from which conclufions may be drawn with clearnefs. This 

 is the iron-ftone, which is commonly found among the argilla- 

 ceous ftrata, attendant upon fo{fil coal, both in Scotland and in 

 England. 



THIS ftone is generally found among the bituminous fchiftus, 

 or black argillaceous ftrata, either in feparate mafles of various 

 fhapes and fizes, or forming of itfelf ftrata which are more or 

 lefs continuous in their direction among the fchiftus or argilla- 

 ceous beds. 



THIS mineral contains in general from 40 to 50 per cent, of 

 iron, and it lofes near one third of its weight in calcination. 

 Before calcination it is of a gray colour, is not penetrable by 

 water, and takes a polifh. In this ftate, therefore, it is perfect- 

 ly folid ; but being calcined, it becomes red, porous, and tender. 



