DOMAIN III. ARGILLACEOUS. 



of sulphate of magnesia, which has led to a be- 

 lief that talc is one of its constituent parts ; as 

 Werner also suspects, from its unctuous feel, and 

 being often of a greenish tint. It not unusually 

 presents two layers ; the upper of a pale greenish 

 yellow, and the under of a blackish brown. The 

 fracture is often scaly, which is seldom observed 

 in other schistose substances ; but that of Bareuth 

 sometimes presents a slaty fracture, partly ap- 

 proaching to the conchoidal, and partly to the 

 earthy. Hone seems nearly related to a highly 

 indurated fullers' earth, and is said by some to 

 decompose into tripoli*. 



* Saussure says, 15p4, that the common touch-stone is com- 

 posed of little white grains of quartz and felspar, enveloped in 

 ferruginous clay. The hard black nodules, which are found in 

 slate, likewise afford very good touch-stones. The little hard grains 

 form a kind of file, which seizes on the substance of the metal, 

 while the black gluten displays the colour. And as acids do not 

 affect the stone, the trace may easily be tried by the nitrous acid, or 

 by the aqua regia. 



