From what has been said in the preceding chapter, it will be gathered that the volcanoes built up by ejections of fragmentary materials differ in many striking particulars from those formed by the outwelling of lavas from volcanic vents. In a less degree, the volcanoes composed of the same kind of volcanic materials also vary among themselves.
CHARACTERS OF SCORIA-CONES.
When masses of scoriæ in a semi-fluid condition are thrown to only a little distance above the volcanic vent, so that they have not time to assume a perfectly solid condition before they fall round the vent, the rugged masses of lava unite to form heaps of most irregular shape. In such cases, the falling fragments being in a semi-plastic state, stick to the masses below, and do not tend to roll down the sides of the heap. Irregular heaps of such volcanic scoriæ abound on the surfaces of lava-streams, being piled up around each 'bocca' or vent which the steam-jets escaping from the lava-currents form at their surfaces. Such irregular accumulations of scoriæ were observed on the lavas of Vesuvius during the eruptions of 1822, 1855, and 1872, and have also been described in the case of many other volcanoes. In [fig. 26] (p. 101) we have given representations of a group of such irregular scoria-cones which was observed by Schmidt on the Vesuvian lava of 1855. It will be seen from this drawing that there is scarcely any limit to the steepness of the sides of such scoria-heaps, in which the materials are in an imperfectly solidified condition when they reach the ground.
But in the majority of cases, the scoriæ ejected from volcanic vents are thrown to a great height, and are in a more or less perfectly solidified condition when they fell to the ground again. In such cases the fragments obey the ordinary mechanical laws of falling bodies, rolling and sliding over one another, till they acquire a slope which varies according to the size and form of the fragments. In this way the great conical mounds are formed which are known as 'cinder-cones,' or more properly as 'scoria-cones.' Scoria-cones usually vary in the slope of their sides from 35° to 40° and may differ in size from mere monticules to hills a thousand feet or more in height. Scoria-cones of this character abound in many volcanic districts, as the Auvergne, where they may be numbered by thousands. The materials forming such scoria-cones vary in size from that of a nut to masses as large as a man's head, and fragments of even larger dimensions are by no means uncommon.
When the lava in a volcanic vent is perfectly glassy, instead of being partially crystalline in structure, we find not scoriæ but pumice ejected. In such cases, as in the Lipari Islands for example, we see cones entirely built up of pumice. Such pumice-cones resemble in the angle of their slope (see [fig. 41], facing p. 124), the ordinary scoria-cones, but are of a brilliant white colour, appearing as if covered with snow.
PRESERVATION OF SCORIA-CONES.
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.
Cinder- or scoria-cones, and pumice-cones, are often found raised by the action of winds to a greater elevation on one side than the other, in the manner already described. One side of the cone is often seen to be more or less completely swept away by an outwelling stream of lava, and thus breached cones are formed (see [fig. 40], p. 123). Not unfrequently we find a number of cones which are united more or less completely at their bases, as in Vulcanello ([fig. 6], p. 43), the several vents being so near together that their ejections have mingled with one another. Cones composed entirely of fragmentary materials often show an approach to the beautifully curved slopes which we have described as being so characteristic of volcanoes, as may be seen in [fig. 41], facing p. 124. In the case of scoria- and pumice-cones this curvature is probably due to the rolling downnwards and outwards of the larger fragments.
We have already pointed out that with the scoriæ there are often ejected fragments torn from the sides of the volcanic vents. Sometimes such fragments are so numerous as to make up a considerable portion of the mass of the volcanic cones. Thus in the Eifel we find hills, of by no means insignificant size, completely built up of small scoriæ and broken fragments of slate torn from the rocks through which the volcanic fissures have been opened. Occasionally we see that few or no scoriæ have been ejected, and the volcanic vents are surrounded simply by heaps of burnt slate.