The short account which we have been able to give in the foregoing pages of the researches that have been carried on concerning the nature of the materials ejected from volcanoes will serve to show that these investigations have already made known many facts of great interest, and that the farther pursuit of them is full of the highest promise. To the scientific worker no subject is too vast for his research, no object so minute as to be unworthy of his most patient study. In some of our future inquiries concerning the nature of volcanic action, we shall be led to an investigation of the phenomena displayed in the sun, moon, comets and other great bodies of the universe; but another road to truths of the same grandeur and importance is found, as we have seen, in an examination of the mode of development of crystallites, and a study of the materials contained in the microscopic cavities of the minutest crystals.
CHAPTER IV.
THE DISTRIBUTION OF THE MATERIALS EJECTED FROM VOLCANIC VENTS.
The escape of great quantities of steam and other gases from the midst of a mass of fluid or semi-fluid lava gives rise to the formation of vast quantities of froth or foam upon its surface. This froth or foam, which is formed upon the surface of lava by the escape of gaseous matters from within it, is made up of portions of the lava distended into vesicles, in the same way that bubbles are formed on the surface of water. It bears precisely the same relation to the liquid mass of lava that the white crest of foam upon an advancing wave does to the sea-water, from the bubbles of which it is formed.
This froth upon the surface of lavas varies greatly in character according to the nature of the material from which it is formed. In the majority of cases the lavas consist, as we have seen, of a mass of crystals floating in a liquid magma, and the distension of such a mass by the escape of steam from its midst gives rise to the formation of the rough cindery-looking material to which the name of 'scoria' is applied. But when the lava contains no ready-formed crystals, but consists entirely of a glassy substance in a more or less perfect state of fusion, the liberation of steam gives rise to the formation of the beautiful material known as 'pumice.' Pumice consists of a mass of minute glass bubbles; these bubbles have not usually, however, retained their globular form, but have been elongated in one direction through the movement of the mass while it was still in a plastic state.
The steam frequently escapes from lava with such violence that the froth or scum on its surface is broken up and scattered in all directions, as the foam crests of waves are dispersed by the wind during a storm. In this way fragments of scoria or pumice are often thrown to the height of many hundreds or thousands of feet into the atmosphere, as we have seen is the case at Stromboli and Vesuvius. Indeed, during violent eruptions, a continuous upward discharge of these fragments is maintained, the ragged cindery masses hurtling one another in the atmosphere, as they are shot perpendicularly upwards to an enormous height and fall back into the vent; or they may rise obliquely and describe curves so as to descend outside the orifice from which they were ejected.
FINENESS OF VOLCANIC DUST.
During their upward discharge and downward fall, the cindery fragments are by attrition continually reduced to smaller dimensions. The noise made by these fragments, as they strike against one another in the air during their rise and fall, is one of the most noteworthy accompaniments of volcanic eruptions. It has been noticed that in many cases there is a constant diminution in the size of the fragments ejected during a volcanic outburst, this being doubtless due to the friction of the masses as they are ejected and re-ejected from the vent. Thus it is related by Mr. Poulett Scrope, who watched the Vesuvian eruption of 1822, which lasted for nearly a month, that during the earlier stages of the outburst fragments of enormous size were thrown out of the crater, but by constant re-ejection these were gradually reduced in size, till at last only the most impalpable dust issued from the vent. This dust filled the atmosphere, producing in the city of Naples 'a darkness that might be felt,' and so excessively finely divided was it, that it penetrated into all drawers, boxes, and the most closely fastened receptacles, filling them completely. Mr. Whymper relates that, while standing on the summit of Chimborazo, he witnessed an eruption of Cotopaxi, which is distant more than fifty miles from the former mountain. The fine volcanic dust fell in great quantities around him, and he estimated that no less than two millions of tons must have been ejected during this slight outburst. Professor Bonney has examined this volcanic dust from Cotopaxi, and calculates that it would take from 4,000 to 25,000 particles to make up a grain in weight.
Various names have been given by geologists to the fragments ejected from volcanic vents, which, as we have seen, differ greatly in their dimensions and other characters. Sometimes masses of more or less fluid lava are flung bodily to a great height in the atmosphere. During their rise and fall these masses are caused to rotate, and in consequence assume a globular or spheroidal form. The water imprisoned in these masses, during their passage through the atmosphere, tends to expand into steam, and they become more or less completely distended with bubbles. Such masses, which sometimes assume very regular and striking forms, are known as 'volcanic bombs.' Many volcanic bombs have a solid nucleus of refractory materials. The large, rough, angular, cindery-looking fragments are termed 'scoriæ.' When reduced to the dimensions of a marble or pea they are usually called by the Italian name of 'lapilli.' The still finer materials are known as volcanic sand and dust.
There are, however, two names which are frequently applied to these fragmentary materials ejected from volcanoes, which are perhaps liable to give rise to misconception. These are the terms 'cinders' and 'ashes.' It must be remembered that the scoriæ or cindery-looking masses are not, like the cinders of our fires, the product of the partial combustion of a material containing inflammable gases, but are, like the clinkers of furnaces and brick-kilns, portions of partially vitrified and fused rock distended by gases. So, too, volcanic ashes only resemble the ashes of our grates in being very finely divided; they are not, like the latter, the incombustible residue of a mass which has been burnt.
VOLCANIC BOMBS AND PELE'S HAIR.