HASWELL — OTf THE DENUDATION OF AETHUR'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, wonld soon be swept aw'ay 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. Eig. 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, \'iz. that after a great mass ot 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 
tlie 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 Eowandthe Dasses, 
and hanging down into the Hunter's Bog, which it must have once 
filled, as it could not stop short there; consequently these vallei/s 
must have been eroded before the ejection of tlie 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 How), 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 tlie coal strata wliich are known to form its 
channel,— Inch- Garvey, Inch-Colm, Cramond Island, Inch-Keith, 
Fidra, Craigleith, and the Bass K-ock. 
The denudation we speak of was not confined to Arthur's Seat 
alone. It extended over a larger area. The Pentland Hills which 
lay submerged under tlie same sea also suffered from it, the Eoman 
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 thev 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. Yll. ^ 
