_Lava-flows_ are masses of molten rock that have welled out from the vent, without being torn to pieces by the explosion of the gases that they contained. The rapidity of their flow depends on their chemical composition, on the amount of gases present, and on the temperature at which they are extruded. The more highly siliceous lavas, for a given temperature, are more viscous than those towards the basaltic end of the series, which contain only about 48 per cent. of silica. A lava of considerable fluidity will consolidate in somewhat thin sheets with smooth and ropy surfaces. A less fluid type will become markedly scoriaceous, where the vapours endeavour to escape from it; the rugged crust formed on its upper cooling surface will be broken up by the continued movement of the more liquid mass below, and the blocks thus formed may become rolled over the advancing front of the flow and entombed in the portion that has not yet consolidated. The surface of ordinary lava-flows remains rough for centuries, and only slowly crumbles down before weathering to form a soil. While tuff-beds provide light and fertile lands, the lava-streams remain marked out among them, as sinuous bands of rock, given over to an irregular growth of woodland. By repeated outflows, lavas tend to fill up the interspaces between the earlier streams, just as these have filled up the hollows in the country over which they spread. A uniform surface thus arises, and _lava-plains_ eventually bury a varied land of hill and dale. Where a number of small vents have opened, perhaps along parallel fissures in the earth, the flooding of the country with igneous rock may lead to an appearance of stratification in masses extending over hundreds of square miles. Sections in the igneous series, however, show that the individual flows dove-tail into and overlap one another, more rapidly than is the case with the lenticular masses that constitute an ordinary sedimentary series. After the constituents of the lava have begun to crystallise, and when the rock may be considered solid, cracks due to contraction are set up. The upper part of the flow, radiating its heat and parting with its gases into the air above, solidifies comparatively rapidly, and cracks arise without much regularity. Now and then, columnar structure, like that of dried starch, appears on a small scale, the columns starting from various oblique surfaces of cooling, and lying in consequence in various directions in the rock.