The cause of this peculiar arrangement of the materials is evident. The sawdust thrown up by the air blast descends in a shower and tends to accumulate in a circular heap around the orifice, the area of this circular heap being determined by the force of the blast. Within this circular area, however, the quantity of falling fragments is not everywhere the same; along a circle surrounding the vent at a certain distance, the maximum number of falling fragments will be found to descend, and here the thickest deposit will take place. As this goes on, a circular ridge will be formed, with slopes towards and away from the centre of injection. As the ridge increases in height, the materials will tend to roll down either one slope or the other, and gradually a structure of the form shown in the figure will be piled up. The materials sliding down the outer slope will tend to increase the area of the base of the cone, while those which find their way down the inner slope will fall into the vent to be again ejected. [Illustration: Fig. 38.--Natural section of a Tuff-cone forming the Cape of Misenum, and exhibiting the peculiar internal Arrangement characteristic of volcanoes composed of fragmentary materials.] Volcanic cones composed of scoriæ, dust, &c. are found to have exactly the same internal structure as is exhibited by the miniature cone of sawdust. The more or less regular layers of which they are made up dip in opposite directions, away from and towards the vent, and thin out in the direction of their dip (see fig. 38). In small cones the crater or central cavity is of considerable size in proportion to the whole mass, but as the cone grows upwards and outwards, the dimensions of the crater remain the same, while the area of the base and the height of the cone are continually increasing. This is the normal structure of volcanic cones formed of fragmentary materials, though, as we shall hereafter show, many irregularities are often produced by local and temporary causes. [Illustration: Fig. 39.--Section of a small Scoria-cone formed within the crater of Vesuvius in the year 1885, illustrating the filling-up of the central tent of the cone by subsequent ejections.] In some cases the central vent of a volcanic scoria-cone may be filled up by subsequent ejections. A beautiful example of this kind was observed by Abich, in the case of a small cone formed within the crater of Vesuvius in 1835, and is represented in fig. 39. [Illustration: Fig. 40.--Volcanic Cones composed of Scoriæ, and breached on one side by the outflow of lava-currents.] [Sidenote: BREACHED CONES.]