HASWELL— ON THE DENUDATION OE ARTHUR'S SEAT. 



97 



more easily exposed to the action of tides and oceanic currents. The 

 upper part of the hill, consisting of softer strata than the traps be- 

 neath, would soon be swept away and the waste material deposited 

 in some other place ; while the lower part of the hill, the traps them- 

 selves, owing to their hardness and tenacity, would resist the action 

 of the waves more, and so stand out in bold relief above the softer 

 Carboniferous strata. This is represented in the accompanying dia- 

 gram; in fact, this is exactly the appearance which a section through 

 the north end of Arthur's Seat would present, if the rocks round the 

 summit were taken away (PL V. Tig. 2). "We shall have occasion to 

 refer to these rocks presently ; but it is now well known that the vol- 

 canic ash or trap-tuff round the summit, the basalt of the summit 

 itself, the columnar basalt of the Lion's Haunch, and the porphyritic 

 felstone which forms part of the ridge descending from the summit 

 into the south end of the Dry Dam, are each and all of them of a 

 more recent age than the other traps of the hill. 



Now if it can be shown that these later igneous rocks cover over 

 and rest upon the denuded edges of the older traps and the Carboni- 

 ferous strata, what other conclusion is open to us but the one already 

 alluded to, viz. that after a great mass of Carboniferous strata had 

 been deposited on the top of the interbedded traps, and after these 

 had been tilted up into their present position by the eruption of St. 

 Leonard's Craig, Salisbury Craigs, and the Dasses, — the whole of 

 these rocks, being still under water, were subjected to marine denu- 

 dation, and the waves, tides, and currents aided by the atmosphere, 

 the springs, and the frosts, and performing their functions exactly in 

 the same way as we see them now, gradually swept away the softer 

 material, and left the harder traps as prominent crags ? We find the 

 later ash filling up the valley between the Long liowandthe Dasses, 

 and hanging down into the Hunter's Bog, which it must have once 

 filled, as it could not stop short there ; consequently these valleys 

 must have been eroded before the ejection of the ash. This tells its 

 own history. Moreover we find the same ash covering over the Long 

 Eow, the Dasses, and the greenstone seen in a section at the south 

 side of the Drive (which is a continuation of the Long Eow), and all 

 these must have existed as prominent crags when the ash and the 

 scoriae fell around them. In the bed of the Firth of Forth also, 

 many traps project from the coal strata which are known to form its 

 channel, — Inch-Garvey, Inch-Colm, Cramond Island, Inch-Keith, 

 Fidra, Craigleith, and the Bass Bock. 



The denudation we speak of was not confined to Arthur's Seat 

 alone. It extended over a larger area. The Pentland Hdls which 

 lay submerged under the same sea also suffered from it, the Roman 

 Camp Hill also, and indeed the whole surrounding area of the Lo- 

 thians. It has been calculated that the Carboniferous strata which 

 covered the Pentlands and Arthur's Seat amounted in thickness to 

 8000 feet, but supposing they were only 5000 feet thick, this, with 

 the present average height of the Pentlands, at 1000 feet above the 

 surrounding country, would give us as the amount of material re- 



TOL. VII. O 



