John W. Judd

“ It has been sometimes argued that as all materials under intense pressure appear to yield to an appreciable extent, and to allow their particles to be packed into a smaller compass, we may find in this fact an explanation of the great density of the internal parts of the globe. It has in fact been suggested that under the enormous pressure which must be exerted by masses of rock several thousand feet in thickness, the materials of which our earth is composed may be compelled to pack themselves into less than one-half the compass which they occupy at the surface. ”

“ The steam frequently escapes from lava with such violence that the froth or scum on its surface is broken up and scattered in all directions, as the foam crests of waves are dispersed by the wind during a storm. ”

“ WHAT VOLCANOES TEACH US CONCERNING THE NATURE OF THE EARTH'S INTERIOR. In entering upon any speculations or enquiries concerning the nature of the interior of our globe, it is necessary before all things that we should clearly realise in our minds how small and almost infinitesimal is that part of the earth's mass which can be subjected to direct examination. The distance from the surface to the centre of our globe is nearly 4,000 miles, but the deepest mines do not penetrate to much more than half a mile from the surface, and the deepest borings fall far short of a mile in depth. ”

“ We have good grounds for believing that the temperature of our globe is continually diminishing by its radiation of heat into space. This cooling of our globe is attended by contraction, which results in movements of portions of its crust. It may at first sight appear that such movements would be so small and insignificant - 347 - as to be quite unworthy of notice. But if we take into account the vast size of our earth it will be seen that the movements of such enormous masses may be attended with the most wonderful results. ”

“ When we have clearly realised the grand effects produced on the surface of the globe by these external forces, through the action of its investing atmosphere and circulating waters, then, and only then, shall we be in a position to estimate the far greater effects resulting from the internal forces, of which the most striking, but not the most important, results are seen in the production of volcanic eruptions and earthquake-shocks. ”

“ Hypothesis is, as we have seen, one of the legitimate and necessary operations in scientific investigation. It only becomes a dangerous and treacherous weapon when it is made to precede rather than to follow observation and experiment, or when being regarded with paternal indulgence, an attempt is made to shield it from the relentless logic of facts. Good and bad hypotheses must be allowed to 'grow together till the harvest;' such as are unable to accommodate themselves to the surrounding conditions imposed by newly-discovered facts and freshly-established laws will assuredly perish ”

“ The earth's form, which mathematicians have shown to be exactly that which would be acquired by a globe composed of yielding materials rotating on its axis at the rate which our planet does, has often been adduced as proving that the latter was not always in a rigid and unyielding condition. In the same way, all the remarkable facts and relations of the bodies of the solar system, which have been shown by astronomers to lend such support to the nebular hypothesis, have been thought, at the same time, to favour the view that our earth is still in a condition of uncompleted solidification. ”

“ What is usually taken for flame during volcanic eruptions is simply, as we have already pointed out, the glowing red-hot surface of a mass of molten rock, reflected from a vapour-cloud hanging over it. ”

“ While, on the one hand, we are led by recent geological investigations to reject the notions which were formerly accepted, by which mountain-ranges were supposed to be suddenly and violently upheaved by volcanic forces, we are, on the other hand, driven to conclude that without the action of these subterranean forces, the irregularities which are exhibited on the earth's surface could not have had any existence. ”

“ If we wish to examine the mode of working of a complicated steam-engine, it will be of little avail for us to watch the machinery when the full blast of steam is turned on, and the rapid movements of levers, pinions, and slides baffle all attempts to follow them, and render hopeless every effort to trace their connection with one another. ”

“ Let us now draw near and examine this wonderful phenomenon of a mountain which seemingly ever burns with fire, and yet is not consumed. The general form of the Island of Stromboli will be gathered from an inspection of the plan, fig. 2, which is copied from a map published by the Italian Government. When we land upon the island, we find that it is entirely built up of such materials as we know to be ejected - 12 - from volcanoes; indeed, it resembles on a gigantic scale the surroundings of an iron furnace, with its heaps of cinders and masses of slag. ”

“ If we examine the freshly-broken surface of a piece of lava, we shall, in most cases, find that it contains a great number of those regular-shaped bodies which we call crystals; in some cases these crystals are so small as to be scarcely visible to the naked eye, in others they may be an inch or more in length. Most lavas are thus seen to be largely made up of crystals of different minerals. The minerals which are usually contained in lavas are quartz, the various kinds of felspar, augite, hornblende, the different kinds of mica, olivine, and magnetite. ”

“ We may speak of such rock-masses, retained in a solid condition by intense pressure, at a temperature far above their fusing point at the earth's surface, as being in a 'potentially liquid condition.' Upon any relief of pressure such masses would at once assume the liquid state, just as the superheated water in a Papin's digester immediately flashes into steam upon the fracture of the strong vessel by which it is confined. ”

“ And it is only by such studies as these that we at last learn to regard the earthquake and volcanic phenomena of our globe, not as the grandest and most important effects of these forces, but as their secondary and accidental accompaniments. 'Volcanoes,' it has been said, 'are the safety-valves of the globe;' and when we come to realise the real extent and nature of the internal forces ceaselessly working in the earth's crust we shall scarcely be disposed to regard the simile as an overstrained one. ”

“ By some other theorists, who have felt the full force of this last objection, an attempt has been made to explain the phenomena of volcanoes by supposing that the globe is solid from its surface to its centre, but that the internal portions of the globe are at such a high temperature that they are only retained in a solid condition by the enormous pressure to which they are subjected. ”

“ A hundred years ago the primary and secondary planets of the solar system could be almost numbered upon the fingers; now we recognise the fact that they exist in countless millions, presenting every variety of bulk from masses 1,400 times as large as our earth down to the merest planetary dust. Between the orbits of Mars and Jupiter, more than 200 small planets have been recognised as occurring, and every year additions are made to the number of these asteroids. Comets have now been identified with streams of such planetary bodies, of minute size, moving in regular orbits through our system. ”

“ The sun is of far greater dimensions than our earth, and is still in great part, if not entirely, in a gaseous condition. The great movements in the outer envelopes of the sun exhibited in the 'sun-spots' and 'solar prominences,' recall to the mind the phenomena of volcanic activity upon our globe. ”

“ 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. ”

“ The red glow observed over a volcano in eruption is indeed precisely similar in its nature and origin to that which is seen above London during a night of heavy fog, and which is produced by the reflection of the gas-lights of the city from the innumerable particles of water-vapour diffused through the atmosphere. Fires, of course, occur when the molten and incandescent materials poured out from a volcano come in contact with inflammable substances, such as - 18 - forests and houses, but in these cases the combustion is quite a secondary phenomenon. ”

“ Thus it has been found that at Vesuvius the lavas erupted in modern times have all a striking similarity to one another in chemical composition, in the minerals which they contain, and in their structure. They are all basic lavas, which when examined by the microscope are seen to consist of a more or less glassy magma, in the midst of which numerous crystals of augite, leucite, olivine, magnetite, and other minerals are scattered. ”

“ 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. ”

“ Around this column of vapour the most vivid lightning constantly plays and adds not a little to the grand and awful character of the spectacle of a volcanic eruption, especially when it is viewed by night. ”

“ So far as observation has yet gone we have reason for believing that not only the members of the solar system, but the more distant bodies of the universe, are all composed of the same elementary substances as those which enter into the composition of our globe. ”