This mode of division is justified from a natural history point of view. The first group includes rocks that have consolidated slowly underground. The second includes rocks cooled more quickly, on the margins of magma-basins, or as offshoots from them, filling cracks in the surrounding rocks, and producing wall-like masses known as _dykes_. The third group appears mostly in dykes and lava-flows. Where a dyke has intruded among heated rocks and undergoes no sudden chilling, it may become coarsely crystalline, even though comparatively small. Some dykes exhibit a chilled margin of glass along their bounding surfaces, and are none the less completely crystalline at the centre, where cooling has been slow. No structure is peculiar to dyke-rocks, nor can a class be established for such rocks on chemical or mineralogical grounds, even though a few special types of igneous rock may at present be known only among these minor intrusive bodies. [Illustration: Fig. 14. Side of a Volcanic Cone. Ash-layer of 1906 on the west flank of Vesuvius. Cliffs of the exploded crater of Monte Somma behind.] The fine-grained layers of _volcanic dust_, commonly spoken of as _ash_, and the coarser _tuffs_, in which lumps of scoriaceous lava are clearly visible, bridge the gap between sedimentary and igneous rocks. The dust, during a great eruption, is distributed by wind over hundreds of square miles of country. The tuffs, deposited nearer the orifice of the volcano, vary in coarseness from day to day, and exhibit marked stratification. Ash-beds and tuffs may be laid out in lakes or in the sea, and their layers may then include organic remains. Waves may round their particles on the shore, and may sift them till only a coarse volcanic sand remains. After an eruption, the newly deposited ash and tuff usually form obvious layers on the surface of the country. Landslips on the side of the volcanic cone may reveal sections of the new coating and of previously stratified material (Fig. 14). In certain districts, sedimentary and other rocks torn off from below form a large part of the fragmental deposits of volcanic action. The characteristic volcanic cone is itself due to the greater accumulation of tuffs and ashes near the vent (Fig. 15). The loose tuffs formed of scoriæ allow water to percolate easily through them, and a cone of fairly coarse material resists the weather well. The remarkable freshness of the extinct "cinder-cones" of Auvergne was thus long ago explained by Lyell. Surfaces of ash, on the other hand, are easily washed down by rain in the form of dangerous mud-flows, which spread across the lowlands, and give rise to compact clays, shrinking as they dry. [Illustration: Fig. 15. Tuff-Cone with Tuff-Beds at the base. Puy de la Vache, Puy-de-Dôme, France.]