Robert L. Jack — On "Wants'' in Ironstone Seams. 389 



disturbed by faults. If it can be shown tbat faults must necessarily 

 produce wants in certain strata, the solution, or at least a solution, of 

 our problem will have been obtained. 



To demonstrate the connexion between wants and faults, it is 

 necessary to premise some facts regarding the latter. 



A single fault, disturbing hitherto unbroken strata, must begin in 

 two zero points, one at each end of the line. The dislocation must 

 reach its maximum somewhere between the two zero points. 



Let us suppose (as is, indeed, generally the case) that only the 

 strata on the downthrow side of the fault have been displaced ; the 

 edge of any particular bed which has been affected by the movement 

 will, on the side of downthrow, "bag" or lie along a curved line; 

 and, on the other side, along a straight line. The curved line will 

 represent a portion of the bed originally of the length of the straight 

 line between the two zero points, but now stretched out to a greater 

 length. For example : a fault whose two zero points are one mile 

 (1760 yards) apart, has a maximum downthrow midway between 

 those points of 220 yards.^ In this case, strata measuring 1760 yards 

 along the so-called "upthrow side" of the line of fault must be 

 stretched out to not less than 1833 yards, or broTcen. 



Few rock masses after consolidation are, strictly speaking, elastic, 

 at least to such an extent that they can be stretched 73 yards in a 

 mile. The source of the capability of being stretched,^ possessed by 

 a set of various stratified beds, as a whole, has to be sought for in the 

 unequal widening (by separation of the solid parts, not by waste), 

 of the vertical joints with which strata of different composition are 

 endowed. Any one who has examined the jointing of Carboniferous 

 shales in a natural section will readily understand how the mass 

 might be, as it were, pulled out horizontally, so as to occupy a 

 greater space, without losing its continuity. Each bed of the shales, 

 as seen from above, is cut by an irregular network of joints into 

 pieces like broken slates in a slate-yard, and only remains in its 

 place in consequence of its resting on lower beds, similarly jointed, 

 although the joints of one bed do not accurately underlie or over- 

 lie those of the neighbouring beds. Seen in a cliff, or other vertical 

 section, the pieces of shale, into which the joints have divided the 

 mass, overlap each other in the manner 

 represented in the diagram (Fig. 1) . A 



glance at the diagram will show that 



by slightly widening each joint the i._portion of a cliff of Car- 



whole mass of the shales might be boniferous Shales, showing the ar- 

 stretched out so as to occupy a greater rangement of the joints, 

 space without losing its continuity. 



Other strata alternating with shales in the Carboniferous Lime- 

 stone series, such as Sandstone, Coal, Limestone, or Ironstone, have 

 systems of jointing peculiar to themselves, which vary, however, 



^ Faults sometimes increase with remarkable rapidity. The above example is not a 

 hypothetical one. 



'■' As neither "elasticity" nor "flexibility" expresses the meaning to be conveyed, 

 I am compelled to use this roundabout phrase. 



