'a is the funnel or neck of the volcano filled with lava; b, b, the crater. The molten lava is highly charged with

Fig. 28.-Diagrammatic Section of Volcano.

elastic fluids, which continually escape from its surface with violent explosions, and rise in globular clouds, d, d, to a certain height, after which they dilate into a dark cloud, C. From this cloud showers of rain, e, are frequently discharged. Large and small portions of the incandescent lava are shot upwards as the imprisoned vapour of water explodes and makes its escape, and, along with these, fragments of the rocks forming the walls of the crater and the funnel are also violently discharged; the cooled bombs, angular stones, and lapilli, as the smaller stones are called, falling in showers, f, upon the exterior

parts of the cone or into the crater, from which they are again and again ejected. Most frequently the great weight of the lava inside the crater suffices to break down the side of the cone, and the molten rock escapes through the breach. Sometimes, however, it issues from beneath the base of the cone. At other times, finding for itself some weak place in the cone, it may flow out by a lateral fissure, g. In the diagram, i, i represents the lava streaming down the outward slopes, jets of steam and fumaroles escaping from almost every part of its surface. Forked lightning often accompanies an eruption, and is supposed to be generated by the intense mutual friction, in the air, of the ejected stones, as well as by that of the steam that always accompanies an eruption (see par. 158).

156. Lava-streams.- Lava issues from the crater white-hot, and flows like melted iron; but the surface soon darkens, hardens, and cracks into fragments like huge black cinders. Before long the surface is firm enough to walk upon, while the mass below is still fluid. The end of a lava-stream has been seen creeping on slowly nine or ten months after the eruption had ceased; and the interior of the mass may retain its heat for twenty years and more. The mass of matter ejected is sometimes enormous. In 1783 the volcano of Skaptur Jökul in Iceland sent forth two streams of lava, the one fifty miles long, and in places fifteen broad; the other forty miles long by seven broad. In some places the lava exceeded 500 feet in thickness.

157. Volcanic Ash.-Not less important than the lava are the scoric and ashes ejected from volcanoes. The fine dust called ash may arise partly from the trituration before spoken of; it may also partly consist of small particles of the molten lava carried up

by the rush of steam-lava spray, in short, cooled in the air.

158. Volcanic Steam.-Steam plays an important part in volcanic eruptions; if indeed it be not the chief cause of them (see par. 162). The steam that issues from a volcano becoming cooled in the air, condenses and falls in torrents of rain. This, mingling with the falling and fallen ashes, descends the slopes in avalanches of mud. It was such mud-lava from Vesuvius in 79 A.D. that overwhelmed the cities of Herculaneum and Pompeii.

159. Bands of Volcanic Action.-About 400 active volcanoes have been enumerated. They are mostly arranged in lines or bands, and as a rule are placed not far from the sea, being partly along the margins of continents, and partly in chains of oceanic islands. The Pacific Ocean is bounded by an almost unbroken line of active volcanoes. Beginning in the New South Shetlands, where there is an active volcano in lat. 62° 55′ S., we pass to Tierra del Fuego, and then on to the Andes, which are, throughout their whole course, volcanic. The line is continued northwards by the burning mountains of Mexico and of North-western America, and the Aleutian Islands carry the chain across to Kamtchatka on the Asiatic side. Here turning southwards, the line may be traced through the Kurile Islands, Japan, Formosa, the Philippines, Moluccas, New Guinea, and the Solomon and New Hebrides groups, to New Zealand. From Celebes, a branch proceeds in a north-westerly direction through Java and Sumatra, to Barren Island in the Bay of Bengal; and even beyond this we find a region in Northern India subject to earthquakes, which may lead us, on the one hand, to the volcanic region in the Thian-shan Mountains, or, on the other, through Asia Minor to the Greek

Archipelago, Sicily, Naples, and on to the Canaries. Two-thirds of the active volcanoes are on islands, and the greatest development of volcanic activity in the globe is manifested in Java and the adjacent islands. Java has a line of 45 vents, 28 of which have been seen in eruption. The only important line of volcanic mountains at a distance from the sea is in the range of Thianshan, in Central Asia.

160. Earthquakes.-The quakings of the earth are of all degrees of strength, from a slight tremor to the most violent commotion. During a heavy shock, trees sway from side to side like the masts of a ship in a swell. Buildings rock to and fro, and are knocked down. Sometimes the ground is rent open in great tracts, miles in length, which engulf whatever is on the surface. The sea is observed to retreat for hundreds of yards, and then to return in a huge billow that bursts over the land, annihilating entire towns. Sometimes abnormal waves come rolling to the shore, caused by an earthquake under the bed of the ocean at too great a distance to be propagated through the solid crust.

Earthquakes of a mild kind are not unknown in Great Britain. The part where they are most frequently experienced is a district in Perthshire, of which Comrie is the centre. The most violent earthquake experienced in the Old World was that which destroyed the city of Lisbon in 1755. A hollow thunder-like sound was first heard, and was immediately succeeded by a violent upheaval of the ground, so that in three minutes the greater part of the houses were overturned. Other shocks followed, and altogether 60,000 lives are estimated to have been lost in Lisbon and its neighbourhood. The ocean first retreated and then advanced in a mass fifty feet high, and a violent flux and reflux of the

con

ocean extended along the coasts of the Peninsula, and was felt as far as the British Islands.-In 1868, a terrible earthquake devastated Peru and Ecuador. The vulsion extended the length of 2000 miles, reducing towns and cities to ruins, and burying the inhabitants by thousands. The shipping in the ports was overwhelmed and wrecked. An American man-of-war was carried a quarter of a mile inland, and lost only one man.-In 1880-81, the city of Agram, the island of Ischia, and that of Chios, were the scenes of violent earthquakes. In Chios upwards of 250 shocks were experienced, 9000 persons perished, and barely twenty houses were left habitable in the whole island.

161. Earthquake Bands.-The regions of the globe where earthquakes are most frequent and violent are those where active volcanoes exist. The most noted earthquake district in the New World lies along the coast on the west side of the Andes, and on the northern declivity of the mountains of Venezuela. In the Old World the earthquake district of the Mediterranean and of Central Asia extends from the Azores and Canaries on the west to Lake Baikal on the east. Within this zone occurred the great earthquake of Lisbon, and the many disturbances that have shaken Italy. An earthquake district connected with Iceland embraces the north of France, Great Britain, Denmark, and Scandinavia.

162. Causes of Earthquakes and Volcanoes.-The two phenomena are evidently closely associated. Not only do they occur in the same regions, but they coincide in time. During violent volcanic eruptions, earthquakes take place; and during earthquake disturbances, volcanoes are observed to be more active. Hence the two things are most likely owing to the same cause. A growing belief among geologists is, that the chief cause