MR MILNE ON THE MID-LOTHIAN AND EAST-LOTHIAN COAL-FIELDS. 391 



of the Roman Camp, — ^there the strata dip SW. — whilst at Bryants they dip 

 NW., and at Edgehead (on the opposite side of the hill) they dip SE. The same 

 strata, therefore, absolutely suiTound this hill in three parts out of four, dipping 

 successively towards three points of the horizon. The North Greens crops out 

 near the top of the hill on aU sides, and at several places the limestone which 

 underlies that coal, may be seen coming up from opposite sides. Now, on examin- 

 ing the direction of the shps running through the strata of the Roman Camp hill, 



it is found, that they all more or less converge to the top of the hiU ; that is, 



they, generally speaiiing, coincide with the dip and rise of the strata. If the hill 

 had been formed by a protrusion of trap in the central parts, so as to occasion a 

 pressure towards the cii-cumference, the facts just stated might be accounted for. 

 The central force acting laterally on the adjoining strata, would also act unequal- 

 ly in consequence of the unequal tenacity of these strata. Dislocations would be 

 produced by this cause. But we have shewn it to be possible, nay probable, 

 that the Roman Camp ridge may have been produced, not by a protrusion of 

 trap (of which there is not a vestige either on the smface or in the underground 

 workings,) — but partly by the subsidence of adjoining strata towards the north, and 

 partly by a thrust of them upwards from the west. This last movement would 

 equally produce dislocations running towards the ridge. Subsidence alone would 

 cause them to assume a different direction, for in that case the tendency of the 

 force being to separate the strata, the fractures would be parallel with the line of 

 crop. This is very clearly shewn by Mr Hopkins in his valuable memoir above 

 alluded to. 



In whichever way, therefore, the quaquaversal dip of the strata at the Ro- 

 man Camp has been produced, — whether by an elevation of the centre by sub- 

 terranean trap as Dr Hibbert supposes — or by a push upwards from a more dis- 

 tant quarter, it is evident that the dislocating forces would act in such a manner, 

 as to cause the slips to converge more or less towards a common centre. 



After these explanations, it must be very apparent why all the dykes, and 

 nine-tenths of the whole sUps, run not far from a direction NW. and SE. The 

 chief eruptions of trap which have elevated and dislocated the strata of the dis- 

 trict lies to the NW. of it, and of coiurse the dislocations have acquired that direc- 

 tion. 



(9.) In connection with this branch of the subject, viz. the direction of the 

 slips, I would remark, that though in general the slips of the district are all 

 either parallel to each other, or converge towards a common centre, there are 

 a few (and some of them are marked on the map) at right angles to each 

 other. This arises (as Mr Hopkins distinctly shews) from the effect of different 

 forces. Those which run parallel with the crop of the strata, shew a diveUent 

 force ; those which are coincident with the dip (and these are the most numerous), 

 shew unequal lateral pressure. 



