296 ^^ MILNE ON THE MID-LOTHIAN AND EAST-LOTHIAN COAL-FIELDS. 



Under these circumstances, it is obvious what extraordinary force and vio- 

 lence the trap must have exerted, before it burst through the sedimentary de- 

 posits that overlaid it. These deposits attain the thickness of nearly 1000 fa- 

 thoms, and how much more there may be below the North Greens coal (to which 

 that measurement extends), it is difficult to say. There was not merely the weight 

 of these accumulated deposits to be overcome before the molten matter could get 

 vent ; — there was also the tenacity arising from their partial consolidation, to l)e 

 overcome. It is impossible, therefore, to imagine that the outburst could have taken 

 place, without there having been previously stupendous upheavings of the strata, 

 at and around this part of the district. The pressure would of course not be 

 confined to one spot. The igneous matter which eventually came up, and came 

 up not in one place, but at several places far removed from each other, and in 

 enormous quantities, — shews, that it must have extended beneath these sedimen- 

 tary strata to a considerable distance. The consequence would be, that during 

 the prevalence of the subterranean pressure, these strata not being every where 

 of equal tenacity or weight, would suffer extensive upheavings and oscillations. 

 The strata of the entire district, would be, in a manner, floated or buoyed up upon 

 the surface of the subjacent volcanic matter, and would undergo tremendous 

 fractures and dislocations. 



When the outburst of Arthur Seat and the trap in its vicinity took place, 

 let us consider what would be some of the most obvious effects on the strata to 

 the east of it. 



(1.) In the first place, there would be a great hollow produced below the sedi- 

 mentary strata, in that part where the trap previously existed. Here, it is im- 

 portant to remark, that a great portion of the trap which flowed out, appears to 

 have come — not straight up from beneath, but rather in a slanting direction from 

 the eastward. It is difficult to account for several of the phenomena of Salis- 

 bury Craigs (especially where the strata have trap interposed between them), ex- 

 cept on that supposition. Now, what would be the consequence of an immense 

 hollow being produced under the coal-measures to the east of Arthur Seat ? There 

 would evidently be a sinking of the sedimentary strata. The tilting or turning 

 up of the edges of the strata would not be the only effect ; — there would be a 

 general sinking of the deposit en masse, in that part at least which was imme- 

 diately over the hoUow caused by the outburst of Arthur Seat, Blackford Hill, &c. 



Before going farther, let us see, whether these effects tally with what is the 

 fact. It has been explained, in the first part of this memoir, that the Esk coal- 

 basin is greatly deeper and steeper than the Tjme basin. The Great Seam of coal 

 runs through both, and the respective levels of that seam, in the centre or trough 

 of each basin, may serve to illustrate in some degree the point now adverted to.* 

 The Great Seam is — near Cockenzie, about 20 fathoms below the sea-level ; whilst 



* See in illustration of what is here stated, the sections on plate XV. 



