246 THE WORLD. 



from the interior of the mountain by the immense power exerted; 

 and they are ejected without having been melted. A stone of 

 109 cubic yards in volume, was ejected by Cotopaxi, and thrown 

 to a distance of nine miles. 



The force which is exerted, to cause the eruptions of lavas, or 

 liquid masses of stone, is almost beyond belief, varying according 

 to the height of the crater. The force of Vesuvius in some of 

 its eruptions has been estimated as equivalent to a pressure of at 

 least 6000 pounds on every square inch ; and of Etna, about 17,- 

 000 pounds on the square inch ; the amount of force requisite to 

 raise melted lava to the crater of Cotopaxi, would be at least 30,- 

 000 pounds on each square inch. The masses of melted matter 

 ejected, are equally incredible ; the amount thrown out by Vesu- 

 vius in 1737, was estimated at 11,839,168 cubic yards, and about 

 twice this amount in 1794. In 1660, the mass of matter disgorged 

 by Etna, according to Mr. Lyell, was twenty times greater than 

 the whole mass of the mountain, and in 1669, when 77,000 per- 

 sons were destroyed, the lava covered 84 square miles. The 

 greatest eruption of modern times, was from Skaptar Jokul, in 

 Iceland, in 1783. Two streams of lava, one fifty miles long and 

 twelve broad, the other forty miles long, and seven bioad ; both 

 avaraging 100 feet in thickness, and sometimes 500 or 600 feet, 

 flowed, in opposite directions, destroying twenty villages, and 

 9000 inhabitants. The velocity with which the melted lavas move 

 varies with the slope of the mountain, and the nature of the 

 ground, as well as the viscidity and quantity of the lava. In 

 general, a velocity of 400 yards an hour is considered quick, al- 

 though sometimes the stream flows much quicker; in flat grounds 

 it sometimes occupies whole days in moving a few yards. Lavas 

 cool extremely slow, the surface becomes soon consolidated, and 

 is such a poor conductor of heat, that the interior remains heated 

 and melted for whole years ; and currents have been mentioned 

 which were flowing ten years after emerging from the crater, 

 and they have been seen smoking twenty years after an eruption 

 of Etna. The currents of lava thrown out by successive erupt- 

 ions being placed one above the other, alternating with beds of 

 sand, scoriee, &c., form a series of inclined beds that give rise to 

 the cone of the mountain. 



