2^8 TRAVELS THROUGH 



The caftle of Montieri is, for a great part, 

 built upon the (lags of the ancient filver mines 

 pf Montieri. Thefe artificial flags or fcorise con- 

 tain, if broken, tranfparent prifmatical cryftalli- 

 ?ations of gypfum or felenites *. 



* Similar cryflallizations in a fmelted fubftance have been 

 lately obferved by Mr. Kair in the common glafs-mafi'es, 

 which for a long while have been expofed to a violent fur.nace- 

 ifire. The glafs-mafs of thofe famples, which I have feen, is 

 of a blueifh tranfparent colour, and the cryilallixations ap- 

 pearing and produced in it are of a white colour, and in a 

 globular lamellous form, exaftly refembling the feed of Na- 

 $urtium. He has prefented an account of this obfervation to 

 the Royal Society; and, together with the above obfervaticn 

 of Mr. ArdHii)i>_ and fome of my own, it greatly countenances 

 Mr. Ferler's hypothefis on the. origin of the cryftallized (herls, 

 found in the lava?. 



LET- 



