Volcanoes: What They are and What They Teach

by John W. Judd

CHAPTER IX.
VOLCANIC ACTION AT DIFFERENT PERIODS OF THE EARTH'S HISTORY.

It is only in comparatively recent times that the important doctrine of geological continuity has come to be generally accepted, as furnishing us with a complete and satisfactory explanation of the mode of origin of the features of our globe. The great forces, which are ever at work producing modifications in those features, operate so silently and slowly, though withal so surely, that without the closest and most attentive observation their effects may be easily overlooked; while, on the other hand, there are so many phenomena upon our globe which seem at first sight to bear testimony to the action of sudden and catastrophic forces, very different to any which appear to be at present at work, that the tendency to account for all past changes by these violent actions is a very strong one. In spite of this tendency, however, the real potency of the forces now at work upon the earth's crust has gradually made its way to recognition, and the capability of these forces, when their effects are accumulated through sufficiently long periods of time, to bring about the grandest changes, is now almost universally admitted. The modern science of geology is based upon the principle that the history of the formation and development of the earth's surface-features, and of the organisms upon it, has been continuous during enormous periods of time, and that in the study of the operations taking place upon the earth at the present day, we may find the true key to the changes which have occurred during former periods.

In no branch of geological science has the doctrine of continuity had to encounter so much opposition and misconception as in that which relates to the volcanic phenomena of the globe. For a long time students of rocks utterly failed to recognise any relation between the materials which have been ejected from active volcanic vents and those which have been formed by similar agencies at earlier periods of the earth's history. And what was far worse, the subject became removed from the sphere of practical scientific inquiry to that of theological controversy, those who maintained the volcanic origin of some of the older rocks being branded as the worst of heretics.

CONTROVERSY CONCERNING ORIGIN OF BASALT.

With the theological aspects of the great controversy concerning the origin of basalt and similar rocks—a controversy which was carried on with such violence and acrimony during the latter half of the eighteenth century—we have here nothing to do. But it may not be uninstructive to notice the causes of the strange misconceptions which for so long a period stood in the way of the acceptance of rational views upon the subject.

At this period but little had been done in studying the chemical characters of aqueous and igneous rock-masses respectively; and while, on the one hand, the close similarity in chemical composition between the ancient basalts and many modern lavas was not recognised, the marked distinction between the composition of such materials and most aqueous sediments remained, on the other hand, equally unknown. Nor had anything been yet accomplished in the direction of the study of rock-masses by the aid of the microscope. Hence there could be no appeal to those numerous structural peculiarities that at once enable us to distinguish the most crystalline aqueous rocks from the materials of igneous origin.

On the other hand, there undoubtedly exist rocks of a black colour and crystalline structure, sometimes presenting a striking similarity in general appearance to the basalts, which contain fossils and are undoubtedly of aqueous origin. Thus on the shore near Portrush, in the North of Ireland, and in the skerries which lie off that coast, there occur great rock-masses, some of which undoubtedly agree with basalt in all their characters, while others are dark-coloured and crystalline, and are frequently crowded with Ammonites and other fossils. We now know that the explanation of these facts is as follows. Near where the town of Portrush is now situated, a volcanic vent was opened in Miocene times through rocks of Lias shale. From this igneous centre, sheets and dykes of basaltic lava were given off, and in consequence of their contact with these masses of lava, the Lias shales were baked and altered, and assumed a crystalline character, though the traces of the fossils contained in them were not altogether obliterated. In the last century the methods which had been devised for the discrimination of rocks were so imperfect that no distinction was recognised between the true basalt and the altered shale, and specimens of the latter containing Ammonites found their way to almost every museum in Europe, and were used as illustrations of the 'origin of basalt by aqueous precipitation.'

Another source of the widely-spread error which prevailed concerning the origin of basalt, was the failure to recognise the nature of the alterations which take place in the character of rock-masses in consequence of the passage through them, during enormous periods of time, of water containing carbonic acid and other active chemical agents. The casual observer does not recognise the resemblance which exists between certain ornamental marbles and the loose accumulations of shells and corals which form many sea-beaches; but close examination shows that the former consist of the same materials as the latter, bound together by a crystalline infilling of carbonate of lime, which has been deposited in all the cavities and interstices of the mass. In the same way, as we have already seen, the vesicles and interstices of heaps of scoriæ may, by the percolation of water through the mass, become so filled with various crystalline substances, that its original characters are entirely masked.

But the progress of chemical and microscopic research has effectually removed these sources of error. Many rocks of aqueous origin, formerly confounded with the basalts, have now been relegated to their proper places among the different classes of rocks; while, on the other hand, it has been shown that the chemical and physical differences between the ancient basalts and the modern basic lavas are slight and accidental, and their resemblances are of the closest and most fundamental character.

VOLCANIC ORIGIN OF 'TRAP ROCKS.'