The breaking up of a lava-field into chaotic masses evidently proceeds from a temporary cessation, either complete or partial, and a subsequent flow of a stream of lava. The surface cools and hardens as soon as the stream slackens; afterwards there is another heaving of the lava, and an onward move, owing to a succeeding ejection or the removing of an obstacle, and the motion breaks up the hardened crust, piling the masses together, either in slabs or huge angular fragments, according to the thickness to which the crust had cooled. If the motion of a lava-stream be quite slow, the cooling of the front of it may cause its cessation, thus damming it up and holding it back, till the pressure from gradual accumulation behind sweeps away the barrier. It then flows on again, carrying on its surface masses of the hardened crust—some, it may be, to sink and melt again, but the larger portion to remain as a field of clinkers. The breaking-up of the ice of some streams in spring gives some idea of the manner in which the hardened masses of a lava-field are piled up as it moves along; but to form a just idea of the greatness of the effect, the mind must bring before it a stream, not of the scanty limits of most rivers, but one, not unfrequently, of several miles in breadth: besides, in place of slabs of pure and clear ice, there should be substituted shaggy heaps of black scoriæ, and a depth or thickness of many yards in place of a few inches. Where volcanic mud-streams have flooded the land, or a rain of ashes and light scoriæ has descended upon the soil, its fertility may soon be restored under the influence of a sunny sky; but as far as the lava reaches, a stony wilderness often remains for ages, particularly in the colder regions of the earth. Thus, though many of the lava-fields of Iceland have existed long before the first Scandinavian colonists settled in the land, their surface is generally as naked as when they first issued from the volcano; and where signs of vegetation may be seen among their fragments, the eye finds nothing to relieve the horrid monotony of the scene but spare patches of lichen and mosses, or here and there some dwarf herb or shrub that hardly ventures to peep forth from the crevice in which it has found a shelter. But in a milder climate, such as that of Italy, and still more rapidly in the torrid zone, the horrid nakedness of a lava-field undergoes a more rapid transformation, provided a sufficient moisture favours the growth of plants. The rains promote the decomposition of the lava, and a rank vegetation succeeds, which in its turn assists the work of decomposition, and thus hastens the accumulation of soil. Ferns and grasses spring up in the nooks and crevices, and finally the vine or the taro flourish luxuriantly, for nothing can exceed the fertility of a disintegrated lava-field.