The cause of the 'setting' of puzzolana and tufa is that rain-water containing a small proportion of carbonic acid acts on the lime in the volcanic fragments, and these become cemented together by the carbonate of lime and the free silica, which are thus produced in the mass. When a strong wind is blowing during a volcanic outburst, the materials may be driven to one side of the vent, and accumulate there more rapidly than on the other. Thus lop-sided cones are formed, such as may frequently be observed in some volcanic districts. In areas where constant currents of air, like the trade-winds, prevail, all the scoria-cones of the district may thus be found to be unequally developed on opposite sides, being lowest on those from which the prevalent winds blow, and highest on the sides towards which these winds blow. [Sidenote: ANGLE OF SLOPE IN VOLCANIC CONES.] The examination of any careful drawing, or better still of the photograph, of a volcanic cone, will prove that the profile of such cones is not formed by straight lines, but by curves often of a delicate and beautiful character. The delineations of the sacred volcano of Fusiyama, which are so constantly found in the productions of Japanese artists, must have familiarised everyone with the elegant curved lines exhibited by the profiles of volcanoes. The upper slope of the mountain is comparatively steep, often exhibiting angles of 30° to 35°, but this steepness of slope gradually diminishes, till it eventually merges in the surrounding plains. The cause of this elegant form assumed by most volcanic mountains is probably two-fold. In the first place we have to remember that the materials falling upon the flanks of the mountain differ in size and shape, and some will rest on a steeper slope than others. Thus, while some of the materials remain on the upper part of the mountains, others are rolling outwards and downwards. Hence we find that those cones which are composed of uniform materials have straight sides. But in some cases, we shall see hereafter, there has certainly been a central subsidence of the mountain mass, and it is this subsidence which has probably given rise to the curvature of its flanks. We have hitherto considered only the methods by which the froth or foam, which accumulates on the surface of fluid lava, is dispersed. But in many cases not only is this scum of the lava ejected from the volcanic vent by the escaping steam, but the fluid lava itself is extruded forcibly, and often in enormous quantities.