90 IGNEOUS AGENCIES. 



the eruption of Krakatoa, August, 1883, 4J cubic miles of material 

 were blown into dust so fine that it was carried by the gas-current 17 

 miles high, and some of it remained suspended for two or three years. 

 The lava which streamed from Skaptar Jokull, Iceland, in 1783, has 

 been computed to be equivalent to about twenty-one cubic miles, or to 

 the whole quantity of water poured by the Mle into the sea in one 

 year! These were, however, very extraordinary eruptions. In the 

 greatest eruptions of Vesuvius the quantity of lava poured out was not 

 more than 600,000,000 cubic feet = one square mile covered twenty- 

 two feet deep. The volume of lava poured out by Kilauea, in 1840, is 

 estimated by Dana as sufficient to cover one square mile of surface 800 

 feet deep. 



Great destruction of life is often produced by volcanic eruptions. 

 The overthrow of Herculaneum and Pompeii by ejections from Vesu- 

 vius is well known. The great eruption of Skaptar Jokull destroyed 

 1,300 human lives and 150,000 domestic animals. The eruption of 

 Etna, in 1669, overwhelmed fourteen towns and villages. In the prov- 

 ince of Tomboro, out of a population of 12,000, only twenty-six persons 

 escaped the great eruption of 1815. 



Monticules. — Eruptions occur not only from the summit- crater, but 

 also frequently from fissures in the side of the mountain. By the im- 

 mense upheaving force necessary to raise lava to the mouth of the 

 crater of a lofty volcano, the mountain is fissured by cracks radiating 

 from the crater in all directions. These cracks are filled with lava, 

 which on hardening form radiating dikes which intersect the successive 

 layers of ejections, and bind them into a stronger mass (Fig. 79, p. 94). 

 Through these fissures the principal streams of lava often pass. During 

 an eruption of Mauna Loa, in 1852, the immense pressure of the lava in 

 the principal crater fissured the side of the mountain, and a fiery fount- 

 ain of liquid lava, 1,000 feet wide, was projected upward through the 

 fissure to the height of 700 feet, and continued to play for several days. 

 Upon these fissures subordinate craters, and finally cones, are formed. 

 These subordinate cones about the base, and upon the slopes of the 

 principal cone, are called monticules or liornitos. There are about 600 

 monticules on Etna — one of them over 700 feet high (Jukes). 



Materials erupted. — As we have already stated, the materials erupted 

 are stones, lav a- streams, cinders, ashes, and gases. 



Stones. — In explosive eruptions the solid floor of the crater is often 

 blown out with violence, and rock-fragments, sometimes of vast size, 

 are thrown to great distances. 



Lava. — the term lava is applied to the liquid matter poured from a 

 volcano during eruption, and also to the same when it has hardened 

 into rock. 



Liquidity of Lava. — At the time of eruption the liquidity of lava 



