When the water issues at a temperature little removed from the boiling point, it is apt to give rise to intermittent springs or geysers, the eruptions of which exhibit a remarkable analogy with those of ordinary volcanoes. Geysers may indeed be described as volcanoes in which heated water, instead of molten rock, is forced out from the vent by the escaping steam. They occur in great abundance in districts in which the subterranean action is becoming dormant or extinct, such as Iceland, the North Island of New Zealand, and the district of the National Park in the Rocky Mountains.

Many attempts have been made to explain the exact mechanism by which the intermittent action of geysers is produced, but it is not at all probable that any one such explanation will cover all the varied phenomena exhibited by them. Like volcanic outbursts, geyser eruptions doubtless originate in the escape of bubbles of steam through a liquid mass, and this liberation of steam follows any relief of pressure. In districts where vast masses of lava are slowly cooling down from a state of incandescence, and surface waters are finding their way downwards while subterranean waters are finding their way upwards, there can be no lack of the necessary conditions for such outbursts. Sometimes the eruptions of geysers take place at short and regular intervals, at other times they occur at wide and irregular intervals of time. In some cases the outbursts take place spontaneously, and at others the action can be hastened by choking up the vent with stones or earth.

Other hot springs, like the Strudel of Carlsbad, rise above the surface in a constant jet, while most of them issue quietly and flow like ordinary springs.

Although the violent and paroxysmal outbursts of volcanic mountains arrest the attention, and powerfully impress us with a sense of the volcanic activity going on beneath the earth's surface, yet it may well be doubted whether the quantity of heat, which the earth gets rid of by their means, at all approaches in amount that which is quietly dissipated by means of the numerous 'stufas,' gaseous exhalations, and thermal springs which occur in such abundance all over its surface. For while the former are intermittent in their action, and powerful outbursts are interrupted by long periods of rest, the action of the latter, though feeble, is usually continuous.

Most people may regard the hot spring of Bath as a very slight manifestation of volcanic activity. This spring issues at a constant temperature of 49° C, or 120° Fahr. As, however, no less than 180,000 gallons of water issue daily from this source, we may well understand how great is the amount of heat of which the earth's crust is relieved by its agency. It may indeed be doubted whether its action in this way is not at least equal to that of a considerable volcano which, though so much more violent, is intermittent in its action.

Nor are thermal springs by any means ineffective agents in bringing materials from the interior of the earth's crust and depositing it at the surface. The Bath spring contains various saline substances, principally sulphates and chlorides, in solution in its waters. These are quietly carried by rivers to the sea, and are lost to our view. The spring has certainly maintained its present condition since the time of the Romans, and I find that if the solid materials brought from the interior of the earth during the last 2,000 years had been collected, they would form a solid cone equal in height to Monte Nuovo. Yet we usually regard the Campi Phlegræi as a powerfully-active volcanic district, and the subterranean action in our own country as quite unworthy of notice.

When we remember the fact that on the continent of Europe the hot and saline springs may be numbered by thousands, and that they especially abound in districts like Hungary, the Auvergne, the Rhine provinces, and Central Italy, where volcanic action has recently become extinct, we shall be able to form some slight idea of the work performed by these agents, not only in relieving the earth's crust of its superfluous heat, but in transporting materials in a state of solution from the interior of that crust and depositing them at the surface. The vast deposits of siliceous sinter and of travertine also bear witness to the effects produced by hot and mineral springs.

Nor is the work of these springs confined to the surface. Mr. John Arthur Phillips has shown that metallic gold and the sulphide of quicksilver (cinnabar) have been deposited with the silica and other minerals formed on the sides of a fissure from which hot springs issue at the surface. There cannot be any doubt that the metallic veins or lodes, which are the repositories of most of the metals employed in the arts, have been formed in cracks connected with great volcanic foci, the transfer of the various sulphides, oxides, and salts which fill the vein having been effected either by solution, sublimation, or the action of powerful currents of steam.

As the igneous activity of the district declines, the temperature of the issuing gases and waters diminishes with it, until at last the volcanic forces appear to wholly abandon that region and to be transferred to another.

Yet even after all or nearly all indications of the volcanic agencies cease to make themselves visible at the surface, occasional tremblings of the earth's crust show that perfect equilibrium has not been restored below, but that movements are taking place which result in shocks that are transmitted through the overlying and surrounding rock-masses as earthquake vibrations.

Such is the cycle of changes which appears to take place at each district of the earth's surface, as it successively becomes the scene of volcanic activity.

The invasion of any particular area of the earth's surface by the volcanic forces appears to be heralded by subterranean shocks causing earthquake vibrations. Presently the origination of fissures is indicated by the rise of saline and thermal springs, and the issuing of carbonic acid and other gases at the surface. As the subterranean activity becomes more pronounced, the temperature of the springs and emitted gases is found to increase, and at last a visible rent is formed at the surface, exposing the incandescent materials below.

From this open fissure which has thus been formed, the gas and vapours imprisoned in the incandescent rock-materials escape with such violence as to disperse the latter in scoriæ and dust, or to cause them to well out in great streams as lava-flows. Usually the action becomes concentrated at one or several points at which the ejected materials accumulate to form volcanic cones.

Sometimes the volcanic activity dies away entirely after these cones are thrown up along the line of fissure, but at others some such centre becomes for a longer or shorter time the habitual vent for the volcanic forces in the district, and by repeated ejections of lavas and fragmentary materials at longer or shorter intervals the cone increases both in height and bulk.

When the height of the cone has grown to a certain extent, it becomes more easy for the volcanic energies below to rend the sides of the cone than to raise the molten materials to its summit. In this way lateral or parasitic cones are thrown up on the flanks of the volcanic mountain, the mass being alternately elevated and strengthened by the ejections from the summit and sides respectively.

When the volcanic energies no longer suffice to raise the fluid materials to the summit, nor to rend the sides of the volcano, fissures with small cones may be formed in the plains around the great central volcano.

At last, however, this energy diminishes so far that rock materials can no longer be forced to the surface, the fissures become sealed up by consolidating lava, and the volcanic cones fall into a condition of extinction and decay.

The existence of heated materials at no great depth from the surface is indicated by the outburst of gases and vapours, the formation of geysers, mud-volcanoes, and ordinary thermal springs. But as the underlying rocks cool down, the issuing jets of gas and vapour lose their high temperature and diminish in quantity, the geysers and mud-volcanoes become extinct, and the thermal springs lose their peculiar character or disappear, and thus all manifestations of the igneous energies in the district gradually die away.

Such a cycle of changes probably requires many hundreds of thousands, or even many millions, of years for its accomplishment; but by the study of volcanoes in every stage of their growth and decline we are able to reconstruct even the minutest details of their history.