Cinder- or scoria-cones, and pumice-cones, are often found raised by the action of winds to a greater elevation on one side than the other, in the manner already described. One side of the cone is often seen to be more or less completely swept away by an outwelling stream of lava, and thus breached cones are formed (see fig. 40, p. 123). Not unfrequently we find a number of cones which are united more or less completely at their bases, as in Vulcanello (fig. 6, p. 43), the several vents being so near together that their ejections have mingled with one another. Cones composed entirely of fragmentary materials often show an approach to the beautifully curved slopes which we have described as being so characteristic of volcanoes, as may be seen in fig. 41, facing p. 124. In the case of scoria- and pumice-cones this curvature is probably due to the rolling downnwards and outwards of the larger fragments. We have already pointed out that with the scoriæ there are often ejected fragments torn from the sides of the volcanic vents. Sometimes such fragments are so numerous as to make up a considerable portion of the mass of the volcanic cones. Thus in the Eifel we find hills, of by no means insignificant size, completely built up of small scoriæ and broken fragments of slate torn from the rocks through which the volcanic fissures have been opened. Occasionally we see that few or no scoriæ have been ejected, and the volcanic vents are surrounded simply by heaps of burnt slate. The smaller fragmentary materials ejected from volcanic vents--such as lapilli and dust--rest in heaps, having a different angle of slope from those formed by scoriæ. In many cases, as we have seen, such finely-divided materials descend in the condition of mud, which flows evenly over the surface of the growing cone and consolidates in beds of very regularly stratified 'tufa' or 'tuff.' [Sidenote: CHARACTERS OF TUFF-CONES.]