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



floating on a mass of liquid volcanic matter, violently pressed up — and suppose 

 farther, that this fracture incline or slope to the north. Let us see how the 

 strata, after being thus severed, | would be supported on each side of the slip. 

 On the north side, it is obvious, that the lower part, or basis of the strata, be- 

 came less in breadth than the top, so that their foundation was narrowed ; — 

 and the question occm-s, whether that foundation was as able as before to bear the 

 weight of the superincumbent strata. It is true that the truncated edges of the 

 strata on the north side of the slip, were resting on the surface of the slip ; and 

 this new support might partly, though not entirely, compensate the other element 

 of weakness just referred to ; but the slip would cease to afford any support what- 

 ever, if there was a continuous moving, or undulation of the strata on either side, 

 arising from unequal or lateral pressure. In that case, the strata on the north 

 side would obviously have a tendency to slide down the upper surface of the slip, 

 — and would continue to sink, until the compression was such as to prevent any 

 farther sinking. 



In regard to the south or under side of the slip, it is obvious that the case 

 would be entirely different. So far from the foundation or lower part of the strata 

 being narrowed, it would be widened ; and so far from the superincumbent weight 

 being increased, it would be greatly lessened. The strata on that side of the slip, 

 would therefore have no tendency to sink. 



We have supposed the slip AB to slope a little from the vertical. If it is 

 exactly vertical, it is evident that no tendency to sink would be occasioned more 

 on one side than on another ; — and this is found to agree with observation. 



Suppose, then, that another shp takes place to the south of the slip AB, viz. 

 through the strata which remained firm, and sloping to the south, it is evident, 

 that the stratified mass between the old slip and the new one CD, will have 

 no tendency to move. It will then become pyramidal in shape, and acquire se- 

 veral qualities calculated to render it firm and stationary. On the other hand, 

 the strata to the south of the slip, will acquire a tendency to sink down on that 

 side, — for the reasons previously explained. 



Suppose that a third slip takes place EF (to the north of the original one), 

 through the strata which sunk down on that side, — and that it slopes to the north. 

 The effect of this new dislocation will, of course, be to throw the strata on the 

 north side of it still more do\^Ti on the north, whilst the strata between this slip 

 and AB can acquire no such tendency ; and even if the slip EF were to take 

 place, so, that the base of the figure between it and AB became less than the top, 

 whereby a tendency to sink would be again produced, still it could not sink so 

 far as the strata immediately north of it. If the slip EF had sloped towards the 

 south, so that the stratified mass between it and AB acquired the form of a cone 

 with the base uppermost, it is evident that, whilst it acquired a great additional 

 tendency to sink, the strata on the north side would remain firm. 



