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 unconformability 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 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, 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 forms. 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 removing most of the cone, has left the top of this thick column projecting as a round knoll upon the hill-top. Arthur Seat presents a good example of 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. [Illustration: Fig. 221.--Dyke rising through the agglomerate of a volcanic vent; Kincraig, Elie.]