33 



Slate. 



Slate is, so far as origin is concerned, merely a form of 

 shale in which a fine, even and parallel cleavage has been de- 

 veloped by pressure. In composition, therefore, it will vary 

 exactly as do the shales considered on previous pages, and so 

 far as composition alone is concerned, slate would not be worthy 

 of more attention, as a Portland cement material, than any 

 other shale. 



Commercial considerations in connection with the slate in- 

 dustry, however, make slate a very important possible source of 

 cement material. Good roofing slate is a relatively scarce mater- 

 ial, and commands a good price when found. In the prepara- 

 tion of roofing slate for the market so much material is lost 

 during sawing, splitting, etc., that only about 10 to 25 per cent, 

 of the amount quarried is salable as slate. The remaining 75 

 to 90 per cent, is of no service to the slate miner. It is sent to 

 the dump heap, and is a continual source of trouble and expense. 

 This very material, however, as can be seen from the analyses 

 quoted below, is often admirable for use, in connection with 

 limestone, in a Portland cement mixture. As it is a waste pro- 

 duct, it could be obtained very cheaply by the cement manufac- 

 turer. 



Composition of American roofing slates. 



