The 'tuff-cones' thus formed differ in many important respects from the scoria-cones already described. The slope of their sides varies from 15° to 30°, and is almost always considerably less than in scoria- and pumice-cones. The tuff-cones undergo much more rapid degradation from rain and moisture than do the scoria-cones; for, though the materials of the former 'set,' as we have seen, into a substance of considerable hardness, yet this substance, being much less pervious to water than the loose scoria heaps, permits of the formation of surface-streams which furrow and wear away the sides of the cones. Sometimes the sides of the crater are found to be almost wholly removed by atmospheric denudation, and only a shallow depression is found occupying the site of the crater; such a case is represented in fig. 59. We not unfrequently find the whole slopes of such cones to be traversed by a series of radiating grooves passing from the summit to the base of the mountains, these channels being formed by water, which has collected into streams, flowing down the slopes of the mountains. The volcanic cone, under these circumstances, frequently presents the appearance of a partially opened umbrella. Owing to the impervious character of the materials composing tuff-cones, their craters are frequently found to be occupied by lakes. [Illustration: Fig. 59.--Summit of the volcano of Monte Sant' Angelo in Lipari exhibiting a crater with walls worn down by denudatioh.] Tufas have usually a white or yellowish-brown colour, and these are the colours exhibited by the cones composed of this material before they become covered by vegetation. Tufas scoriæ, and lavas usually crumble down to form a very rich soil, and many of the choicest wines are produced from grapes grown on the fertile slopes of volcanic mountains. When, however, as not unfrequently happens, the materials are finely divided and incoherent, they are so easily driven about by the winds that cultivation of any kind is rendered almost impossible. In the Islands of Stromboli and Vulcano the gardens have to be surrounded by high fences to prevent them from being overwhelmed by the ever-shifting masses of volcanic sand. [Sidenote: CHARACTERS OF LAVA-CONES.] There are some cones which are composed in part of scoriæ and in part of tufa. Hence we are sometimes at a loss whether to group them with the one class of cones or the other. But in the majority of cases, scoria- and tuff-cones present the sufficiently well-marked and distinctive characters which we have described. Lava-cones differ quite as greatly in their forms as do the cones composed of fragmentary materials, the variations being principally determined by the degree of liquidity of the lavas.