ARGILLACEOUS SCHIST. 355 



The prevailing colour in these varieties, 

 which form roofing slate, is lead blue. It is also of 

 a pale grey, or of an obscure purple, or greenish 

 grey, or mottled with two colours. The greener 

 varieties are not abundant, are coloured by chlo- 

 rite, and pass into smooth chlorite schist. It 

 occurs, but rarely, of very pale greyish and 

 greenish hues, or nearly white. 



B. Imperfectly fissile : massive slate. 



a. Amorphous, but breaking more readily in 



one direction than in another. 



b. Amorphous, considerably harder than the 



former, and breaking neatly alike in 

 many directions. 



c. Fibrous. 



d. Massive, but splitting in many directions, 



with curved surfaces. 



e. Black, imperfectly fissile, of a dull earthy 



aspect. 



f. Black, and so soft as to mark paper : black 



chalk, or drawing slate. Some varieties 

 of black chalk are fissile. 



g. In spheroidal concretions imbedded in the 



laminar varieties. 



C. More compact, with a smooth splintery, or 

 minutely granular and splintery fracture, translucent 

 on the edges. Whet slate, or hone. 



A A 2 



