MR MILNE ON THE MID-LOTHIAN AND EAST-LOTHIAN COAL-FIELDS. 299 



I allude now to the formation of dykes and slips. 



It is obvious, that consolidated strata subjected to the subterranean and 

 lateral violence above explained, must have suffered very numerous and very ex- 

 tensive cracks and dislocations. If they subsided at all, their mere subsidence 

 would alone be sufficient to occasion such effects ; — and observe, where these frac- 

 tures would commence. When in the act of bending, the fracture would necessa- 

 rily commence at the external or lower side of the arch ; — and thus it follows, that 

 both slips and dykes would originate from below, and not from the upper part 

 of the basin. It is clear, therefore, that any slips or dykes which are seen at 

 the surface must be unfathomable, that is to say, they must reach to the very 

 lowest of the sedimentary strata ; and accordingly (as mentioned in the first part 

 of the memoir) no instance exists where, in going down from the surface, a slip 

 is found to stop or cease. On the other hand, one instance was mentioned of a 

 shp, which does not rise so high as the surface. This is easily accountable on the 

 principles just noticed. 



I may here take notice of another phenomenon, which appears to me to de- 

 pend on the same principles as the slip just aUuded to. In a pit called the Mucklits 

 pit, in Sir John Hope's workings at New CraighaU, the coal-seam there (the Sjylint) 

 is intersected by frequent dykes or " gullets " * of clay — which, however, rise no 

 higher than the roof, consisting there of a micaceous sandstone. The pavement 

 of the coal is clay, in quality exactly the same with the gullets. These clay dykes 

 run for considerable distances, and vary in width from a few feet to 15 fathoms. 

 They have all the appearance of being a portion of the pavement squeezed up into 

 the substance of the coal ; and this idea is strongly confirmed by the fact, that 

 they make their appearance exactly in that part of the basin where the splint 

 coal is most curved and fractured, — that is to say, in the very trough of the basin 

 where the strata rise to the NW. and to the SE. They are known in no other part 

 of the district. The coal in bending there has, on account of its brittleness, cracked, 

 and as the cracks would commence at the lower part of the seam, the clay of the 

 pavement, in consequence of the enormous pressure, instantly rose up, so as to 

 fiU and widen the cracks. The clay could not, however, rise higher than the 

 roof, because the sandstone which lies above the coal, from its slaty and mica- 

 ceous qualities, would bend without cracking. 



(7.) If the formation of slips and dykes is attributable to the violence of the 

 subterranean and lateral action, which preceded and accompanied the eruption 

 of the trap-hUls, we should naturally expect to find them most numerous near the 

 eruption ; and there also the slips and dykes should produce the greatest derange- 

 ment or dislocation of strata. 



This inference is consistent with the fact. On inspecting the accompan3dng 

 map it will be seen, that all the dykes, and the greatest number of the slips, are 



* This is one of the terms given by the pitmen to these clay-dykes — another term is " lunker." 



