205 



and argillaceous varieties effervesce with diffi- 

 culty. Almost all those which contain organic 

 remains, may safely be classed with the secondary 

 rocks ; as may the highly argillaceous ; but as 

 some interesting cases occur in which those of 

 the primary and secondary class cannot be dis- 

 tinguished by their mineral characters, it is ne- 

 cessary to have recourse to their geological 

 positions. In how far some of those which con- 

 tain organic remains, and which have been 

 called transition limestones, may belong to the 

 primary class, is not yet determined, 



. 



SHALE. 



The shales consist of schistose indurated clay, 

 commonly grey, or blackish ; occasionally, of 

 other and brighter colours. In general, they 

 may be distinguished from the argillaceous schists, 

 by their comparative fragility and want of the 

 continuous schistose structure. But, as formerly 

 remarked, cases also occur, where the characters 

 of the two so nearly resemble each other, that 

 these cannot be satisfactorily determined without 

 consulting their geological positions. 



