_a._ Crater. _b b._ Lava-streams. _c._ Dykes which have clearly formed the ducts, through which the lava has risen to the crater. _d d._ Stratified volcanic scoriæ. _e._ Talus of fallen materials.] In the second place, we have lava-streams which have been ejected from the crater or from fissures on the flanks of the cone, and flowed down its sides. And thirdly, we find masses of lava filling up cracks in the cone; these latter are called 'dykes.' Of these three kinds of structures most volcanic mountains are built up, but in different cases the part played by these several elements may be very unequal. Sometimes volcanoes consist entirely of fragmentary materials, at others they are made up of lavas only, while in the majority of cases they have been formed by alternations of fragmentary and fluid ejections, the whole being bound together by dykes, which are masses of lava injected into the cracks formed from time to time in the sides of the growing cone. If we direct our attention in the first place to the fragmentary ejections, we shall find that they affect a very marked and peculiar arrangement, which is best exhibited in those volcanic cones composed entirely of such materials. [Sidenote: INTERNAL STRUCTURE OF VOLCANIC CONES.] Everyone who examines volcanoes for the first time will probably be struck by the regular stratification of materials of which they are composed. Thus the tuffs covering the city of Pompeii are found to consist of numerous thin layers of lapilli and volcanic dust, perfectly distinct from one another, and assuming even the arrangement which we usually regard as characteristic of materials that have been deposited from a state of suspension in water. The fragmentary materials in falling through the air are sorted, the finer particles being carried farther from the vent than the larger and heavier ones. The force of different volcanic outbursts also varies greatly, and sometimes materials of different character are thrown out during successive ejections. These facts will be illustrated by fig. 36, which is a drawing of a section exposed in a quarry opened in the side of the Kammerbühl. In this section we see that the falling scoriæ have been arranged in rudely parallel beds, but the regular deposition of these has been interrupted by the ejection of masses of burnt slate torn from the side of the vent, probably during some more than usually violent paroxysm of the volcano. In those volcanoes which are built up of tuffs and materials which have fallen in the condition of a muddy paste, the perfect stratification of the mass is often very striking indeed, and large cones are found built up of thin uniformly-spread layers of more or less finely-divided materials, disposed in parallel succession. Such finely-stratified tuff-cones abound in the district of the Campi Phlegræi. [Illustration: Fig. 36.--Section in the side of the Kammerbühl, Bohemia.