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of rock thus formed has not always been allowed to repose in its ancient bed; but the force due to the fire underneath has again and again shattered the earth's crust, and lifted the internal rocks high into the air, so as to form volcanoes and ordinary mountains. The rocks thus formed by the agency of fire are termed igneous rocks (Lat. ignis, fire). Rocks of this class are, of course, void of fossils.

Thus we have two distinct classes of rocks, differing in character and origin,-the one formed by the agency of water, and the other by the agency of fire; the one stratified, the other not so; the one abounding in fossils, the other having none. It is also clear that fire and water are two forces, serving to counteract each other to some extent in their action upon the crust of the earth; for while water is always - wearing down the mountains and carrying the waste material into the ocean, where it collects at the bottom, the fire within the bowels of the earth often upheaves, by its energy, rivers of melted lava, and occasionally gives birth to a new mountain. Thus, in 1759 a celebrated volcano near Mexico, called Jorullo, made its first appearance.

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The rocks

2. Cambrian System. are chiefly sandstones and flagstones,

and fossils of a marine character.

3. Silurian System.-Roofing slates and flagstones, with ores of mercury, gold, and silver. Fossils chiefly shell-fish.

4. Devonian System.The rocks chiefly sandstone of a reddish colour, which is due to the iron found in them. Among the fossils are various fishes.

5. Carboniferous System. The rocks consist of sandstones, clays, shales, and limestones. The minerals are very valuable: marble, lead, and silver, and especially iron and coal. The fossils are mainly pines, palms, tree ferns, reeds,

and mosses, from which our coals have been formed.

6 & 7. Permian System and Triassic System. Often classed together, as the New Red Sandstone. Traces have been discovered of footprints of a kind of lizard and of a gigantic bird, allied to the ostrich. include marble, alum, jet (a hard variety 8. Oolitic System.-The minerals of coal) and fuller's earth. Traces of marsupial (pouched) animals allied to the opossum and kangaroo. Fossils of huge reptiles very abundant.

9. Cretaceous System.-It is composed chiefly of sands, dark clays, and thick beds of chalk (L. creta, chalk). There are remains of certain mammals, supposed to be those of monkeys.

10. Tertiary System.-This includes four groups of most recent formation. It is so-called, because originally the other stratified rocks were divided into two systems only, under the terms Primary and Secondary. In the Tertiary system we meet with fossils of the higher species of plants and animals.

VOLCANOES.

VOLCANOES are mountains like huge conical furnaces, which belch forth, at certain times, volumes of smoke, sheets of flame, blocks of rock, showers of cinders, and rivers of lava. When the volcano is thus in a state of eruption, it is an object of terror to all spectators; for the lava pours over the sides of the mountain like so many cascades of fire, consuming everything in its path. Indeed, it often happens that the lava makes its way out before reaching the crater (as its fire-spouting mouth is called), and having burst through the flank of the mountain, near its base, forms a small additional volcano, and spreads havoc on all the country round.

In high volcanic mountains we often find at the base of the great cone a series of small volcanoes: Etna, for instance, which is 11,000 feet high, has quite a family of small volcanoes scattered over its flanks. In fact, it is these that have in particular ravaged the surrounding district.

The most frightful eruption of Etna in modern times was produced by one of these young volcanoes, the Monte Rosa. From it issued in 1771 the long river of lava which rolled its burning waves over a distance of nine leagues, fired great part of Catania, and only ceased to destroy when it plunged into the sea amidst a most tumultuous struggle between the waves and fire.

Within the last

Vesuvius has worked still greater ruin. century, this volcano, in one of its throes, has ejected lavafloods which have annihilated small towns and villages. In 1794 its burning waves submerged all the houses in Torre del Greco, even rising above their roofs. One eruption of Vesuvius, which took place A.D. 79, is especially celebrated for the destruction of two rich and important cities which rose on its sides, Herculaneum1 and Pompeii. The former was in part invaded by a stream of lava, the second was completely buried under a prodigious shower of ashes.

The loftier volcanoes are, the less frequent are their eruptions. The lava which they vomit forth has to ascend a much higher chimney, and so a much greater force is required than in the others. Thus one of the smallest of all, Stromboli, is always throwing out flames; since the days of Homer1 it has served as a beacon to sailors approaching the Æolian Islands.

On the contrary, the volcanoes of the

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Cordilleras, which are six or eight times as high, seem condemned to long intervals of repose, and in many cases only break out from century to century.

The lofty volcanoes which lord it over the frozen summits of the Andes present a spectacle of awful grandeur to the traveller who has the perseverance and courage to reach the top. The crater of some of these fiery giants is a pro

digious basin, large enough “for a hundred horsemen to manœuvre in the presence of a thousand persons." Descending far enough to look down into the funnel of the mountain, the traveller is sometimes appalled with the sight of a burning wave rising, as if to destroy him, and then sinking again on the lake of fire within.

Most volcanoes, however, are as harmless as old lions that have become toothless; for the majority of them have spent all their energy, their fires have died out or become extinct. The number of active volcanoes is about 250, and most of these are found in the great volcanic belt which fringes the Pacific Ocean. Volcanoes, indeed, are almost always situated in the neighbourhood of the sea.

This fact has suggested the cause of volcanic action. The eruption of a volcano is the result of some violent explosion in the bowels of the earth. This explosion is very probably caused by steam. Water, we know, finds its way to great depths; for it sometimes rises again in the form of boiling springs. Now if, through the cracks and crevices which abound in the earth, water sank far enough into the interior of the earth to come in contact with the fire which always rages there, it would be converted into steam; and steam, when exposed to great heat, becomes as explosive as gunpowder. An explosion, or series of explosions, is the inevitable result; and the burning lava, or liquid rock, is ejected where it can most easily find vent, and that is through the gullet of the nearest volcano. Volcanoes, therefore, may be regarded as safety-valves, giving a free outlet to the burning matter driven on by the steam, which would otherwise rend the crust of the earth, and find an outlet for itself, causing meanwhile a destructive earthquake.

'Herculaneum.-It was buried A.D. 79, under showers of ashes and streams of lava, from 70 to 100 feet below the surface. It was accidentally discovered by the sinking of a well in 1720; many buildings and works of art have been brought to light.

2 Pompeii.- It was buried A.D. 79 by successive showers of ashes from the volcanoes. The excavation of it in modern times has thrown great light upon the life of the old Romans; such as the construction of their houses, and the kind

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EARTHQUAKES are due to the same cause as volcanic eruptions, but they are more terrible in their action. No phenomenon, indeed, offers such a dreadful spectacle as a great earthquake. The traveller who explores a dangerous volcano has generally timely warnings of its intention to emit liquid. fire; but an earthquake, in the twinkling of an eye, topples down towers and overturns houses.

The celebrated earthquake at Lisbon began on the 1st November, 1755, without the least warning. A sound like that of thunder was heard underground, and immediately afterwards a violent shock threw down the greater part of the city; and in the course of about six minutes sixty thousand persons perished. The sea at first retired, and left dry the bar at the mouth of the Tagus; it then rolled in, rising fifty feet above its former level. A large crowd of people had fled for safety to the new quay, where they might be beyond the reach of the falling houses. Suddenly the quay sank down, with all the people on it, and not one of the dead bodies was ever recovered. A great number of boats and small vessels, anchored near the spot, were also swallowed up, and no fragment of these ever rose to the surface. It is very probable, therefore, that a deep narrow

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