CHAP. XXXI 
THE VENTS OF EASTERN FIFE 
83 
Fig. 220. — Section of part of crater rim, Island of Volcano. 
tuffs, on this view of their origin, must be regarded as remains of tire beds 
of dust and stones which gathered within the crater and volcanic orifice, 
and which, on the cessation of volcanic action, sometimes remained in their 
original position, or were 
dislocated and slipped down 
into the cavity beneath. 
That the tuff's consolidated 
on slopes, perhaps cpiite 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 direc- 
tion 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. 
If the inference be correct, that the stratification here described belongs 
to the old craters or the upper parts of the funnels, it furnishes additional 
evidence of the wide interval of time that elapsed between the deposition of 
the Carboniferous strata and the outbreak of these vents. During that 
interval prolonged denudation reduced the upturned Carboniferous Limestone ’ 
series to nearly its present form of surface, and any materials discharged 
from the vents over the surrounding ground would obviously lie with a 
violent unconfonnability on the rocks below. 
The frequent great disturbance in the bedding of the tuff within the 
vents may be connected with some kind of collapse, subsidence or shrinkage 
°f 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, which has been already described. 
The minor vents for the most part contain only fragmentary materials ; 
but those of larger size usually present masses of lava in some characteristic 
torrns. In not a few cases, the lava has risen in the central pipe and has 
hardened there into a column of solid rock. Subsequent denudation, by 
lenioving most of the cone, has left the top of this thick column projecting 
as a round knoll upon the hilltop. Arthur Seat presents a good example 
°t this structure. Where the denudation has not proceeded so far, we may 
still meet with a remnant of the cake of lava which sometimes overflowed 
the bottom of a crater. The summit of Largo Law affords indications of 
this arrangement, the cone of tuff being there capped with basalt, evidently 
the product of successive streams, which welling out irregularly covered the 
crater bottom with hummocks and hollows (Fig. 226). The knolls are 
beautifully columnar, and sometimes show a divergent arrangement of the 
prisms. 
But the most frequent form assumed by the lava in the necks is that 
°f veins or dykes running as wall-like bands through the tuff or agglomerate. 
