The structure of these composite cones may be understood by an inspection of the accompanying fig. 47, which shows the appearances presented in a cliff on the coast of the Island of Madeira. We see that the mass is made up of numerous layers of volcanic scoriæ, alternating with sheets of lava. The latter, which are represented in transverse section in the drawing, are seen to thin out on either side, and to vary greatly in breadth. Besides the alternating masses of scoriæ and the lava-sheets, there are seen in the section, bands of a bright-red colour, which are represented in the drawing by black lines. These are layers of soil, or volcanic dust, which, by the passage of a lava-stream over their surface, have been burnt so as to acquire a brick-red colour. These bands of red material, to which the name of 'laterite' has been frequently applied, very commonly occur in sections of composite volcanic cones. Crossing the whole of the horizontally-disposed masses in the section, we find a number of 'dykes,' which are evidently great cracks filled with lava from below. Some of these run vertically through the cliffs, others obliquely. In some cases the lava, rising to fill a dyke, has flowed as a lava-stream at the surface. Last of all, we must call attention to the fact that the section exhibits evidence of great movements having taken place subsequently to the accumulation of the whole of the materials. A great crack has been produced, on one side of which the whole mass has subsided bodily, giving rise to the phenomenon which geologists call a 'fault.' In the section, fig. 27, p. 104, copied from a drawing of a sea-cliff in the Island of Vulcano, a transverse section of a lava-stream is represented on a somewhat larger scale. The upper and under surface of the lava-stream is seen to have a scoriaceous structure, but the thick central mass is compact, and divided by regular joint-planes. This section also illustrates the fact that, before the lava-stream flowed down the sides of the mountain, a valley had been cut by meteoric agencies on the flanks of the volcano, the dykes which traverse the lower beds of tuff being abruptly truncated. In mountain ravines, upon the slopes of ancient volcanoes, and in the cliffs of volcanic islands, we are often able to study the way in which these great mountain masses are built up of alternating lava-currents, beds of volcanic agglomerate, scoriæ, tuff and dust, and intersecting dykes. In fig. 48, the features above described are illustrated by a section in the sides of the great volcano of Mont Dore. [Illustration: Fig. 48.--Section seen at the cascade. Bains du Mont Dore.] [Illustration: Fig. 49.--Section in the Island of Ventotienne, showing a great stream of andesitic lava overlying stratified tuffs.] [Sidenote: SECTIONS IN THE PONZA ISLANDS.]