Wonders of Creation: A Descriptive Account of Volcanoes and Their Phenomena. UnknownЧитать онлайн книгу.

      Wonders of Creation: A Descriptive Account of Volcanoes and Their Phenomena

      CHAPTER I

      Volcanoes in general—Origin of the Name—General Aspect—Crater—Cone—Subordinate Cones and Craters—Peak of Teneriffe—Lava-Streams—Cascades and Jets of Lava—Variations in its Consistency—Pumice—Different Sorts of Lava—Obsidian—Olivine—Sulphur—Dust, Ashes, &c.—Volcanic Silk—Volcanic Islands—Volcanic Fishes—Hot Water, Mud, Vapours, &c—Volcanic storm—Explosions—Number of Volcanoes—King of the Volcanoes—Artificial Volcano.

      Among the many wonderful works of God, none exhibits so much of awful grandeur as an active volcano. This name for a burning mountain was first applied to that which exists in the island anciently called Hiera, one of the Lipari group. It is derived from the name of the heathen god Vulcan, which was originally spelt with an initial B, as appears from an ancient altar on which were inscribed the words BOLCANO SAC. ARA. This spelling indicates the true derivation of the name, which is simply a corruption of Tubal-cain, who was "an instructer of every artificer in brass and iron" (Gen. iv. 22). The ancient heathen, having deified this personage, imagined, on first seeing a burning mountain, that Tubal-cain, or Vulcan, must have established his forge in the heart of it, and so, not unnaturally, named it Volcano—an appellation which the Island of Hiera retains to the present day.

      The Cyclops—the supposed descendants of Vulcan, who were fabled to have been of gigantic stature, and to have had each only one eye in the centre of the forehead—were imagined to be the workmen who laboured in these underground forges. The noises, proceeding from the heart of the mountain, were attributed to their operations. It is to the Island of Hiera that Virgil alludes in the Æneid, lib. viii. 416. The passage is thus rendered by Dryden:—

      A volcano generally presents itself to the imagination as a mountain sending forth from its summit great clouds of smoke with vast sheets of flame, and it is not unfrequently so described. The truth is, however, that a real volcano seldom emits either true smoke or true flame. What is mistaken for smoke consists merely of vast volumes of fine dust, mingled with much steam and other vapours—chiefly sulphurous. What appears like flames is simply the glare from the glowing materials which are thrown up towards the top of the mountain—this glare being reflected from the clouds of dust and steam.

      The most essential part of a volcano is the crater, a hollow basin, generally of a circular form. It is often of large dimensions, and sometimes of vast depth. Some volcanoes consist of a crater alone, with scarcely any mountain at all; but in the majority of cases the crater is situated on the top of a mountain, which in some instances towers to an enormous height. The part of the mountain which terminates in the principal crater is usually of a conical form—much like a glass-house chimney, and is therefore named the cone. It is generally composed of loose ashes and cinders, with here and there masses of stone, which have been tossed into the air by the volcanic forces. In some mountains the cone rises out of a hollow at a considerable height from the base. A hollow of this kind is generally regarded as having been a former crater, which had become extinct before the existing cone was raised. There are sometimes formed lower down the mountain subordinate craters, smaller than that which occupies the summit of the cone. Within the crater itself there are frequently numerous little cones, from which vapours are continually issuing, with occasional volleys of ashes and stones.

      One of the largest and most perfect of the volcanic cones in the world is that of the Peak of Teneriffe, of which you have here a representation. It conveys a good idea of the general form of the cone, and has long been a conspicuous and useful landmark to mariners. It is upwards of twelve thousand feet in height, and is said to be visible in very clear weather at a distance of a hundred miles.

      The most interesting products of an active volcano are the streams of lava which it pours forth—sometimes from the principal crater on the summit—sometimes from the smaller craters lower down. This lava consists of melted stone. When it issues from the mountain its heat is intense and it glows like a furnace, so that, during the night especially, these fiery rivers present a grand yet awful spectacle. The streams spread themselves till they sometimes attain a breadth of several miles, with a depth of several hundred feet, and they flow onward till their length sometimes reaches fifty miles.

      Lava, not being so liquid as water, does not flow so rapidly: nevertheless, when it is careering down the sides of a mountain, or where the slope of the ground is considerable, it advances with great speed. Even when at its hottest, it is somewhat viscid, like treacle, and this viscidness increases as it cools. Hence on a level plain, and at some distance from its source, the lava-stream advances at a leisurely pace. In such circumstances the cooling proceeds so quickly that a crust of considerable thickness is soon formed on the top of the current, and persons who are bold enough may cross the stream by means of this natural bridge. Even where the current continues flowing rapidly, this crust may be formed on its surface; and a man, whose curiosity exceeds his prudence, may stand on the top of it, bore a hole through the crust, and see the lava flowing underneath his feet!

      Nothing can resist the progress of the lava-flood; trees, houses, everything yields to its massive assault, The trees take fire before its approach, and when it reaches them they emit a hissing noise almost amounting to a shriek, and then plunging into the molten flood are seen no more. Even the sea cannot withstand the lava-stream, but retires on its approach; so that promontories stretching to a considerable distance from the shore are formed in this manner, when the molten matter hardens into stone.

      The eruptions of lava are sometimes attended by peculiarities which impart to them much additional grandeur. Instances have occurred in which the fiery stream has plunged over a sheer precipice of immense height, so as to produce a glowing cascade exceeding in breadth and perpendicular descent the celebrated Falls of Niagara. In other cases, the lava, instead of at once flowing down the sides of the mountain, has been first thrown up into the air as a fiery fountain several hundred feet in height. This happens when the great crater at the summit of the cone is full of liquid lava but does not overflow. Then, on the formation of an opening in the side of the cone, a good way down, the lava issuing from it is projected upwards to nearly the same height that it occupies in the interior of the crater at the top of the cone. It is hardly possible for the fancy to picture to itself anything so magnificent as such a fountain of liquid fire must be. A simple jet of water of considerable volume, thrown into the air to the height of a hundred feet, is itself a beautiful spectacle. What then must be a huge jet of glowing white lava projected to the height of several hundred feet, and with what an awful thundering sound must it come tumbling to the ground, thence to rush as a roaring torrent down the mountain's side!

      Lava, when congealed, differs in its consistency according as it is near the top or near the bottom of the stream. When near the top it is porous, owing to its rapid cooling; when near the bottom it is dense, owing to its slow cooling and the great pressure to which it is subjected. When the lighter superficial lava is brought suddenly into contact with water, as when a lava-stream enters the sea, it becomes still lighter and more porous—forming the well-known substance called pumice, so much used for polishing. It may be regarded as the solidified froth of lava, and is so light that it floats on the surface of water.

      The lavas of different mountains, when cooled and hardened, differ much in their appearance and composition. Among those of Iceland is found the beautiful black volcanic glass named obsidian. It is a good deal used for ornamental purposes; for it possesses the peculiar property of presenting a different appearance according to the manner in which it is cut. When cut in one direction it is of a beautiful jetty black; when cut across that direction it is glistering gray. The lavas of Vesuvius are generally of a brown colour, and are also used in the arts. In them are found the beautiful olive-green