The subject of the production of volcanic cones, whether by eruption or by elevation, or partly by both, has been adverted to in the pre ceding considerations, and the remarkable example of Jorullo, re peatedly mentioned in the descriptive part of this article, cited from Humboldt as one of elevation. Since their original publication, much research, in which have been observed numerous facts explicatory of the subject, and much reasoning and discussion relating to it, has been instituted and made public, especially by Mr. Serope, in continua tion, or rather resumption of his arguments in support of the reality of the production of volcanic cones exclusively by eruption, by Sir C. Lyell, and by M. Abich, all on the same aide.
The most recent view of the subject, in relation to these researches and discussions, taken by Professor John Phillips, F.R.S. (' Anniv. Address to Geol. Soc.' 1859), which it is just to him to quote in this place, is as follows " In general, it appears probable that cones of elevation are at least of rare occurrence, while cones of eruption are numerous ; but as vertical movement of the ground is an essential condition for volcanio excitement at the outset, we must be prepared to admit the possibility of its occurrence as a part of volcauic history : and the only questions which remain for calm and serious study in reference to a given volcano are—How much! and at what epoch I Questions not to be answered hastily." On this point, however, we submit that, however theoretically probable it may be, that vertical movement of the ground is essential to the beginning of volcanic excitement, no proof that it is so has yet been adduced. beyond that required to produce fissures, and ceasing with their production ; while induction from the late observations of earthquake-phenomena already noticed, is opposed to it, and some of the results obtained by Mr. W. Hopkins, in his researches in physical geology, appear to indicate that the theory is itself defective, if not erroneous.
Mr. Scrope's recapitulation on this question (' Quart. Journ. of Geol. Soc.' vol. xv. p. 545), states its present aspect in these terms : " My argument then is, that the elevation-crater' or ' upheaval' theory, as applied to volcanic action by MM. de Humboldt, De.Buch, de Beaumont, and Dufreenoy, and to some extent by Dr. Daubeny and Professor James Forbes, as well as in several popular geological com pilations, is an assumption irreconcilable with the appearances it professes to account for, and wholly hypothetical—such a process never having been witnessed ; while there is nothing in the form, structure, or composition of any of the cones or craters to which it is applied by its advocates Inconsistent with the supposition that they owe their origin to the simple, ordinary, normal, and perfectly intelligible phenomena of volcanic eruptions, as witnessed repeatedly by competent observers as well in the present day as through all past historical times." ' Subsequently to the promulgation of these arguments, evidence of the most precise nature, proving that the volcano of Jorullo is truly a cone of eruption, as contended long ago, even on the evidence of Humboldt himself, by Mr. Scrope and Sir C. Lyell, and that it was
formed in no degree by upheaval—of which process it had been brought forward by Humboldt as almost a crucial instance—has been given by M. H. de Saussure, a geologist of Geneva, the third in descent, we presume, of a name dear to science. He communicated this evidence to the SocitRe Vaudoiee des Sciences Natnrelles,' in 1859, exactly a century after the surface of the fertile valley of Jorullo had been transformed into a barren sheet of lava, by a catastrophe, having " for one of its results a perfectly characterised volcanic mountain, which suddenly sprung up on the surface of the globe, with un exampled rapidity and grandeur of proportions." Described about half a century after its formation by the great traveller under the influence of an imposing hypothesis, it became one of the subjects, for another half•centnry, of a controversy among the most eminent investigators of the globe's physical structure. This controversy, we think, as respects Jorullo, must now cease. M. de Saussure states that the sheets of lava surrounding the mountain called Malpais.—which Humboldt regarded to be the result of a softening of the pre-existing surface soil by gases, and its inflation by them from beneath like a bladder,—are nothing else than vast outflows of incandescent matter, which have lined the whole valley, filling its cavities and forming pro montories, just as a mass of molten lead would spread when formed on an uneven surface. The edges of the Malpais are not a section or broken edge of an elevated tract, but only the lateral or terminal borders of currents of lava. Tho cone which forms the mountain of Jorullo Itself is the simple result of the heaping up of cinders and scorire ejected by gaseous explosions from the principal orifice of eruption. after the outbursts of lava had ceased. The eruption took place originally from an axial fissure running north and south, but there is no trace of the elevation of the beds along this axis. The volcanic pres sure from beneath had only forced an exit through this fault for the escape of the liquid and aarifonn matters erupted. In short, the phenomena of Jorullo show that the most powerful volcanic outbursts can take place without the slightest derangement of the superficial beds. (' Quart. Journ. of Geol. Soc.' vol. xvii. ; Tranalations; p. 13,1d.) The subject of subterranean heat and its consequences is so vast, and its ramifications in science are ao unlimited, that the preceding view of reasonable explanations of Plutonic and volcanic action is far from having exhausted the catalogue. Not merely geologists, mineralogists, and chemists, but astronomers and mathematicians have attempted the solution of the problems involved; the former directing their attention chiefly to special structures, producta, or phenomena, the latter to such probable causes as point to hypotheses of wide generality, embracing the entire system of the relations between the earth as a planet to the heat of its interior regions, and to that which it receives from the solar radiation.