MODE I. ALUM ROCK. 243 



scarcely any visible horizontal beds ; but there 

 are some fissures filled with quartz, yielding 

 what are called Tolfa diamonds*. He after- 

 wards proceeds: 



" The alum hills are very high, shining, white Toifa. 

 rocks, separated by a long valley, and large ex- 

 cavations, which are made in the following man- 

 ner. The workmen descend by ropes to the 

 steep rocks; thus suspended, they bore blasting- 

 holes, fill them with cartridges, free the rocks 

 which by former blastings are loosened, and 

 then are pulled up again. The firing of the 

 powder is done by dry branches and leaves, 

 which experience has taught them to throw 

 from on high to any place below. 



" The alum rock is whitish grey, or chalk- 

 white ; extremely compact, and remarkably 

 hard. Scraped with a knife it yields an argil- 

 laceous powder, which does not ferment with 

 any acid, as it is penetrated by the vitriolic 

 acid, and composed of an argillaceous substance. 

 There are some bluish grey shivery pieces, which 

 are rejected as unfit, and probably are the re- 

 mains of the natural argillaceous stone, before it 

 was sufficiently imbibed and whitened by the 

 vitriolic acid. In some cracks appears a chalk- 



* Italy, 206. 



