SCARBA, GEOLOGY. 181 



mens may differ, and even entire beds of graywacke, 

 occurring in the immediate vicinity of secondary rocks, 

 may be remarkable for the same laxity of texture by 

 which these are characterized ; while the same substance 

 involved among the primary, may present an unusual 

 degree of comparative hardness and compactness, But 

 the general character remains unaltered, and the corre- 

 spondence of the specimens with the definition continues 

 perfect. If geologists are desirous of distinguishing them 

 by their position as well as their mineral character, a prac- 

 tice often convenient, and even necessary, they may receive 

 appropriate names ; or those who are still inclined to 

 retain the terms of primitive and transition may have 

 recourse to them as adjective terms. The term quartzi- 

 ferous clay slate has indeed been applied to these rocks, 

 but there are other substances possessing a distinct cha- 

 racter which have a stronger claim to this appellation. 



