DOMAIN III. ARGILLACEOUS. 



order to facilitate the precipitation of a reddish 

 selenite; and in the boiling they mix in the 

 liquor some lime and urine*. 



" The supports of the pans are made of a grey 

 lava, with large white crystalline schorl-prisms, 

 whose quantity exceeds the mass of the ierru- 

 minating lava. It is found, as they told rne, in 

 large loose pieces, at nine or ten miles' distance 

 from Tolfa ; and it resembles much the lava of 

 a volcanic hill called St. Fiora, in Tuscany, 

 which I have seen, and shall describe in my fol- 

 lowing letters. 



" The Tolfa alum-mines are said to have been 

 discovered in former times by a man, who, hav- 

 ing been long time a slave in Turkey, and worked 

 there in some alum works, guessed by the ilex 

 aquifolivm, common about Tolfa, that there 

 must be alum in the neighbourhood. But this 

 shrub is found in many places where no alum is 

 discovered." 



It is evident from this account that the alum 

 rocks of Tolfa are very different from aluminous 

 slate, which shall be afterwards described. 



* " If this be the case, the selenite is in no respect a substantial 

 part of the Tolfa alum -stone, as the author seems inclined to sup- 

 pose." Raspe the translator, whose language is far from pure. 



