I have already referred to the frequently abundant pieces of stratified tuff, found as ejected blocks in vents filled with tuff, and to the derivation of these blocks from tuff originally deposited within the crater. There can, I think, be little hesitation in regarding the stratification of these Fife vents as a record of successive deposits of volcanic detritus inside the vents. The general dip inwards from the outer rim of the vent strikingly recalls that of some modern volcanoes. By way of illustration, I give here a section of part of the outer rim of the crater of the Island of Volcano, sketched by myself in the year 1870 while ascending the mountain from the north side (Fig. 220). The crater wall at this point consists of two distinct parts--an older tuff (_a_), which may have been in great measure cleared out of the crater before the ejection of the newer tuff (_b_). The latter lies on the outer slope of the cone at the usual angle of 30°. It folds over the crest 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. [Illustration: Fig. 218.--Section across the great vent of Kincraig, Elie, on a true scale, vertical and horizontal, of six inches to a mile. 1, Sandstones, shale, etc., of Lower Carboniferous age, plunging down toward the neck T; B, columnar basalt, shown also in Figs. 223 and 225.] [Illustration: Fig. 219.--Dyke in volcanic neck, on the beach, St. Monans.] [Illustration: Fig. 220.--Section of part of crater rim, Island of Volcano.] Applying modern analogies of this kind, I have been led to conclude that the stratification so conspicuous in the tuff of the vents in the east of Fife and in the Carboniferous series of the Lothians belongs to the interior of the crater and the upper part of the volcanic funnel.[103] 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, sometimes 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. [Footnote 103: Further illustrations of this characteristic structure of some vents will be found in the account of the Tertiary vents of the Faroe Isles in Chapter xli. See also the remarks in the introductory chapters, vol. i. p. 63.]