The sudden swelling of an extensive plain in Mexico and the formation of volcanic mountains, varying, from three to sixteen hundred feet in elevation, where no volcanos had before existed, seems scarcely credible; yet this has happened in modern times and is established on unequivocal testimony: while it is a fact in history that in 1538, a hill four hundred and forty feet high (Monte Nouvo, near Naples) was thrown up in twentyfour hours. The latter instance is near that active vent, Vesuvius; but in Peru, where only one volcano in the whole country is known, great changes have been effected in the surface by internal violence, and scarcely a week passes without an earthquake. Hot springs are seldom absent in volcanic countries, and in the regions of extinct volcanos they are generally abundant.

Various hypotheses have been suggested to account for the phenomena of volcanos. Heat, it is well known, is merely an intense chemical action, which may be excited in matter in a variety of ways, independent of actual contact with ignited substances-by electricity, rapid oxidation, friction, and other processes. The brilliant discovery of Sir Humphry Davy of the metallic bases of the earths and alkalis, which has proved that the greater part of the substances constituting the earth, consist of certain delicate metals, united with the all-pervading principle oxygen, has been suggested as affording a clue to the process by which internal heat is generated. These metallic substances, when separated from their kindred element, are so eager to re-unite with it, that the moment oxygen is placed

* Jorullo, in 1750.

within their reach, they seize it, as it were, with incredible violence, and light and heat of the most intense description attend their embrace. * Now, although

potassium, sodium, calcium, and the other metallic bases of the earths, are never found on the exterior of the globe uncombined with oxygen, it was supposed by Sir Humphry Davy, that at great depths in the interior, far removed from the action of the atmosphere, they might exist in a separate state: in which case, the accidental admission of oxygen by the percolation of water, would be attended with the production of that intense heat, whose expansive force we witness in the volcanic energy. He also suggested, that the proximity of volcanos to the sea, seemed to indicate that its waters were concerned in producing the effect: and a careful analysis of the vapours, the sublimations, and the other products of Vesuvius, seemed to confirm his conjecture. It is clear that water is present, from the immense quantity of aqueous vapour, or steam, constantly evolved, but the mere contact of water with the incandescent mass would be sufficient to account for this, without supposing that the heat is generated and maintained by its decomposition. Indeed, if the constant decomposition of water were going on in the manner supposed, water being composed of eight parts of oxygen united with one of hydrogen, when the oxygen is absorbed, an immense volume of hydrogen would be set free, and ought to be evolved either in flame or gas from the crater, in much greater quantities than it is found to be. That the vicinity of

It is this principle by which the intense light of the oxyhydrogen microscope is produced.

the ocean is not essential, is proved: for although Ætna and Vesuvius, and Stromboli and Teneriffe, rise near the ocean, there are extensive volcanic regions, comprising an area of two thousand five hundred square miles, in the heart of Asia, at a distance of more than a thousand miles from the nearest sea; and in the central chain of the Andes, in South America, there are raging volcanos, far removed from the coast. To whatever cause, however, they may owe their origin-whether to chemical decomposition, or to a permanent central heat-the fact is certain, that enormous furnaces, in a state of frightful ignition, exist at intervals, in every part of the earth, at no great distance from its surface, whose action is slowly, but constantly, altering the physical features of the globe; fusing and ejecting from beneath the hardest materials, upheaving mountains on the existing land, and raising islands from the depth of the ocean. Indeed, so extensive is the dominion of this powerful agent, that one is inclined to wonder with the elder Pliny-a great Roman naturalist, who lost his life in watching the remarkable eruption of Vesuvius in the year 79—not at the frequency and magnitude of these awful phenomena, but that “a single day should pass without an explosion."

Many of these furnaces have gone out at different periods of the earth's history. At an era, historically remote but geologically recent, Europe was illuminated

from one end to the other with volcanic fires, compared with which, those of Etna and Vesuvius in modern times, are feeble coruscations. An examination of these deserted forges of Vulcan in Italy, the South of France, and Germany, has presented us with some of the most

interesting results of geological enquiry; and we may here witness all the effects of volcanic phenomena without their attendant danger and inconvenience. They belong, however, to a period antecedent to what we have regarded as contemporary times, the events of which we are considering.

*

An active agent of change in the structure of the earth's surface, constantly but quietly at work, is found in the organic creation. Rock-formations of vast extent, are in progress in many parts of the ocean, effected by the labours of the well-known coral animals, or animal plants as they are termed, from the circumstance of their branch-like habitations, always being found springing from rocks. These polyparia, in com mon with the other shelly inhabitants of the ocean have the extraordinary property of secreting (from whence derived is not known) an enormous quantity of carbonate of lime in the construction of their dwellings. Affixing themselves to the submarine mountains or elevated portions of the bed of the ocean, they spread their myriad arms, and rapidly build up the ramified substance, which is in this country admired for its beautiful forms. In their progress upwards one generation builds upon the ruined and deserted habitations of another, calcareous sand and other cementing matter furnished by the ocean, is mixed with the mass, and the whole becomes a consolidated limestone; which, as it emerges from the water, decomposes and becomes eventually the abode of vegetation, birds make it their resort, animals accidentally transported by the waves find in it a refuge

* Zoophytes.

from a watery grave, and man at last finds his way thither, erects his habitation, cultivates the decomposing soil, now enriched with nutritive matter and adorned with vegetable productions, and calls himself "lord" of this new creation. This creative process is going on to an incredible extent in various parts of the world. Reefs, as these newly-built islands are called, extending many hundred miles, are forming in the tropical regions of the Pacific. The Indian ocean teems with this world-building population, and its insidious encroachments are fast filling up the Arabian gulph.

Springs, although upon a small scale, are actively engaged in adding to the mineral masses in the earth's superstructure. All waters issuing from beneath the surface of the ground, contain more or less of earthy matter in solution, and it is to these extraneous ingredients that the "crystal spring" owes its clearness and agreeable taste. Lime is the predominating substance in mineral waters, in which it is held in solution by means of carbonic acid-and where this element is abundant as in the case of thermal, or hot springs, the water is sometimes saturated with the carbonate. The carbonic acid being, in these cases, withdrawn by the abstraction of heat, and other circumstances, a large deposit of limestone takes place, as in the Solfatara, and on the banks of the Anio, at Tivoli, and in many other situations in the neighbourhood of Rome: the principal buildings in "the eternal city," in fact, are built of this rock of modern origin. The rapidity with which this concretionary deposit is effected is not less incredible than the great extent of its formation. I have seen specimens of travertin or tufa, as it is termed, equal in