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cracks up into irregular fragments, which tumble and grind over each other with a rattling noise like huge black and red cinders. Though the surface gets black and firm enough even to be walked upon, the rock only a few inches below is still intensely hot, so slow is the transmission of heat through the hardened crust. The lower end of a stream of lava has been seen tardily creeping onward, nine or ten months after the close of the eruption. Eight or ten years after a lava-stream has been poured forth, its crevices are sometimes still too hot to allow the hand to be put into them.
When first erupted, thé motion of a lava-stream is sometimes very rapid, owing to the extreme liquidity of the mass; but as the outer portions which touch the ground and the air cool and harden, the rate of motion speedily lessens. The stream then flows on like a river of rough slag, its progress getting feebler, until, after flowing, it may be, many miles, it comes to rest. The resistance which these hardened portions oppose to the motion of the still fluid parts within, sometimes enables even a tree or a large stone to arrest for a time, or to deflect the course of the lava-stream. The rough slag is piled up against the opposing object, and adds to the obstacle, so that the lava has either to accumulate behind until it surmounts or sweeps away the latter, or to make a detour. Thus, when Etna was in eruption in the year 1669, a stream of lava flowed for fifteen miles, enveloping a number of small towns and villages, and at last reaching the sea with a black rugged front 40 feet high and 1800 feet broad. Near the end of its journey it reached the walls of the city of Catania, against which it gradually accumulated, until, gaining the top of the rampart, it rolled over and enveloped a portion of the town.
The size of lava-streams, and the distance to which they flow, vary indefinitely in different volcanos and in different eruptions from the same cone. Sometimes the currents are like mere brooks, and stiffen into immobility before they get many yards away from their source; at other times they roll on for many miles like large rivers, enveloping towns, farms, fields, and woods, filling up valleys and ravines, and spreading a black barren mass of rough rock over many a square league of country. During the great eruption of 1783 in Iceland, two lava-streams were poured out from the same mountain: one of these reached to a distance of forty-five, the other to fifty miles. Their breadth averaged, the one seven, and the other twelve to fifteen miles, with an average thickness of one hundred feet, which occasionally increased sixfold. This is probably the largest mass of melted matter which has been at one time poured out from the interior of the earth since man began to observe and chronicle these phenomena.
Some of the recent eruptions of Etna, Vesuvius, Hawaii, and other volcanic districts, have been watched by careful observers, and the details thus collected have made us fully acquainted with the leading features of volcanic action. In the year 1865, Etna, which had shewn symptoms of uneasiness for several years previously, at length broke out into eruption. After many tremblings, and even the emission of lava and cinders, a fissure at last opened for a distance of more than a mile and a half to the north of Monte Frumento -one of the numerous minor (though still often large) secondary craters which dot the flanks of the great central mountain. The fissure gave out lava for a short while, but in the end the energy of the eruption shewed itself in the production of a number of cones of scoriæ, some of which reached a height of 300 feet, rising on the surface of a gentle slope. The upper cones threw out scoriæ, bombs, and ashes ; from those further down the slope, jets of liquid lava were ejected. Clouds of steam and heavy showers of dust and cinders rose into the air to a height of more than a mile, with a constant succession of explosions and subterranean noises. During this time lava was constantly pouring out from the lower orifices and descending the slope. Its rate of motion was at first about twenty feet per minute ; but as it advanced and spread out, and its course became impeded by the accumulating crust of scoriæ and other obstacles, the motion decreased till it was not more than twenty inches. The principal lava-stream, or coulée—1000 to 1600 feet broad, with an average thickness of about fifty feet-flowed for a distance of 34 miles, when it reached the edge of a gorge more than 160 feet deep. Here the fiery mass threw itself over in a glowing cascade of rock. Gradually it filled up the ravine, and the lava rolled on as if no ravine had ever been there. In about a fortnight it had advanced upwards of six miles, and its motion had become very feeble. There was still, however, a mass of melted rock within the volcano seeking an escape to the surface. Unable to get away rapidly enough along the main stream, the imprisoned lava broke through the higher part of the coulée, and formed a new stream, which took another direction, but soon stopped.
The damage done by these eruptions was not inconsiderable. A number of farm-houses were destroyed, many hundred acres of pasture and cultivated land were covered ; a strip of forest, containing from 100,000 to 130,000 trees, was demolished. The blaze of the burning wood, added to the lurid glow of the volcano, formed at night a scene of terrible grandeur. By daylight, the dark rugged surface of the lava-current could be seen stretching over the buried fields and pasturage—a surface bristling with rough slags and blocks of scoriæ, studded here and there with charred trunks of trees, and sending out clouds of steam from its still red-hot interior. Sometimes the lava had surrounded a tree without demolishing it, leaving it standing as a sort of measure of the thickness of the coulée. At other points, a few trees standing near each other, or even a single tree so overthrown as to fall athwart the course of the current, sufficed to divert the lava. In one recorded instance this happy
accident was the means of preserving a beautiful little wooded valley from being engulfed under the molten flood.
By this eruption, a wide space of the wooded slopes of Etna was bared and burnt up. Yet so vast is the mountain, and so abundant are its cones and craters, that all the material erupted in 1865 forms, after all, but a mere pellicle added to a part of one side of the great volcano. Etna reaches a height of almost 11,000 feet above the sea, with a circumference of nearly ninety miles, and consists of lavas and ashes erupted from many different points round the central
How enormous a mass of material has here been poured out from the interior of the earth! Yet, in a geological sense, Etna is a very modern volcano. Beneath all its lavas there lie beds of marl and clay which are of the same age as some of the youngest deposits in the British Islands. All our British mountains are immensely older than Etna.
Vast as is the amount of material which has there been erupted, the volcano shews no signs of quiescence. Towards the close of 1868, immense quantities of dust, ashes, and stones were projected high' into the air, and fell to the ground even at a distance of upwards of forty miles. Lava poured from the cone in every direction, and the glowing mountain presented a grand spectacle at night even to the natives of Valetta-a distance of more than 120 miles.
Several hundreds of active volcanos are enumerated as occurring on the surface of the globe. They are arranged for the most part in lines placed not far from the sea, partly along the margins of the great continents, and partly in chains of oceanic islands. Of the former class, the most remarkable example is shewn in the long chain of volcanic vents which ranges along the western border of America from Tierra del Fuego into Mexico-a region which, as we have seen above, has likewise been convulsed by earthquakes. Of the insular volcanos, the most wonderful development occurs in Java and the adjacent islands. There, a line of vents, forty-five in number, runs along the island of Java. Some of these form mountains more than 11,000 or 12,000 feet in height. Twenty-eight of them have been seen in eruption-a greater number of active volcanos than occurs in any other district of the same size upon the globe.
Though active volcanos are still grouped more or less abundantly on all the continents, there is evidence that they once existed in many places where they have long been extinct. Indeed, there is probably no region of any considerable size which does not contain traces of former volcanic action. From the earliest geological times, so far as we know, there have been volcanos, though the seat of the activity has been often shifted, the eruptions dying out in one place only to break out afresh at another. Of such extinct volcanoes we can trace the remains in every stage of decay. In one district they remain still so fresh, their cones and craters are so perfect, and the currents of lava descending from them so black and sterile, that it seems as if the outbursts might be expected to be renewed at any moment. In other places, the craters have disappeared, the massive lava-streams have been cut through by streams, and only a few fragments are left to guide an experienced eye to trace the reality and extent of the ancient eruptions.
In Auvergne, in Central France, a well-known and instructive district of extinct volcanos occurs. In its southern portion, the older lavas are piled up into lofty mountains, which have been deeply trenched by running water. Northward, a long plateau of granite has been pierced by some hundreds of separate points of eruption. Numerous cones of loose cinders rise into conspicuous hills, each with its crater, and some with long coulées of lava, which flow down the plateau into the valleys on either side. The craters are sometimes coated with grass and wild-flowers; and now, instead of the roar of the volcano and the rumbling of the earthquake, there ascends from these cavities only the murmur of the bee, or the tinkle of the bells upon the cattle, which find there their sweetest pasture.
Along the borders of the Rhine, too, numerous cones of slag, ash, and lava mark the position of extinct volcanic hills.
In some cases, the craters are densely clothed with copse or brushwood ; in others, they are filled with water.
Volcanic rocks abound in the British Islands ; but they are so old, and have been so long exposed to the wasting of the elements, that no actual crater now exists among them. The sheets of lava, however, are still conspicuous, and form some of the finest hilllandscapes in the country. The earliest rocks of volcanic origin which we possess are probably those of which Snowdon and other mountains in Wales are in great measure made up. Later than these, and belonging to a long succession of widely separated eruptions, are the chains of hills and detached eminences which diversify the surface of the central valley of Scotland from Arran to Montrose, and from the borders of the Highlands southward to the edge of the great southern uplands. Still later, come the enormous piles of basalt which form the high plateau of Antrim, and stretch thence through the chain of the Western Islands to Faröe, and thence into Iceland. There is good reason to believe that at one time, and that, in a geological sense, not very remote, a great volcanic bank or chain of volcanic islands extended all the way from the north of Ireland to Iceland. Possibly the present active volcanos of the latter island are the lineal descendants of those whose eruptions formed the hills of Antrim, Mull, Eigg, Skye, Faröe, and the older plateaux of Iceland.
At the time when so much subterranean activity was manifesting itself along the north-western sea-board of Britain, the country lying to the south-east underwent some remarkable changes. The solid crust of the earth was rent by thousands of long, straight, parallel fissures, running to the east and south-east of the volcanic area. These fractures extended often for many miles—in one case, at least, to a distance of sixty miles. They were filled up with melted lava which rose from below, and, solidifying between the walls of the cracks, formed what are known as dikes. These long lines of hard rock may be followed, running in their persistent course over hill and dale, regardless alike of the changing nature of the rocks beneath and of the contour of the surface. One conspicuous example begins at Robin Hood's Bay, and crossing the Yorkshire wolds, extends inland for sixty miles. Another dike stretches from the shore, south of Alnwick, westward through the Cheviot Hills. In the Scottish coal-fields many of these dikes occur ; while all along the western coast, from the mouth of the Firth of Clyde up into Ross-shire, they may be counted by thousands.
SLOW RISING AND SINKING OF LAND. The subterranean movements which have been described in the foregoing pages are all more or less of a violent and paroxysmal kind. Between the sudden upheaving of an earthquake shock, and the commotions of an active volcano, there is often a close connection. Both point to the existence underground of forces which, after long intervals of quiescence, gather renewed strength, and seek relief towards the upper surface, shaking the solid framework of the land, or splitting it open.
But over and above these movements, there are others in progress of so gentle and insensible a kind, that it is only by the results which they bring about in a long course of years that we can detect their presence. They give rise to no vibrations and rendings of the ground; yet in the end, they succeed in elevating vast tracts of land above the sea, or in depressing them beneath it. We do not see the process in progress : its march is so slow that the longest human life may be too short to detect any sensible change. Yet, since man began to observe and record such phenomena, many proofs have been obtained that some wide regions are gradually rising from the ocean, while others are as slowly sinking under its waters. It is not merely that the sea wastes the margin of the land so as to roll now over spaces that were once covered with corn-fields or towns; nor that tracts of sand and mud are here and there laid down in such a way that the sea is beaten back and the land advances : these are, indeed, marked changes, of which our own coast-line furnishes many, and often sad, illustrations; but they affect merely the surface, while the changes which now fall to be described take their origin far within the interior of the earth, and continue whether the sea wastes the land or not.
If a mass of land, such, for instance, as the British Islands, be