J30MAIN III. ARGlLLACEpUS. 



white ductile clay. Some pieces are bluish 

 grey, with white spots, produced hy the acid. 

 They much resemble the half-dissolved black 

 lava in Solfaterra t with white, garnet-like, 

 schorls ; with this difference, that in Solfaterra 

 the subterraneous acid worked upon lava, and 

 here upon an argillaceous bluish stone. The 

 acid seems in this place likewise to be produced 

 by subterraneous steams, which, penetrating the 

 argillaceous stones, changed them into alum 

 ore. I could not ascertain whether there be 

 near Tolfa ancient volcanoes ^ but I saw lava- 

 fragments in the wall under the boiling-pans, 

 and therefore they cannot be far distant. 



" By all this it appears that the aluminous 

 rock at Tolfa is an indurated clay, having im- 

 bibed and been whitened by a vitriolic acid, and 

 contains some few calcareous particles, which, 

 in the alum manufactories, precipitate in the 

 wooden rills or troughs, under the form of sele- 

 nites. It is a compact and sound rock, neither 

 stratified nor shivery and slaty. Some nearly 

 perpendicular white-grey quartz veins, three or 

 four inches wide, cross it from top to bottom ; 

 and in some places appears in the midst of the 

 white rock a red mixture, as it were, of a colco- 

 thar vitrioli, or crocus martis, or spotted pieces, 

 which resemble red and white marbled soap. 



