ITALY. LETTER XV* 20p 



are bkreHh-grey ivith white fpots, produced by the 

 acid. Thefe are much refembling the half- 

 diffblved black lava in Solfatara with white garnet- 

 like (herls , with this difference, that in Solfatara 

 the fubterraneous acid worked upon lava^ and here 

 upon an argillaceous blueifli (tone. The acid 

 feems in this place likewife to be produced by 

 fubterraneous fleams, which, penetrating the argil- 

 laceous (tones, changed them into alum-ore. I 

 could not afcertain whether there be near Tolfa 

 ancient volcanos ; but 1 faw lava-fragments in the 

 wall under the boiling-pans ; and therefore they 

 cannot be far diftant. 



By all this it appears, that the aluminous rock at 

 Tolfa is an indurated clay, having imbibed and been 

 whitened by a vitriolic acid, and contains fome 

 fcarce calcareous particles, which in the alum- 

 manufactories precipitate in the wooden rills or 

 troughs under the form of felenites. It is a com- 

 pact and found rock, neither ftratified nor fhivery 

 and flaty. Some nearly perpendicular white-grey 

 quartz-veins, three or four inches large, crofs it 

 from top to bottom : and in fome places appears in 

 the midft of the white rock a red mixture, as it 

 were of a colcothar vitrioli, or crocus martis, or 

 fpotted pieces, which refemble red and white 

 marbled foap. 



P Tfit 



