Volcanoes: What They are and What They Teach

by John W. Judd

All these facts point to one conclusion—namely, that during all past geological periods, materials similar to those which are now being extruded from volcanic vents were poured out on the earth's surface by analogous agencies. If we could trace the lava-streams of the present day down to the great subterranean reservoirs from which their materials have been derived, we should doubtless find that at gradually increasing depths, where the pressure would be greater and the escape of heat from the mass slower, the rocky materials would by degrees assume more and more crystalline characters. We should thus find obsidian or rhyolite insensibly passing into quartz-felsite and finally into granite; trachyte passing into orthoclase-porphyry and syenite; and basalt passing into dolerite, augite-porphyry, and gabbro.

On the other hand, if we could replace the great masses of stratified rocks which must once have overlain the granites, syenites, diorites, and gabbros, we should find that, as we approached the original surface, these igneous materials would gradually lose their crystalline characters, and when they were poured out at the surface would take the forms of rhyolite, trachyte, andesite, and basalt—all of which might occasionally assume the glassy forms known as obsidian or tachylyte.

ALTERED FORMS OF ANCIENT LAVAS.

But while we insist on the essential points of similarity between the lavas poured out upon the surface of the earth during earlier geological periods and those which are being extruded at the present day, we must not forget that by the action of percolating water and acid gases, the mineral constitution, the structure, and sometimes even the chemical composition of these ancient lavas may undergo a vast amount of change. In not a few cases these changes in the characters of a lava may be carried so far that the altered rock bears but little resemblance to the lava from which it was formed, and it may be found desirable to give it a new name. Among the rocks of aqueous origin we find similar differences in the materials deposited at different geological periods. Clay, shale and clay-slate have the same composition, and the two latter are evidently only altered forms of the first mentioned, yet so great is the difference in their characters that it is not only allowable, but desirable, to give them distinctive names.

In the same way, among the deposits of the earlier geological periods we find rocks which were doubtless originally basalts, but in which great alterations have been produced by the percolation of water through the mass. The original rock has consisted of crystals of felspar, augite, olivine, and magnetite distributed through a glassy base. But the chemical action of water and carbonic acid may have affected all the ingredients of the rock. The outward form of the felspar crystals may be retained while their substance is changed to kaolinite, various zeolites, and other minerals; the olivine maybe altered to serpentine and other analogous minerals; the magnetite changed to hydrous peroxide of iron; the augite may be changed to uralite or hornblende; and the surrounding glassy mass more or less devitrified and decomposed. The hard, dense, and black rock known as basalt has under these circumstances become a much softer, earthy-looking mass of a reddish-brown tint, and its difference from basalt is so marked that geologists have agreed to call it by another name, that of 'melaphyre.' Even in their ultimate chemical compositions the 'melaphyres' differ to some extent from the basalts, for some of the materials of the latter may have been removed in solution, and water, oxygen, and carbonic acid have been introduced to combine with the remaining ingredients.

But if we carefully study, by the aid of the microscope, a large series of basalts and melaphyres, we shall find that many rocks of the former class show the first incipient traces of those changes which would reduce them to the latter class. Indeed, it is quite easy to form a perfect series from quite unaltered basalts to the most completely changed melaphyres. Hence we are justified in concluding that all the melaphyres were originally basalts, just as we infer that all oaks were once acorns.

Now changes, similar to those which we have seen to take place in the case of basaltic lavas, are exhibited by the lavas of every other class, which have been exposed to the influence of the same agencies,—namely, the passage of water and acid gases. But inasmuch as the minerals composing the basic lavas are for the most part much more easily affected by such agencies than are the minerals of acid lavas, the ancient basic rocks are usually found in a much more highly altered condition than are the acid rocks of equivalent age.

NAMES GIVEN TO ALTERED LAVAS.

We thus see that each of the classes of modern lavas has its representative in earlier geological periods, in the form of rocks which have evidently been derived from these lavas, through alterations effected by the agency of water and acid-gases that have permeated their mass. Thus, while the basalts are represented among the ancient geological formation by the melaphyres, the andesites are represented by the porphyrites, and the trachytes and rhyolites by different varieties of felstones. And, as we can form perfect series illustrating the gradual change from basalt to melaphyre, so we can arrange other series demonstrating the passage of andesites into porphyrites, and of trachytes and rhyolites into felsites.

It must be remembered, however, that these changes do not take place in anything like determinate periods of time. Occasionally we may find lavas of ancient date which have undergone surprisingly little alteration, and in other cases there occur lavas belonging to a comparatively recent period which exhibit very marked signs of change.