390 Robert L. Jack — On ''Wants" in Ironstone SeatJis. 



in consequence of differences in thickness, or in fineness of material, 

 of individual beds, and other causes. Generally speaking, the joints 



cut each bed, of whatever material, 

 from top to bottom. A bed of, say- 

 ironstone, a foot in thickness, is cut up 

 by the system of joints peculiar to it 

 into parallelepipeds, separable from 

 each other (named "lunkers"by the 

 miners), something like paving-flags. 

 In the bed of shale above or below the 

 ironstone there are probably a score of 

 joint planes for one in the ironstone ; 

 while in a thick bed of sandstone or 

 limestone there is probably not one 

 joint for a thousand in the shale. If 

 a number of beds of ironstone, sand- 

 stone, or the like, lie together, they 

 will no doubt be capable of being 

 stretched as a whole (though clumsily 

 and with difficulty), in the same 

 manner as a number of shale-beds. 

 But in the case of a single stratum of 

 such material (jointed from top to 

 bottom) occurring in the midst of a 

 thickness of shales, it is evident that 

 as the stretching of the latter goes on, 

 the single stratum — which cannot be 

 stretched — must yawn along one or 

 more of the joints which cut it in 

 lines forming angles (probably right 

 angles) with the line along which the 

 pulling force is exerted. When the 

 pulling force is the weight of the 

 strata let down by a fault, it is of 

 course exerted along the line of the 

 fault ; and the yawning in the stratum 

 which cannot be stretched will take 

 place along a line forming an angle 

 with the line of fault. 



It is hardly necessary to remind 

 either the miner or the geologist who 

 hunts for fish-teeth that by far the 

 greatest number of the workable iron- 

 stones in the Carboniferous Limestone 

 series occur as isolated seams among 

 considerable thicknesses of dark Car- 

 bonaceous shales. The vast heap of 

 "blaise" which has to be excavated as "holing" is almost as 

 familiar an object at the pit mouth as the engine-house itself. 



Wants in ironstone seams are, therefore, the gaps necessarily 



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