Volcanoes: What They are and What They Teach

by John W. Judd

A piece of granite and a piece of pumice may at first sight appear to present so many points of difference, that it would seem quite futile to attempt to discover any connection between them. Yet, if we analyse the two substances, we may find that in ultimate chemical composition they are absolutely identical. There is nothing irrational, therefore, in the conclusion that the same materials under different conditions may assume either the characters of granite on the one hand, or of pumice on the other; the former being consolidated under circumstances in which the chemical and crystalline forces have had the freest play and have used up the whole of the materials to form crystallised minerals, while the latter has cooled down and solidified rapidly at the surface, in such a way that only incipient crystallisation has occurred, and the glassy mass has been reduced to a frothy condition by the escape of steam-bubbles from its midst This conclusion receives the strongest support from the fact that examples of every stage of the change, between the glassy condition of pumice and the crystalline condition of granite, may be detected among the materials of which the globe is built up.

There is still another class of facts which may be adduced in support of the same conclusion. Many lavas, as we have seen, contain crystals of much larger dimensions than those constituting the mass of the rock, which is then said to be 'porphyritic' in structure. The porphyritically embedded crystals, when carefully examined, are often seen to be broken and injured, and to exhibit rounded edges, with other indications of having undergone transport. When examined microscopically, too, they often present the cavities containing liquids which distinguish the crystals of plutonic rocks. All the facts connected with these porphyritic lavas point to the conclusion that while the crystals in their groundmass have separated from the liquefied materials near the surface, the large embedded crystal, have been floated up from great depths within the earth's crust, where they had been originally formed.

GRANITIC REPRESENTATIVES OF OTHER LAVAS.

The careful consideration of all the facts of the case leads to the conclusion that where pumice, obsidian, and rhyolite are now being ejected at the surface, the materials which form these substances are, at various depths in the earth's interior, slowly consolidating in the form of quartz-felsite, granite-porphyry and granite. It may be that we can nowhere point to the example of a mass of rock which can be traced from subterranean regions to the surface, and is, under such conditions, actually seen to pass from the dense and crystalline condition of granite to the vesicular and glassy form of pumice; but great granitic masses often exhibit a more coarsely crystalline condition in their interior, and the offshoots and dykes which they give off not infrequently assume the form of quartz-felsite; while, on the other hand, the more slowly consolidated rocks found in the interior of some rhyolite masses are not distinguishable in any way from some of the true quartz-felsites.

That which is true of the lavas of acid composition is equally true of the lavas of intermediate and basic character. The andesites, the trachytes, the phonolites, and the basalts have all their exact representatives among the plutonic rocks, and these have a perfectly crystalline or granitic structure. The plutonic and the volcanic representatives of each of these groups are identical in their chemical composition, and numerous intermediate gradations can be found between the most completely granitic and the most perfectly vitreous or glassy types. In illustration of this fact, we may again refer to the series of microscopic sections of rocks given in the frontispiece.

Another objection to the conclusion that the volcanic products of earlier periods of the earth's history were identical in character with those which are being ejected at the present day is based on the fact of the supposed non-existence of the scoriaceous and glassy materials which abound in the neighbourhood of the active volcanic vents. Where, it is asked, do we find among the older rocks of the globe the heaps of lapilli, dust, and scoriæ, with the glassy and pumiceous rocks that now occur so abundantly in all volcanic districts?

In reply to this objection, we may point out that these accumulations of loose materials are of such a nature as to be capable of easy removal by denuding agents, and that as they are formed upon the land they will, if not already washed away by the action of rain, floods, rivers, &c., run great risk of having their materials distributed, when the land sinks beneath the waters of the ocean and the surface is covered by new deposits. With respect to the glassy rocks it must be remembered that the action of water, containing carbonic acid and other substances, in percolating through such masses has a tendency to set up crystalline action, and these glassy rocks easily undergo 'devitrification'; it would therefore be illogical for us to expect glassy rock-masses to retain their vitreous character through long geological periods, during which they have been subjected to the action of water and acid gases.

But careful observation has shown that the scoriaceous and vitreous rocks are by no means absent among the igneous materials ejected during earlier periods of the earth's history. Their comparative infrequency is easily accounted for when we remember, in the first place, the ease with which such materials would be removed by denuding forces, and in the second place, the tendency of the action of percolating water to destroy their characteristic features, by filling up their vesicles with crystalline products and by effecting devitrification in their mass.

SIMILARITY OF ANCIENT AND RECENT LAVAS.

If we go back to the very oldest known rock-masses of the globe, those which are found underlying the fossiliferous Cambrian strata, we find abundant evidence that volcanic action took place during the period in which these materials were being accumulated. Thus, in the Wrekin, as Mr. Allport has so well shown, we find clear proofs that before the long-distant period of the Cambrian, there existed volcanoes which ejected scoriæ, lapilli, and volcanic dust, and also gave rise to streams of lava exhibiting the characteristic structures found in glassy rocks. In these rocks, which have undergone a curious alteration or devitrification, we still find all those peculiar structures—the sphærulitic, the perlitic, and the banded—so common in the rhyolites of Hungary, with which rocks the Wrekin lavas, in their chemical composition, precisely agree. Prof. Bonney, too, has shown that the rocks of Charnwood Forest, which are also probably of pre-Cambrian age, contain great quantities of altered volcanic agglomerates, tuffs, and ashes. I have found the sphærulitic, perlitic, and banded structures exhibited by British lavas of the Cambrian, Silurian, Devonian and Carboniferous periods, as well as in those of Tertiary age; and in connection with these different lavas we find vast accumulations, sometimes thousands of feet in thickness, of volcanic agglomerates and tuffs which have undergone great alteration.