DOMAIN HI. ARGILLACEOUS. 



rocks being alum, it seems the most na- 

 tural progress to begin with those sub- 

 stances which chiefly supply commerce 

 with that earth. 



MODE I. ALUM ROCK. 



Of this there are two very different structures; 

 the alum rock of Tolfa, which yields what is 

 called the Roman alum, and the common alumi- 

 nous slate. 



Alum of It has been said by some that the rock alum 

 of the middle ages derived its name from a town 

 in Syria, called Roch or Roque, Rocca ; but a 

 pilgrim having observed the same kind of rocks 

 near Civita Vecchia, the Pope founded the ce- 

 lebrated manufactory which supplied Europe 

 for some time*. The description of the latter 

 has been given by several mineralogic authors 

 under the class of salts ; but it may be interest- 

 ing to present the accurate account of Ferber, 

 who mentions, that the rocks which yield the 

 Roman alum constitute white, high, and argil- 

 laceous hills, of a compact structure, and with 



* Wall. ii. p. 43. Alum is classed among the salts by chemical 

 writers, and is called sulphate of argil. 



