DOMAIN 111. ARGILLACEOUS. 



occasional bits of hard clay slate, and sometimes 

 of schistose keralite, are imbedded in an argilla- 

 ceous cement, of the nature of common grey clay 

 slate. When the particles are very fine, it as- 

 sumes the slaty structure, and forms the grauwack 

 slate of the Germans. It is the chief of Werner's 

 transitive rocks, nearly approaching to the primi- 

 tive; while at the same time it sometimes con- 

 tains shells, and other petrifactions of the se- 

 condary. 



This important rock was formerly considered 

 as being almost peculiar to the Hartz, where il 

 contains the richest mines ; but as the science has 

 advanced, it has been observed in many othei 

 countries. The slaty grison, or Bergmanite, haf- 

 been confounded with a clay slate; and we arc 

 obliged to Mr. Jameson for the following dis- 

 tinctions : 1. It is commonly of a bluish, ash, or 

 smoke grey, and rarely presents the greenish or 

 light yellowish grey colour of primitive clay slate. 

 2. Its lustre is sometimes glimmering from specks 

 of mica, but it never shows the silky lustre of clay 

 slate. 3. It never presents siderite nor garnets. 

 4. It alternates with massive grauwack. But i? 

 not the chief distinction its aspect of a sand-stone, 

 which has led to the trivial French name of gr&s- 

 gris, and the English rubble-stone, which may 

 imply that it was formed of rubbed fragments, or 



