Ordinary scoriæ are usually of a black colour when first ejected, but after a short time the black oxide of iron (magnetite) which they contain, attracts the oxygen of the air and moisture, and assumes the reddish-brown colour of iron-rust. Under such circumstances the heaps of black material gradually acquire the red-brown colour which is characteristic of so many of the scoria-cones around Etna, and in the Auvergne and the Eifel. The moisture of the air, and the rain falling upon these loose cindery heaps, cause them to decompose upon their surfaces; the action is facilitated by the growth of the lower forms of vegetation, such as mosses and lichens, and thus at last a soil is produced on the surfaces of these conical piles of loose materials which may support an abundant vegetation. Cinder- or scoria-cones are not uncommonly found retaining in a most perfect manner their regular, conical form, the lips of their craters being sharp and unbroken as if the cone were formed but yesterday, while their slopes may nevertheless be covered with a rich soil supporting abundant grass and forest-trees. It may at first sight seem difficult to understand how a loose mass of scoriæ could have so long withstood the action of the rain and floods, retaining so perfectly its even slopes and sharp ridges. A little consideration will, however, convince us that it is the very loose and pervious nature of the materials of which scoria-cones are composed, which tends to their perfect preservation. The rain at once sinks into their mass, before it has time to form rivulets and streams which would wear away their surfaces and destroy the regularity of their outlines. Scoria- and pumice-cones are frequently found to be acted upon by acid vapours to such an extent that the whole of the materials is reduced to a white pulverulent mass. In these cases the oxides of iron and the alkalis have united with the sulphuric or hydrochloric or carbonic acids, the compounds being carried away in solution by the rain-water falling on the mass; the materials left are silica, the hydrated silicate of alumina, and hydrated sulphate of lime (gypsum), all of which are of a white colour.