MODE XII. SLATE. 



" At fourteen or fifteen feet from the surface 

 is found the good slate, which has been quarried 

 to the perpendicular depth of about 300 feet, 

 the remaining thickness being unknown. 



" The operations are conducted by open quar- 

 ries, by successive foncees, trenches, of about 

 nine feet deep, gradually narrowed, in order to 

 preserve a slope sufficient to prevent lapses of 

 the rock ; so that a trench, four hundred feet in 

 width at the opening of the quarry, shall be re- 

 duced to nothing at the thirtieth foncce, that is 

 the depth of 270 feet. There is every reason to 

 presume that a far greater depth might be at- 

 tained, and with more advantage, as the lower 

 they have gone the more perfect is the slate. 

 They have only been stopped by the difficulties 

 presented by the method of quarrying hitherto 

 adopted, which appears not to have been the 

 best, in one respect particularly, which is, that 

 the quantity of slate diminishes as the quality 

 becomes better, so that in the total mass those 

 of a middling quality are far more numerous. 

 It would seem that the method of subterranean 

 galleries would prevent the inconveniencies bf 

 the present plan ; there would not at least be 

 lost and overwhelmed a prodigious quantity of 

 excellent slate. The slate-mines of Charleville 

 might serve as an example; where, in spite of 



VOL. i. i 



