CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 465 
of the rim, and dips down to the flat tuff-covered crater bottom, at an angle 
of 37°. These are its natural angles of repose. 
Applying modern analogies of this kind, I have been led to conclude that 
the stratification so conspicuous in the tuff of the Carboniferous vents through- 
out central Scotland belongs to the interior of the crater and the upper part of 
the volcanic funnel. These stratified tuffs, on this view of their origin, must be 
regarded as remains of the beds of dust and stones which gathered within the 
crater and volcanic orifice, and which, on the cessation of volcanic action, some- 
times remained in their original position, or were dislocated, and slipped down 
into the cavity beneath. That the tuffs consolidated on slopes, perhaps quite 
as steep as those of Volcano, is now and then indicated by an interesting 
structure. The larger stones imbedded in the layers of tuff may be observed 
to have on their fronts in one direction a small heap of coarse gravelly debris, 
while fine tuff is heaped up against their opposite side. This arrangement 
doubtless points to deposit on a slope of loose debris, from which the larger 
blocks protruded so as to arrest the smaller stones, and allow the fine dust to 
gather behind. 
The frequent evidence of great disturbance in the bedding of the tuff within 
the vents may be connected with some kind of collapse, subsidence, or shrinkage 
of the materials in the funnel below. That a movement of this nature did take 
place is shown by the remarkable bending down of the strata round the 
margins of the vents, as will be described in the sequel. 
Dykes, Pipes, and Cakes of Lava in Necks of Tuff and Agglomerate.—The minor 
vents for the most part contain only fragmentary materials ; but those of larger 
size usually present masses of lava in some characteristic forms. In not a few 
cases the lava has risen in the central pipe and hardened there into a column of 
solid rock. Subsequent denudation, by removing most of the cone, has left the 
top of this broad column projecting as a round knoll upon the hill top. 
Arthur Seat presents a good example of this structure. Where the denuda- 
‘tion has not proceeded so far, we may still meet with a remnant of the cake of 
lava which sometimes overflowed the bottom of acrater. The summit of Largo 
Law affords indications of this arrangement. That cone of tuff is capped with 
basalt, evidently the product of successive streams, which welling out irregularly 
covered the crater bottom with hummocks and hollows. The knolls are beauti- 
fully columnar, and sometimes show a divergent arrangement of the prisms. 
But the most frequent form assumed by the lava in the necks is that of veins 
or dykes running as wall-like bands through the tuff or agglomerate. Many 
admirable examples might be cited; the most striking and accessible being 
those of the Fife coast. The shores between Largo and St Monans abound 
with them. These intruded masses vary in breadth from mere threadlike 
veins up to dykes several yards in breadth, which sometimes expand into large 
VOL. XXIX. PART I. 6 D 
