minutes; others have reached the sea in three hours from the summit of the mountain, a distance of 3200 yards. The stream which destroyed Catania, in 1669, was fourteen miles long and five wide. In Etna, currents have been traced forty miles in length; and a stream that issued from Mount Hecla, in Iceland, is computed at ninety-four miles in length, and fifty in its greatest breadth; and its depth, where there were obstacles to its progress, was in some places several hundred feet.

Lava-currents retain a high temperature for a long period; some have been observed to flow slowly ten years after their eruption. A mass of lava on the flanks of Vesuvius ignited wood thrust into it four years after its motion had ceased.

The cooled lavas and other mineral products which form the great mass of a volcanic mountain are rent and torn by the earthquakes which generally precede every fresh eruption; and these fissures and chasms become filled by subsequent injections of molten rock. In this manner dykes and veins are formed in the trachytic and scoriaceous masses of Vesuvius, Etna, and other active volcanos (Lign. 198), resembling on a small scale the intrusive trap-dykes in the ancient strata, of which we have already noticed many examples.

Fig. 1. Veins and dykes of slaggy lava in volcanic tuff; Stromboli.

The loose sand, scoriæ, and ashes which have been either wafted by the winds and fallen into the sea, or washed down by torrents on the plains, become agglutinated together and form an earthy conglomerate, which is termed volcanic tuff. This substance is frequently traversed by veins and dykes of lava, thrown up by subsequent eruptions. It often happens that the beds thus permeated, being formed of materials that readily decompose, are partially or wholly worn away, while the durable intruded dykes remain, and stand out in relief, sometimes forming vertical

walls or buttresses, of great thickness and extent; of which the celebrated Val del Bove, of Etna, to be noticed in the sequel, affords some of the most remarkable examples.

*

10. ERUPTIONS OF VESUVIUS.-In the early periods of activity, violent gaseous explosions, with showers of scoriæ, ashes, and sand, characterized the eruptions of Vesuvius ; but since the existence of the present crater lava-currents have generally been ejected. The appearance of an ordinary eruption, seen by night, has been thus picturesquely described by a traveller :—

"It was about half-past ten when we reached the foot of the craters, which were both tremendously agitated; the great vent threw up immense columns of fire, mingled with the blackest smoke and sand. Each explosion was preceded by a bellowing noise, like thunder, in the interior of the mountain. The smaller vent was the most active; and the explosions followed each other so rapidly that we could not count three seconds between them. The stones which were emitted were fourteen seconds in falling back to the crater; consequently, there were always the discharge of five or six explosions-sometimes more than twenty—in the air at once. These stones were thrown up perpendicularly, in the shape of a wide-spreading sheaf, producing the most magnificent effect imaginable. The smallest stones appeared to be of the size of cannon-balls; the greater were like bomb-shells; but others were pieces of rock, five or six cubic feet in size, and some of the most enormous dimensions: the latter generally fell on the ridge of the crater, and rolled down its sides, splitting into fragments as they struck against the hard and cutting masses of cold lava. The smoke emitted by the smaller cone was white, and its appearance inconceivably grand and beautiful; but the other crater, though

* The craters of Auvergne, (vol. i. p. 272), that exhibit no traces of lava-currents, are also supposed to have been produced by explosions.

less active, was much more terrible; and the thick blackness of its gigantic volumes of smoke partly concealed the fire which it vomited. Both vents occasionally burst forth at the same instant, and with the most tremendous fury, the ejected stones intermingling in the air.

"If any person could accurately fancy the effect of 500,000 sky-rockets darting up at once to a height of three or four thousand feet, and then falling back in the shape of red-hot balls, shells, and large rocks of fire, he might have an idea of a single explosion of this burning mountain; but it is doubtful whether any imagination can conceive the effect of one hundred of such explosions in the space of five minutes, or of twelve hundred or more in the course of an hour, as we saw them! Yet this was only a part of the sublime spectacle before us.

"On emerging from the darkness, occasioned by the smaller crater being hidden by the large one, as we passed round to the other side of the mountain, we found the whole scene illuminated by the river of lava, which gushed out of the valley formed by the craters and the hill on which we now stood. The fiery current was narrow at its source, apparently not more than a few feet in breadth; but it quickly widened, and soon divided into two streams, one of which was at least forty feet wide, and the other somewhat less: between them was a sort of island, below which they reunited into one broad river, that was at length lost sight of in the deep windings and ravines of the mountain.” *

In an eruption witnessed by Sir W. Hamilton, jets of liquid lava, mingled with stones and scoriæ, were thrown up to a height of ten thousand feet.

The streams of lava, which issue with great velocity, are in a state of perfect fusion; but as they cool on the surface, they crack, and the matter becomes vesicular or porous; at a considerable distance from their source, they resemble a

* From the Saturday Magazine.

heap of scoriæ, or cinders, from an iron-foundry, rolling slowly along, and falling, with a rattling noise, one over the other.

The eruptions of Vesuvius, its cones and craters, and the still older and partly encircling crater of Somma, have afforded continuous and important lessons to geologists as to the mode of action and nature of volcanic forces. In 1856, Mr. Poulett Scrope read before the Geological Society a very interesting and instructive résumé of his views on the mode of production of volcanic craters, and on the nature of the liquidity of lavas, mainly illustrated by the Vesuvian phenomena, and originally advanced in his work on Volcanos in 1824. In the Memoir referred to, the first point insisted on is the formation of all volcanic cones and craters by the simple process observed in habitually active volcanos, namely, the eruptive ejection of lavas and fragmentary matter from a volcanic vent; the accumulation of which around it cannot fail to give rise to the cone-shaped mountain so characteristic of a volcano, and to the crater usually contained in it. The author showed, by the history of Vesuvius, that the cone of that mountain has, within the last hundred years, been at least five several times emptied by explosions of a paroxysmal character, and as often refilled by the products of subsequent minor eruptions; while throughout this time the exterior of the cone has been gradually increasing in bulk, and the old crater of Somma as gradually being filled up, by accretions from the volcanic matter ejected beyond the lip of the Vesuvian crater. He refuses to believe that any other process originally formed the outer cone and crater of Somma, than that which he and others have seen to be continually augmenting the inner cone of Vesuvius, and which before his eyes in 1822 scooped out of its heart a crater concentric to that of Somma, three miles in circumference, and some 2000 feet in depth. And generally of other great craters, ancient or modern, such as Palma, Santorini, the Val del Bove, &c., he considers, that no argument in favour of their having any other than a similarly "eruptive" origin can be derived from the fact of their dimensions exceeding those of the crater of Vesuvius. The authentic accounts of enormous quantities of ejected pumice, scoriæ, or ashes hrown out by many eruptions from Polynesian or American volcanos, reaching to distances of above a thousand miles, and of course spreading over the whole intermediate space, to a thickness sometimes of 10 or 12 feet at more than twenty-five miles from the volcano, would amply account for the dispersion, by explosive eruptions, of the contents of the largest craters ever observed.

At the same time the author guards himself from being supposed to have ever denied that some amount of elevation has taken place in the

external cone of a volcano through the occasional injection of lava from within into rents broken across its framework, and hardened into dykes, which may be called a process of gradual distension. This, in fact, was suggested by him in 1824. All he contends against is the theory of Von Buch, that volcanic mountains are the result of the elevation of nearly horizontal beds of lava and conglomerates by some sudden expansion. He maintains, on the contrary, that the growth of a volcano by accretion, through eruptive ejections on the exterior, and partial distension from within, is a gradual, though intermittent, normal process, which may be watched almost like the growth of a tree.

The author next referred to the opinion published by him in 1824, that the liquidity of the stony and crystalline lavas (excluding the vitreous varieties) at the time of their protrusion is owing, not to complete fusion, but to the entanglement between their component granular or crystalline particles of some fluid, chiefly water, at an intense heat, of course, but unvaporized by reason of the extreme pressure to which they are subjected while beneath the earth, and escaping in vast bubbles of steam, when, by the opening of a fissure of escape, its discharge is permitted, and also by a kind of exudation through the pores and crevices of the expelled lavas as they cool.

The author originally extended this theory of the combination of aqueous with igneous agency in lavas to all the crystalline plutonic rocks, which he considered to be derived from a mass existing beneath the crust of the globe under the above circumstances, in a state of extreme tension, such as on the occurrence of any sufficient local relaxation of the restraining pressure from above, or increase of temperature from within, must occasion its partial intumescence, and the consequent fracture and elevation of the overlying rocks, with or without extravasations of the intumescent crystalline matter through rents, in the form either of volcanic eruptions, or the protrusion of the granitoidal axes of mountainchains.

These ideas on the character of the liquidity of lavas and the hypogene crystalline rocks, promulgated by the author in 1824-1826, were considered unchemical at that time, and little regarded. They have, however, of late been reproduced by M. Scheerer of Christiania, and adopted by M. Elie de Beaumont, and have received much confirmation from recent researches into the conduct of water under pressure at high temperatures, its power of taking silex into solution, &c.

The author further asks the attention of geologists to the ideas developed by him in the same early work, and founded on actual and careful observations, as to the change of position occasioned in the component crystals of a matter moving in the pasty state here attri