

DIFFERENT TYPES OF VOLCANIC ERUPTIONS. 181 



short interval, on the 2Qth of September, the ground opened, and thick 

 clouds of ashes and rock fragments burst into the air, with the 

 appearance of flames along the fissures. So sudden was the outburst, 

 that it became known to the farm-servants by ashes falling on their 

 hats. The two rivers were swallowed up in the open chasms, and in a 

 short time the eruption built up six volcanic cones which extend in a 

 line. One of these, called Jorullo, rises to a height of 1600 feet above 

 the plain, which was formed around the volcano from ashes and 

 basalt, poured out to a depth of 500 feet, in a deposit which thins 

 away so as to give a convex appearance over an area of about four 

 square miles. 



When Baron Humboldt, twenty years later, crossed this district, 

 termed the Malpais, the heat in a fissure in lava was still sufficient to 

 light his cigar ; and he records that the rivers, which had disappeared 

 on the day of the eruption, flowed out again a mile and a quarter 

 farther west as hot springs, which then had a temperature of 126 F. 



The whole surrounding plain was covered with thousands of small 

 cones, 6 to 9 feet high, which gave off vapour rising to a height of 20 

 to 30 feet; and the ground gave out the peculiar resonant sound 

 when struck which, is known to the Italians as the rimbombo. Since its 

 formation Jorullo has shown no signs of activity, and in this respect 

 is comparable to the Monte Nuovo, produced in the Phlegraean Fields 

 in September 1538. 



Structure of a Volcano. Although it is not possible to examine 

 far into the structure of active volcanoes, yet it is not difficult to 

 conceive of the steps by which a cone is built up. Although dust 

 may sometimes be ejected, according to Mr. Whymper, to a height 

 of 20,000 feet, and carried, as we have already seen, to great distances, 

 it as a rule is simply shot into the air, and falls down again, so that 

 most of the particles descend round the spot from which they were 

 thrown up; and the thickness of the deposit becomes less and less 

 farther out from the eruptive throat. But although the materials of the 

 flanks of the cone may thus come to be arranged in layers, something 

 like coarse gravel, which are inclined outward, some of the material 

 falls within the sloping throat, so that the layers dip towards its 

 centre; and wherever a volcano has been naturally dissected by 

 denuding agencies, as in the Auvergne and Eifel, this condition is 

 well seen. The whole mass then shows an irregular stratified appear- 

 ' ance, crossed and bound together transversely by the walls of lava, 

 termed dykes, which have been injected into fissures, opening 

 downward. 



Sequence of Events in an Eruption. The succession of events 

 which constitutes a volcanic eruption is variable in different volcanoes, 

 and at different times in the same volcano. Professor Dana tells us 

 that in 1789 Kilauea, in the Sandwich Islands, poured out an 

 enormous quantity of light pumice-like scoriae and sand which 

 darkened the air. Whereas the well-known outbursts of that volcano 

 have consisted of fluid lavas, which have sometimes run down to 

 the sea. Similarly, in the grand eruption of Vesuvius in the year 79, 



