and Scrope, as well as by Dana in this country. According to this theory, most great volcanoes consist of two portions, very distinct from each other in their mode of formation. The lower part, or base of the mountain as it might be called, consists of strata inclined at a less angle than the upper, and has not been formed by the accumulation of ejected materials, but consists, rather, of stratified masses, which may have been sedimentary beds deposited horizontally, or volcanic materials erupted from fissures under the ocean. In either case these beds are supposed, by the upholders of Buch's theory, to have been brought into their present inclined position by a "bubbleshaped elevation of the ground," caused by pressure of the volcanic forces confined beneath. On this inflated mass, through the centre of which the lava afterwards found its way, the cone of eruption is supposed to have been formed in the ordinary manner. In many cases, however, the process was a more complicated one. After the formation of the flattened dome-shaped mass, the volcanic energy, exerting itself at the base of the chimney by which the dome was penetrated, would fracture it in all directions, force lava into these fissures, swell out the mass, and gradually open a great crater at the summit, around the edge of which the strata would stand at a much greater angle than they originally had, it being maintained by Buch and the upholders of the elevation theory that lava could not consolidate in thick beds on steep slopes, an assertion which has been abundantly disproved by observations in different parts of the world. It is in this condition of dome-shaped elevation, caused by pressure from beneath, that Vesuvius is supposed to have been at the time Spartacus camped in its crater, just before the great eruption of 79. At the time the explosion took place, and the hidden forces obtained an outlet, one side of the crater of elevation was blown off, and an ordinary ash and cinder cone began to form in the cavity. The same mode of formation is claimed for Etna by Élie de Beaumont, one of the most zealous supporters of Buch's theory, who maintained that the lower portion of this great volcano was quite distinct in its formation from the upper; that the one was formed beneath the sea by the elevation of horizontally deposited - strata, while the other, or the cone proper, which is eleven hundred feet high and has as steep an angle as thirty-two degrees, was built up by subaerial accretions exclusively. Buch applied his theory to the Peak of Teneriffe, of which he made a most detailed examination, and endeavored to explain by it the formation of the great semicircular wall which encloses the peak itself and the cone of Chahorra. This encircling precipice is, in places, full two thousand feet high and no less than eight miles in its longest diameter. Buch also visited and described with minuteness the beautiful island of Palma, a little west of Teneriffe, which is another of these great truncated cones, with a huge and deep cavity in the centre, called by the natives a caldera (kettle), from three to four miles in diameter, and walled in by a precipice varying from fifteen hundred to twenty-five hundred feet in vertical height. This boundary wall is so steep and unbroken that there is only one place where a descent is possible even on foot. This kind of structure- namely, an encircling ring, of enor mous dimensions compared with those of ordinary craters, with a cone in the centre -is quite common, and is especially well seen on some volcanic islands, where the internal structure is revealed by breaches made by the sea in the exterior wall. The interesting island of Santorin, in the Grecian Archipelago, is a good instance of this kind of arrangement, the volcanic fires here having been active of late, and the region one which has furnished material for a considerable number of volumes, as already mentioned. The island of Nisyros has a similar structure, the nearly circular crater being three miles in diameter and surrounded by a rim which rises from two thousand to twenty-three hundred feet above the sea. The island of St. Helena is described by Mr. Darwin as a trachytic volcano, encircled by a broken ring of basalt, measuring eight miles in diameter one way and four the other; the internal cliff faces are nearly perpendicular, except that they have in some places flat projecting shelves or ledges cut around them in parallel curves. Barren Island, in the Bay of Bengal, and the Mauritius, are other excellent examples of the same interesting type of structure. The encircling crater ring of the last-named island measures no less than thirteen miles in diameter. Lyell, in the tenth edition of his "Principles of Geology," published two years since, has gone pretty thoroughly into the question of the applicability of Buch's theory to both Vesuvius and Etna, giving the results of his own repeated and recent examinations of these classic volcanoes, and pointing out that many important facts had been misapprehended by those geologists who had endeavored to show that the crater-of-elevation theory was the only one applicable to explain their form and structure. Hoffmann, many years ago, after a careful study of Vesuvius, abandoned the theory of Buch, which he had previously maintained. Of eminent French geologists, Cordier and Constant Prévost were also opposed to the idea of the building up of volcanoes in any other way than by the piling of one layer of ejected materials upon another. The principal difficulty which those who do not support the crater-of-elevation theory have to meet is the enormous size of some of these great encircling rings, which would seem at first too large to be the result of explosive forces, implying as they do an astonishingly violent action and areas of vast dimensions over which the volumes of vapor must have been driven upwards. There are craters of gigantic size, however, in regard to which it seems clearly demonstrated that they were formed in the ordinary way, that is, by the aggregation of materials erupted from a central orifice. Thus Kilauea does not bear any marks of being a crater of elevation; neither does the grand Haleakala, on the island of Manui, which is estimated to be some thirty miles in circumference. Junghuhn, who has made such a careful examination of the volcanoes of Java, gives it as the result of his observations that the great cones of that island have all been formed by eruption, and not by elevation; and. he gives most excellent reasons for drawing this inference, such reasons, indeed, as could only be successfully opposed by proving him to have misstated the facts. Similar conclusions have been arrived at by the writer of this article, after examining several of the great cones on the Pacific coast of North America. If we consider what prodigious masses of material are thrown out, as already mentioned, in such eruptions as that of TemVOL. CIX.- - NO. 224. 17 boro or Coseguina, it will not be difficult to understand that a cavity of corresponding size must be left behind; and, as a means of enlarging such a cavity to an almost indefinite extent, we may call in both subaerial and submarine erosion, although the former has probably been usually by far the most effective agent in this respect. That any such great blister-like uplift of the superficial crust as was imagined by Humboldt to account for the dome-shaped base of Jorullo ever occurred seems, on the whole, highly improbable. His idea of a hollow crust or roof blown up over a vast empty space beneath will hardly be adopted by any geologist at present. Everything indicates, on the contrary, that, instead of there being a vacuum or a space filled only with gaseous substances under or over the centre of the volcanic action, there is much more likely to be a crowding together in that region of fluid material, seeking to find a vent. That great areas of stratified deposits might, under such conditions, be elevated into domeshaped masses, is certainly not impossible; and yet it is questionable whether the fact of any such occurrence has ever been demonstrated. a man It is indeed curious that the great name of Buch once the very leader of geological science, and to whom Humboldt dedicated his Kleinere Schriften in these words: "Dem geistreichen Forscher der Natur, dem grössten Geognosten unseres Zeitalters, Leopold von Buch"- should for many years back have been most frequently quoted in order to bring forward fresh evidence against some one of his favorite theories, or to show how thoroughly he misapprehended some great geological phenomenon, like that of the distribution of the glacial boulders in Switzerland. Still the fact, however discouraging it may seem to those looking simply to permanence of personal reputation, is, in reality, an indication of progress in the science. Had Buch made a thorough examination of the geologically classic region of Southern Tyrol, he never would have given to the world a theory so entirely unsupported by facts as that by which he sought to explain the formation of the wonderfully picturesque cliffs of dolomite which have made that country so celebrated, and the origin of the rock of which they are composed. The day of generalizations of a magnitude entirely disproportionate to the slender base of facts on which they rest has passed away; or, at least, the practice of bringing such theories forward with the positiveness, and upholding them with the obstinacy, of a Buch is one which is no longer in vogue. There are, indeed, many geological phenomena the theory of which is obscure and difficult, and for whose final elucidation the stock of accumulated observation is still insufficient. If, with the view of directing attention to deficiencies in this stock, rather than of parading his actual knowledge, the geologist groups these facts together, and endeavors to show in what direction they seem to point or what the ultimate solution of the problem will probably be, he will, if his work be done in the right spirit, not incur the charge of rashly generalizing or of endeavoring to force his opinions on others. Among the obscurest and yet most attractive topics of geological investigation it would be safe to include the theory of volcanoes and earthquakes, and especially the connection of their phenomena with those movements of the earth's crust, which have resulted in the formation of continents and mountain-chains, and which, by altering the relative level of land and sea, have played the principal part in the long series of events that have been going on since our planet became the theatre of geological changes. This article, and one in the preceding number, may be considered as leading the reader to a point from which he will be able, with profit, and, it is to be hoped, not without pleasure, to survey the indicated field, and we shall endeavor at a future time to act as his guide in such a survey. Before closing, we must add a few pages to what has been said in a previous article, in regard to the geographical distribution of volcanoes, or their arrangement upon the earth's surface. By far the most interesting fact in this connection is the proximity to the ocean of almost all active volcanic vents. Probably nine tenths of them are distributed around the Pacific, forming what has been aptly called a "circle of fire" full twenty thousand miles in length. The islands on the west side of that ocean form almost a continuous chain, beginning with the Aleutians on the north, and extending to New Zealand on the extreme south. This is pre-emi |
