74

HOT SPRINGS. VOLCANOS.

[CHAP.

Another well-known spring

well known as the Sita-Khúnd. of hot water is one that gushes out at Gangútri at the source of the Ganges, and many others are known in different parts of India. All these springs come from a great depth, and their heat is derived from rocks deep in the earth, through which the water has passed before coming to the surface. In volcanic countries, hot springs are very common; and they occur also among hills and mountains, where the rocks have been much disturbed at some former period; as is the case both in the Karakpúr hills and the Himálaya. They are never met with in the middle of great river plains, where the deposits formed by the river are of considerable thickness. It would be in vain therefore to search for such springs in the rice fields of Lower Bengal.

But the most striking proof of the high temperature of the earth's interior is afforded by volcanos. A volcano is the orifice or opening of a vent or channel, which communicates with the interior of the earth; and through which heated water and steam, various gases, and stony matters frequently liquefied by heat, are ejected, in some cases continually, in other cases at intervals of many years; while the intervening periods are periods of rest during which the orifice is completely or partly closed.

In India there is at the present day no active volcano. The nearest is that of Barren Island, an isolated volcano forming a small island in the Bay of Bengal, not far from the Andaman Islands. The accompanying figure is a representation of this island, reduced from one given in the IVth Volume of the "Asiatic Researches." It consists of a conical hill, surrounded by a ring-shaped mountain, once continuous, but now broken down on one side, as shown in the figure; and at the summit of the former is an orifice, through which sulphurous gases are emitted, and which is termed the crater. Both the conical and ring-shaped hills are formed of stony matter which has been ejected from the crater during former periods of activity; for there has been no eruption of this particular volcano for the last seventy years. In 1795, Captain Blair describes the smaller

v.]

'BARREN ISLAND.'

75

central cone, as throwing out showers of red-hot stones of several tons weight, and enormous volumes of smoke; and Horsburg states that, in 1803, the volcano was observed to explode every ten minutes, projecting each time a column of black smoke perpendicularly to a considerable height; and in the night a fire of considerable size continued to burn on the east side of the mountain.

The whole island is formed of the materials thus ejected, including melted rock, termed lava, which has poured out from the lower part of the mountain, and is now solidified as a hard, black, cindery-looking mass. The whole of the cavity within the ring-shaped mountain is an old crater; and the cone within, which is nearly 1,000 feet high, has been formed by the matters ejected when the activity of the volcano had much declined. Were it again to become as active as it once was, the first act of the eruption would probably be to destroy the greater part of this cone, and it

76

VESUVIUS.

TUMBORO.

[CHAP.

might be, even a part of the outer ring, a new crater being perhaps formed in a different spot. Such has been the history of the well-known volcano of Vesuvius near Naples; which, up to the year 79 of the Christian era, was thought to be extinct, and was covered with cultivation, large towns being built around its foot. In that year it broke out again in a very violent eruption: a great part of the old ring was destroyed, a new cone and crater formed, and the cities Herculaneum and Pompeii were completely buried beneath the ashes ejected from the mountain. For more than 1,500 years, their existence remained a mere historical tradition, but they have now been excavated, and not only have the public buildings, the theatres, baths, dwelling-houses, and shops of the ancient Roman inhabitants of Pompeii been laid open to us, with their wall paintings and sculpture as fresh as they were 1,800 years ago, but all the details of their domestic appliances are brought to light; and by pouring plaster of Paris into the cavities left by the bodies of the buried inhabitants, casts have been taken, which reproduce the external forms of persons who perished in a catastrophe 1,800 years ago. That this city has been so completely preserved, is owing to its having been buried. beneath the light volcanic ash or dust which forms a large part of the ejected matter of volcanic eruptions. Cases are on record, in which this has been poured forth in such vast quantities as completely to obscure the sky. Such was the case in the great eruption of Tumboro, a volcano in the island of Sumbawa, in 1815. It is stated that "the darkness occasioned in the daytime, by the ashes of this eruption, in Java, (more than two hundred miles to the westward,) was so profound, that nothing equal to it was ever witnessed in the darkest night;" and some of the finest particles were transported to the islands of Amboyna and Banda; which last is 800 miles east from the site of the volcano. The cinders floating on the sea to the west of Sumatra "formed a mass two feet thick, and several miles in extent, through which ships with difficulty forced their way."

v.]

VOLCANIC ZONES. EARTHQUAKES.

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Volcanos rarely occur isolated. Most frequently they are arranged in a linear series, and some of these lines of volcanos are many thousand miles in length. Thus, Barren Island is now the northernmost of a line of volcanos which extends through the whole length of Sumatra, Java, and the smaller islands Sumbawa, Timor, &c., up to New Guinea. Another line runs through Japan and the Philippine Islands to the same point; and a third extends to the eastward, through the Solomon Islands to the New Hebrides. In the Andes of South America, which contain some of the loftiest volcanos of the world, the arrangement is of the same character, and so also in Mexico.

Another very general character is, that most of them occur either on islands or at no great distance from the sea; and it has been thought that the percolation of water to the heated interior of the earth is one of the causes which determine eruptions. Vast quantities of steam and heated water are among the substances poured forth on such occasions, and the expansive power of highly heated steam is such as certainly might exert the enormous explosive force manifested in volcanic eruptions.

Closely connected with volcanos are those disturbances of the ground, which I have already spoken of as earthquakes. Volcanic districts are especially liable to them, and great eruptions are generally preceded by earthquakes; as if those explosive forces which produce the eruption. were struggling to find an outlet. But they are not restricted to the neighbourhood of active volcanos. Some of the most violent, indeed, have occurred in places where there is now no active volcano, and there are few countries in which slight shocks are not occasionally felt. There is no active volcano for instance in the neighbourhood of Kachh; and in Bengal we sometimes have slight earthquake shocks, although, as I have already stated, there is no volcano nearer to us than Barren Island. Most of those that we feel in Bengal proceed from the eastward; from the Irawadi valley and the country adjacent. One which was felt in August, 1858, was most severe in Ava, and

78

NATURE OF EARTHQUAKES.

[CHAP. another on the 10th January, 1869, was extremely severe in Kachar and the hills to the northward. But below Ava there is a large extinct volcano called Puppa Doung; and another, Hawshuenshan, also extinct, exists in the immediate neighbourhood of Momien. These prove that, at no distant date, (in a geological sense,) the line of volcanos, which now terminates in Barren Island, has extended far up the line of the Irawadi valley and the country adjacent to it; and the earthquakes, which proceed from this region, may be regarded as due to the same causes which were once powerful enough to produce large volcanos in the same.

tract.

An earthquake is a trembling of the ground, a kind of molecular wave or vibration, propagated through the solid earth; in the same way that if a long massive bar of iron be struck at one end with a heavy hammer, the vibration is propagated through the bar to the further end. The original cause is, very likely, that which Mr. Mallet has suggested, viz., a sudden crushing of the rocks, deep down in the earth, just beneath the place where the earthquake is most violent. The changes that precede and cause this crushing will be explained in the next chapter. Great earthquakes are very destructive, especially to buildings; which, in slight cases, are cracked and thrown out of the perpendicular, and in severe cases, thrown down in ruins.

One of the most destructive earthquakes of modern times was that which occurred at Lisbon on the 1st November, 1755. A sound like that of thunder was heard underground, and immediately afterwards a violent shock threw down the greater part of the city. In the course of six minutes, sixty thousand persons perished. The destruction was much increased by a gigantic wave, (the effect of the earthquake,) which rolled in from the sea; the water at first retired and laid the bar of the river dry; it then rolled in, rising fifty feet or more above its ordinary level. This earthquake was felt over an extent of the earth's surface, which Humboldt computes to be four times greater than