CLAYS, SHALES, AND SLATES. 363 



TEXAS. Kennedy, W. Texas clays and their origin. Science, 



vol. 22, pp. 297-300. 1893. 



WASHINGTON. Landes, H. Clays of Washington. Vol. 1, Rep. Wash- 



ington Geol. Survey, pt. 2, pp. 13-23. 1902. 



WISCONSIN. Buckley, E. R. The clays and clay industries of Wis- 



consin Bulletin 7, Wisconsin Geol. Survey, 304 pp. 

 1901. 



WYOMING. Knight, W. C. The building stones and clays of Wyoming. 



Eng. and Min. Jour., vol. 66, pp. 546-547. 1898. 



Slates. 



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

 a fine, even, and parallel cleavage has been developed by pressure. In 

 composition, therefore, it will vary exactly as do the shales considered 

 in the last section, and so far as composition alone is concerned, slate 

 would not be worthy of more attention, as a Portland-cement mate- 

 rial, than any other shale. 



Commercial considerations in connection with the slate industry, 

 however, make slate a very important possible source of cement mate- 

 rial. Good roofing slate is a relatively scarce material and commands 

 a good price when found. In the preparation 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 product, it could be obtained 

 very cheaply by the cement manufacturer. 



Geographic distribution of slates. The principal areas in the United 

 States in which roofing slate is at present quarried are briefly noted 

 below. For more detailed information on the subject, reference should 

 be made to the papers and reports listed on page 366. 



Beginning in the northeast, slates are extensively quarried in the 

 Brownsville-Monson area in northern Maine, but no satisfactory lime- 

 stones occur in this district The next important slate area lies in 

 western Vermont and eastern New York, a region well supplied with 

 good limestones. In New Jersey and Pennsylvania slates are worked 

 just north of the Lehigh cement-rock belt, as noted in Chapter XXIV. 

 The Peach Bottom slate district, located in southern Pennsylvania and 

 northeastern Maryland, is also important, but is poorly supplied with 



