INTERNAL STRUCTURE OF LAVA-CONES.
Fig. 45.—Volcanic Cone (Mamelon) composed of very viscid lava. (Island of Bourbon.)
Fig. 46.—Another Mamelon in the Island of Bourbon, with a crater at its summit.
But while some volcanoes are composed entirely of the fragmentary ejections and others are wholly formed by successive outflows of lava, the majority of volcanoes, especially those of larger dimensions, are built up of alternations of these different kinds of materials.
Fig. 47.—Cliff-section in the Island of Madeira, showing how a composite volcano is built up of lava-streams, beds of scoriæ, and dykes.
NATURAL SECTIONS OF CONES.
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.'