DOMAIN III. ARGILLACEOUS. 241 



primitive schisti however there is still a 

 great reponderance of sand ; and the 

 glossy appearance may sometimes proceed 

 from decomposed mica. 



The argillaceous rocks are mostly of a 

 simple and uniform appearance, and do 

 not admit the numerous modifications of 

 some other substances. This earth is chiefly 

 eminent in gemmology, where it consti- 

 tutes some of the most beautiful varieties. 

 The argillaceous rocks are never crystal- 

 lised, and present but small splendour in 

 their appearance*; hence they are very 

 seldom used in the ornamental arts, and 

 are chiefly important in a geological point 

 of view, where they often rank among the 

 most important primitive substances. Yet 

 even in this light they have not been 

 treated with the attention and minute in- 

 vestigation which have been bestowed upon 

 the Siliceous and Calcareous Divisions. 



The essential part of the argillaceous 



* Brongniart, i. 512, informs us, that as the argils never crystal- 

 lise, they afford no species. A further proof that this term is foreign 

 to mineralogy. 



VOL. I. R 



