of the section, at a and c they diminish upwards. In all these cases, the layers or beds are not disturbed further than by fracture, those on either side of the dykes preserving their continuous lines of original accumulation. This need not always be the case, as will be readily inferred, since after a fracture is made, should the liquid or viscous lava rise with much force from considerable pressure of a column of molten rock with which it may be connected, with a comparative cooling of the upper part of the lava as it rose, increasing the solidification of the particles, the upper layers of tuff or lava broken through may be heaved upwards by the friction of the uprising lava, this even overflowing, as appears to be frequently the case. Of this kind the section beneath (fig. 128), taken from a view by Dr. Abich, of a dyke exposed by the fall of part of the crater of Vesuvius, is probably an instance.

The observer has next to consider the magnitude and direction of these fissures. Perhaps volcanic islands, such as those in the Atlantic and Pacific, are as favourable situations for studying volcanic fissures as are to be found, not, however, that great facilities do not present themselves elsewhere.* The rent or series of rents which traversed the side of Kilauea, during the ejection of lava in 1840, is not only remarkable for its length, but also for the com

* Respecting the Hawaiian group, Mr. Dana, (Geology of the United States' Exploring Expedition, p. 282), infers:-1. That there were as many separate rents or fissures in the origin of the Hawaiian Islands as there are islands. 2. That each rent was widest in the south-east portion. 3. That the south-easternmost rent was the largest, the fires continuing there longest to burn. 4. That the correct order of extinction of the great volcanos is nearly as follows (leaving out Molokai and Lanai, which were not visited by Mr. Dana)-a, Kauai; b, Western Oahu; c, Western Maui, Mount Eeka; d, Eastern Oahu; e, North-western Hawaii, Mauna Kea; f, South-east Maui, Mount Hale-a-Kala; and g, South-east Hawaii, Mauna Loa. "This order," he observes," is shown by the extent of the degradation on the surface. Each successive year, since the finishing of the mountain, has carried on this work of degradation, and the amount of it is, therefore, a mark of time, and affords evidence of the most decisive character." (p. 283.)

paratively tranquil manner in which it was effected, the inhabitants not being aware of its formation by any earthquake motion, but from finding a torrent of liquid lava poured out.* Fissures so produced would seem to point to much softening of the subjacent rocks, so that when fractures were formed, though many miles in length, comparatively little resistance, from cohesion, remained in the rocks. From an examination of Maui (Hawaiian group), Mr. Dana infers that at its last eruption, a huge segment of that volcano must have been broken off, by which two great valleys were formed (one two miles wide), through which great opening the lavas were poured out. Here would appear to have been far greater resistance and a more sudden overpowering of it by the force exerted. According to the accounts given, Jorullo was the result of an uprise of ground, finally traversed by a fissure (p. 346). With respect to fissures traversing active volcanos from which lava has issued, there is abundant evidence. The great outflow of lava from Skaptar-jökull, in 1783 (p. 344), was from fissures at the base of the mountain. Great fissures have been made in Etna, and the numerous subordinate craters seem little else than points in those formed at various times through which volcanic matter has been ejected. Respecting the fissures on Etna, M. Élie de Beaumont remarks, that they occur, for the most part, in nearly vertical planes, often so cutting the crater, as it were, to star it, the lower part of the fissures usually filled with lava, the higher with scoriæ, and with pieces of tuff and lava fallen from the upper part. He mentions that the fissure formed in 1832, was so far a shift, or fault, that one of the sides of the dislocation rose about a yard higher than the other.§ The great fissure, which in 1669 traversed the slope of the great gibbosity of Etna and the Piano del Largo, is described as having ranged from near Nicolosi to beyond the Torre del Filosofo, and to have been about two yards wide at the surface, a livid light being emitted from the incandescent lava rising in it. An observer should carefully ascertain the directions of such rents or fissures whether large or small, and

* Dana, "Geology of the United States' Exploring Expedition," p. 217. + Ibid, p. 259.

It is stated that there are 52 of these subordinate volcanic hills on the west and north of the summit of Etna, and 27 on the east side; "some covered with vegetation, others bare and arid, their relative antiquity being probably denoted by the progress vegetation has made upon their surface."-Daubeny, Volcanos, 2nd edition, p. 272. § Recherches sur la Structure et sur l'Origine du Mont Etna, par M. L. Elie de Beaumont;"Mémoires pour servir a une Description Géologique de la France," 1838, t. iv., p. 111.

Ibid. p. 108.

always with reference to the complication which may arise from variable resistances, even in the prolongation of the same fissures, to the force employed, seeing especially if the greater fractures have continued to preserve any definite directions at different intervals of time.

M. Élie de Beaumont has called attention to the fact that these fractures so often starring Etna, and into which the molten lava is introduced, there hardening and remaining, must produce a tumefaction or elevation of the whole, each eruption of the mountain, so characterized, having a tendency to elevate the mass of the volcano. The same reasoning is applicable to all volcanos rent and fissured in a similar manner, and the abundance of the resulting dykes of lava is often a prevailing feature in many. They are sometimes interlaced so as to show differences in date, hose of one time cutting those of another, exhibiting proofs of repeated fractures through the same general mass of volcanic matter.

In examining some volcanic regions, the observer will have to consider, as above noticed (p. 323), the probable differences which would arise in the structure and arrangement of the accumulations from a part or the whole of them having been produced beneath water. At considerable depths in the ocean, beyond those at which an equal temperature of 39°.5 (p. 96) would appear to prevail, not only the pressure but also the constancy of that temperature and the mass of water possessing it have to be borne in mind. Assuming a communication made, whether by an elevation of the seabottom and the bursting of a tumefaction formed by forces acting from beneath, or by one of those adjustments of the earth's surface by which more or less considerable fractures are produced, he will have to recollect that a great volume of water, with a low temperature, would be at once brought to bear upon it, and that not only are the usual volcanic gases absorbed by water, but that the very pressure itself might tend to drive them into the liquid state.† It would be out of place here to enter into the probable effects produced under such conditions, further than to notice that, supposing the communication made, and the elevatory force sufficient to lift a body of molten lava, so that it could pass out of the volcanic orifice or crack, the observer has to consider the effects which would follow. However any intense heat might permit the existence of

* "Mémoires pour servir a une Description Géologique de la France," t. iv., p. 118. + Dr. Faraday has shown (Philosophical Transactions, 1823), that sulphurous acid gas becomes liquid under the pressure of two atmospheres, at a temperature of 45° Fahr.; sulphuretted hydrogen under that of 17 atmospheres at 50°; carbonic acid under 36 atmospheres, at 32°; and hydrochloric acid gas under 40 atmospheres at 50°.

the vapours and gases observed at subaërial volcanos, before the rupture was effected, as soon as the water came into contact with them, a ready supply of that at 399-5 pouring in, the more heated water ascending, as so heated, they would disappear as they rose. If disseminated amid the lava thrown out, the great pressure upon the latter would produce its effects upon them, while the low temperature would soon act on the external liquidity of the lava itself.

From such a state of things to the minor depths and surface of the water, great modifications would be expected, solid lava, (supposing the struggle between the forces brought into action to be such, as, on the whole, to permit a gain on the side of the volcanic products), probably prevailing beneath, while there was an admixture of more scoriaceous matter above, as the accumulations rose into the atmosphere. The whole mass would be liable at all times to be cracked and fissured, molten lava rising into the rents according to the pressure of the time upon it, and the tumefaction mentioned by M. Elie de Beaumont progressing.

The extent to which sheets of matter could be spread in various directions and at different times around submarine volcanic vents, would necessarily depend upon circumstances; among them, the absence of piles of cinders and ashes into cones, such as are formed by the discharge of vapours and gases through lava into the atmosphere, being important, so that when fissures were produced, molten matter flowed more freely out, in the manner, so far as liquidity and the absence of cinders and ashes are regarded, of the streams which poured out on the flanks of Mauna Loa, in 1843.

It has been seen that volcanic vents may remain for a long time dormant or closed, and then lava, cinders, and ashes be driven out of them. The probable differences which would arise in such cases with volcanic accumulations beneath the sea and those above its level, require attention. It may be inferred that, beneath given depths of water, where the rents have been more frequent, and the lavas have more frequently been thrown out, there may be such a mechanical resistance to the new application of an elevatory force, that an increase of heat may soften and even melt some of the priorformed accumulations, for the most part readily fusible. Thus a dome-shaped mass may be raised, not finally splitting, in given localities, at its surface, until even above water, and the quiet cracking of Mauna Loa for the length of many miles, would appear to show us that conditions may arise, even in subaerial volcanos, permitting the heating and softening of a volcanic crust to within. comparatively moderate distances from that crust.

CHAPTER XX.

VOLCANOS AND THEIR PRODUCTS CONTINUED.—THE CALDERA, ISLAND OF PALMA. SECTIONS OF ETNA AND VESUVIUS.-FORM AND STRUCTURE OF ETNA.-ORIGIN OF THE VAL DEL BOVE, ETNA.—FOSSILIFEROUS VOLCANIC TUFF OF MONTE SOMMA.-MIXED MOLTEN VOLCANIC ROCKS AND CONGLOMERATES.-MODIFICATION OF SUBMARINE VOLCANIC PRODUCTS.

PEAK OF TENERIFFE.-SANTORIN GROUP.-ISLAND OF ST. PAUL, INDIAN OCEAN.-DISTRIBUTION OF VOLCANOS IN THE OCEAN-ON CONTINENTS AND AMID INLAND SEAS.-VARIABLE PROXIMITY OF VOLCANOS ΤΟ WATER.-EXTINCT VOLCANOS.-MINERAL AND CHEMICAL COMPOSITION AND STRUCTURE OF BASALT.

THAT dome-like elevations of volcanic products have been formed, MM. Von Buch, Élie de Beaumont, Dufrénoy, and others consider they have sufficient proof. Some notice has been above given (p. 318), of the equivocal appearances which may be presented by the "craters of elevation" and the "craters of eruption." The Caldera, in the Island of Palma, Canaries, is adduced by Von Buch as a good example of the "craters of elevation." A large precipitous cavity or crater is there surrounded by beds of basalt and conglomerate, composed of basaltic fragments, dipping regularly outwards, and is broken only by a deep gorge on one side, through which access can be obtained to this central cavity. White trachyte, and a compound of hornblende and white felspar, are also noticed among these rocks. There being no mixture of scoria or ashes, and the beds of molten rock as well as the conglomerate presenting a uniform stratification, it is inferred that the whole was formed under different circumstances, such as beneath water, from the ordinary eruptive accumulations of a volcano, and had been upraised in a dome-like manner, until finally the rupture was effected, and the least resistance being in one direction, the lateral gorge was produced, the whole presenting the appearance of the pear-shaped termination of the fissures in figs. 115 and 116.

M. Élie de Beaumont has given a valuable description of Etna,