Volcanoes: What They are and What They Teach

by John W. Judd

With respect to leucite, on the other hand, it must be remembered that it is a very unstable mineral which appears to be easily changed into felspar. It is by no means improbable, therefore, that some ancient igneous rocks which now contain felspar were originally leucitic rocks.

To the view that the action of volcanic forces upon the globe during past geological times was similar in kind to that which we now observe going on around us, still another objection has been raised. It has been asserted that some of the deposits of igneous rock associated with the older geological formations are of such a nature that they could not possibly have been accumulated around volcanic vents of the kind which we see in operation around us.

VOLCANIC ORIGIN OF ANCIENT IGNEOUS ROCKS.

Mr. Mallet has declared that the igneous products of the Palæozoic period differ fundamentally in character from those materials formed by volcanic action during the later Secondary and the Tertiary periods. Upon what observations these generalisations are based he has given us no information, and the enormous mass of facts which have been collected in recent years concerning the structure of the lavas and fragmental volcanic deposits of the pre-Cambrian, Cambrian, Silurian, Devonian and Carboniferous periods, all point to a directly opposite conclusion. The more carefully we carry on our investigations concerning these ancient lavas, by the aid of chemical analysis and microscopic study, the more are we convinced of the essential identity of the ancient and modern volcanic rocks, both in their composition and their minute structure. Of great masses of dust produced by crushing, such as Mr. Mallet has supposed to have been formed during the earlier geological periods, there is not the smallest evidence; but we everywhere find proofs, when the rocks are minutely examined, of the vesicular structure so characteristic of materials produced by explosive volcanic action.

It has frequently been asserted that in the great districts covered by basaltic lavas which we find in the Rocky Mountains of North America, in the Deccan of India, in Abyssinia, and even in the Western Isles of Scotland, we have proofs of the occurrence, during earlier geological periods, of volcanic action very different in character from that which at present takes place on our globe. It has been asserted that the phenomena observed in these districts can only be accounted for by supposing that great fissures have opened in their midst, from which lavas have issued in enormous floods unaccompanied by the ordinary explosive phenomena of volcanoes.

It must be remembered, however, that none of the districts in question have been subjected to careful and systematic examination with a view to the discovery of the vents from which these masses of lava have issued, with the exception of that which occurs in our own islands. In this case, in which superficial observers have spoken of the district as being covered with horizontal lava-sheets piled upon one another to the depth of 3,000 feet, careful study of the rock-masses has shown that the accumulations of basalt really consist of a great number of lava-currents which have issued at successive epochs covering enormous periods of time. During the intervals between the emission of these successive lava-currents the surfaces of the older ones have been decomposed, and formed soils upon which forests have grown up; they have been eroded by streams, the valleys so formed being filled with gravels; and lakes have been originated on their surfaces in which various accumulations have taken place.

TRULY-VOLCANIC ORIGIN OF LAVA PLATEAUX.

It has been demonstrated, moreover, that the basal-wrecks of no less than five volcanic mountains, each of which must have rivalled Etna in its proportions, existed within this area, and the connection of the lava-currents, which have deluged the surrounding tracts, with these great volcanoes has been clearly proved. It is probable that when more careful and systematic researches are carried on in the other districts, in which widely-spread sheets of basaltic rocks exist, similar volcanic vents will be discovered. It must also be remembered that if such a country as Iceland were subjected to long-continued denudation, the mountain peaks and cones of loose materials would be worn away, the whole island being thus reduced to a series of plateaux composed of lava-sheets, the connection of which with the crystalline materials filling the great volcanic vents, a superficial observer might altogether fail to recognise.

But even where we cannot trace the former existence of great volcanic mountains, like those which once rose in the Hebrides, it would nevertheless be very rash to conclude that the vast plateaux of lava-rock must have been formed as gigantic floods unaccompanied by ordinary volcanic action. Mr. Darwin has pointed out that in crossing districts covered by lava, he was frequently only able to determine the limits of the different currents of which it was made up, by an examination of the age of the trees and the nature of the vegetation which had sprung up on them. And everyone who has travelled much in volcanic districts can confirm this observation; what appears at first sight to be a great continuous sheet of lava proves upon more careful observation to be composed of a great number of distinctly different lava-currents, which have succeeded one another at longer or shorter intervals.

We must remember, too, how various in kind are the volcanic manifestations which present themselves under different circumstances. Sometimes the amount of explosive action at a volcanic vent is very great, and only fragmental ejections take place, composed of the frothy scum of the lava produced by the escape of gases and vapours from its midst. But in other cases the amount of explosive action may be small, and great volumes of igneous materials may issue as lava-streams. In such cases, only small scoriæ-cones would be formed around the vents, and one half of such cones is commonly swept away by the efflux of the lava-currents, while the remainder may be easily removed by denuding action or be buried under the lava-currents issuing from other vents in the neighbourhood. Thus it may easily come to pass that what a superficial observer takes for an enormous mass of basaltic lava poured out from a great fissure at a single effort, may prove upon careful observation to be made up of innumerable lava-currents, each of which is of moderate dimensions; and it may further be found that these lava-currents, instead of being the product of a single paroxysmal effort from one great fissure, have been accumulated by numerous small outbursts taking place at wide intervals, from a great number of minor orifices.