"MODE II. CLAY SLATE. 249 



Aspect 2. Glossy. This, as already mention- 

 ed, has rather a metallic appearance, and is some- 

 times tarnished like peacock coal. In the north 

 of France it is sometimes found singularly hard 

 and compact. 



Aspect 3. Alum earth. This is found com- 

 pact, and of a brownish black colour. 



MODE II. CLAY SLATE. 



This must not be confounded with the argil- Distinction* 

 laceous schistus of Kirwan, which is here called 

 slate, and assigned to the Siderous Domain ; 

 while clay slate is the schistose clay of Kirwan, 

 which he also calls shale, and which is often 

 found over coal, bearing vegetable impressions. 



The argillaceous schistus, or argillite, of Kir- 

 wan, is the thonrchieffer of Werner ; while, by 

 too nice a distinction, his schieffertho?i, the slate 

 clay of Kirwan, is our clay slate. Brochant 

 terms it argile schisteuse. It is less hard and 

 weighty than siderous slate, adheres to the 

 tongue, and softens in water. But all their de- 

 scriptions chiefly refer to that kind which is 

 found in coal-mines ; while the most important 

 division is that which forms entire mountains, as 



