Volcanoes: What They are and What They Teach

by John W. Judd

The notion of the aqueous origin of basalt, which was so long maintained by the school of Werner, has now been entirely abandoned, and the so-called 'trap-rocks' are at the present day recognised as being as truly volcanic in their origin as the lavas of Etna and Vesuvius.

There is, however, a vestige of this doctrine of Werner, which still maintains its ground with obstinate persistence. Many geologists in Germany who admit that volcanic phenomena, similar to those which are going on at the present day, must have occurred during the Tertiary and the later Secondary periods, nevertheless insist that among the earlier records of the world's history we find no evidence whatever of such volcanic action having taken place. By the geologists who hold these views it is asserted that while the granites and other plutonic rocks were formed during the earlier periods of the world's history, true volcanic products are only known in connection with the sediment of the later geological periods.

Some geologists have gone farther even than this, and asserted that each of the great geological periods is characterised by the nature of the igneous ejections which have taken place in it. They declare that granite was formed only during the earliest geological periods, and that at later dates the gabbros, diabases, porphyries, dolerites and basalts, successively made their appearance, and finally that the modern lavas were poured out.

A little consideration will suffice to convince us that these conclusions are not based upon any good evidence. The plutonic rocks, as we have already seen, exhibit sufficient proofs in their highly crystalline character, and in their cavities containing water, liquefied carbonic acid, and other volatile substances, that they must have been formed by the very slow consolidation of igneous materials under enormous pressure. Such pressures, it is evident, could only exist at great depths beneath the earth's surface. Mr. Sorby and others have endeavoured to calculate what was the actual thickness of rock under which certain granites must have been formed, by measuring the amount of contraction in the liquids which have been imprisoned in the crystals of these rocks. The conclusions arrived at are of a sufficiently startling character. It is inferred that the granites which have been thus examined must have consolidated at depths varying from 30,000 to 80,000 feet beneath the earth's surface. It is true that in arriving at these results certain assumptions have to be made, and to these exception may be taken, but the general conclusion that granitic rocks could only have been formed under such high pressures as exist at great depths beneath the surface, appears to be one which is not open to reasonable doubt.

If, then, granites and similar rocks were formed at the depth of some miles, it is evident that they can only have made their appearance at the surface by the removal of the vast thickness of overlying rocks; and the sole agency which we know of that is capable of effecting the removal of such enormous quantities of rock-materials, is denudation. But the agents of denudation—rain and frost, rivers and glaciers, and sea-waves—though producing grand results, yet work exceeding slowly; and almost inconceivably long periods of time must have elapsed before masses of rock several miles in thickness could have been removed, and the subjacent granites and other highly crystalline rocks have been exposed at the surface.

ANCIENT AND MODERN VOLCANIC ROCKS.

It is an admitted fact that among the older geological formations, we much more frequently find intrusions of granitic rocks than in the case of younger ones. It is equally true that among the sediments formed during the most recent geological periods, no true granitic rocks have been detected. But if, as we insist is the case, granitic rocks can only be formed at a great depth from the surface, the £acts we have described are only just what we might expect to present themselves under the circumstances. The older a mass of granitic rock, the greater chance there is that the denuding forces operating upon the overlying masses, will have had an opportunity of so far removing the latter as to expose the underlying crystalline rocks at the surface. And, on the other hand, the younger crystalline rocks are still, for the most part, buried under such enormous thicknesses of superincumbent materials that it is hopeless for us to search for them. Nevertheless, it does occasionally happen that, where the work of denudation has been exceptionally rapid in its action, such crystalline rocks formed during a comparatively recent geological period, are exposed at the surface. This is the case in the Western Isles of Scotland and in the Pyrenees, where masses of granite and other highly crystalline rocks are found which were evidently formed during the Tertiary period.

The granites which were formed in Tertiary times present no essential points of difference from those which had their origin during the earlier periods of the earth's history. The former, like the latter, consist of a mass of crystals with no imperfectly crystalline base or groundmass between them; and these crystals include numerous cavities containing liquids.

Between the granites and the quartz-felsites every possible gradation may be found, so that it is impossible to say where the one group ends and the other begins; indeed, many of the rocks called 'granite-porphyries' have about equal claims to be placed in either class. Nor is the distinction between the quartz-felsites and rhyolites any more strongly marked than that between the former class of rocks and the granites; some of the more crystalline rhyolites of Hungary being quite undistinguishable, in their chemical composition, their mineralogical constitution, and their microscopic characters, from the quartz-felsites. The more crystalline rhyolites are in turn found passing by insensible gradations into the glassy varieties and finally into obsidian.

RELATIONS BETWEEN GRANITE AND PUMICE.