SHALE. 



461 



and terminating in white, but the latter variety is 

 rare. Acquiring red and yellow tints, it passes 



into the following variety. 



B. Ferriferous shale. 



a. Laminar, simple, and of various colours, 



pink, red, purple, brown, obscure 

 blue, yellow, or mottled variously 

 with different tints. 



b. Laminar and short columnar : the co- 



lumnar ironstone of some mineralo- 

 gists. The surfaces are sometimes 

 remarkably channelled on the mar- 

 gins, as noticed in the chapter on the 

 Forms of Rocks. 



c. Containing red oxide of iron in excess, 



and passing to common ironstone. 



C. Adhesive slate. Opaque, adheres to the 

 tongue, pale yellowish, or smoke, grey. The slaty 

 structure is only visible in consequence of the loss of 

 its water by exposure, and is rendered again invisible 

 by wetting it. 



This substance is, for convenience, here 

 ranked with the shales, but it belongs to that 

 limited series of rocks occurring in the vicinity 

 of Paris which is supposed to owe its origin to 

 fresh water. 



D. Polishing slate. White or yellowish white, 

 opaque, brittle, and lighter than water. 



