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



305 



means, the trap would in several places, be able to find its way even to the very 

 surface of these strata, though in most instances, it would not reach that height 

 before cooling and becoming thereby arrested in its course. Now, it is obvious 

 that the trap when forced up into cracks and openings in the strata, would act as 

 wedges on the adjoining strata, and prevent them from slipping down, as they 

 might otherwise have done. The broadest or thickest part of the wedge would 

 be below, — so that it would occupy the position, and assume the form, best cal- 

 culated to produce the effect referred to. This explanation is strongly confirmed 

 by what is observed along the Niddry dyke, and especially at that end of it where 

 it thins away, and at last becomes a mere slip. Exactly in that part of its 

 course, viz. where tJie dyke turns into a slip, — a derangement of the strata commences. 

 Along the slip, — into which the trap had not flowed, the strata are dovm on the 

 north side, 15 fathoms below the corresponding strata on the opposite side. 



There is one other phenomenon, which I must notice before I altogether take 

 leave of the subject of slips. In the fii'st part of my memoir, I alluded to the 

 very ciu-ious ch-cumstance, that if a slip dipped or sloped towards any quarter of 

 the horizon (instead of being exactly vertical), it was invariably found, that the 

 strata (if deranged in position) were thrown dovra on that side towards which 

 the slip dipped. The slip, for example, last mentioned, namely, the prolongation 

 of the Niddry dyke, dips or slopes towards the north ; and it is on that side that 

 the strata are lowest, or have sunk down. If it had sloped towards the south, 

 the strata would have been lowest on that side. It appears to me, that this phe- 

 nomenon is susceptible of the following explanation ; but I offer it with great 

 difBdence, knowing that no satisfactory explanation has been reached by those 

 who are far better qualified to treat of these matters : 



NorA 



South 



Let AB, CD, EF, represent three slips, respectively inclined from the ver- 

 tical plane. The strata, by these slips, must have been (as above explained) 

 Iractm-ed from the very lowest member of the series. Suppose, that, in the first 

 instance, the slip AB was produced, — among strata previously unfractured, and 



VOL. XIV. PART I. Q q 



