LAVA-FLOWS OF VESUVIUS AND ETNA. 185 



rock to a height of 25,000 feet, and some, thrown up from Heckla, 

 have been seen by ships 180 miles out at sea. Humboldt speaks of 

 masses weighing 200 tons ejected from Cotopaxi, while the plain 

 around the volcano is strewn for many miles with great masses of 

 lava which have been thus accumulated. 



Lava Streams. All this time the lava has been slowly rising in 

 the volcanic throat ; for in place of the pressure of superincumbent 

 rock which formerly confined it, there is now no pressure above but 

 the earth's atmosphere. The reduced pressure must operate on the 

 liquid rock much in the manner of a pump ; while below, there is 

 the lifting power of the steam, which Bischoff states to be alone 

 sufficient to expel the lava ; and above all there is lateral contraction 

 thrusting the rocks together so as to force the fluid to the surface. 

 We need not now inquire into the height to which lava streams are 

 stated to have been thrown ; it is enough to recognise the fact that 

 the molten rock rises in increasing volume. If the volcano is low, 

 lava fills the crater till the stream overflows its rim, or bursts down 

 its weakest side, and escapes like a torrent. If the volcano is high, 

 fissures may appear on the flanks, and give exit to the molten rock, 

 which then usually disappears from the crater. 



The character of the lava stream depends upon the fluidity of the 

 lava, the amount thrust out, and many circumstances of the eruption. 

 \Vhen the streams are short they are frequently permeated with 

 vapour, which expands the rock into a cindery mass ; and as these 

 vesicles burst on its surface, the rock acquires a rough or reticulated 

 and stringy aspect. Such vapour cavities are always elongated in the 

 direction in which the lava flows, and by their expansion help to 

 arrest its movement. They usually disappear when the stream is 

 large ; and then the current, instead of dividing and subdividing as it 

 descends, is apt to move irresistibly over all obstacles and spread 

 itself in a wide sheet. In the last three centuries many lava streams 

 from Vesuvius have* reached the sea, frequently descending upon 

 the town of Torre del Greco. In 1631 twelve or thirteen branches 

 reached the coast in broad masses, and still cap the cliffs. Some of 

 these streams were five miles long, and their extreme distance apart, 

 on the coast, was seven and a half miles. Among other well-known 

 streams in this district was the lava flow of 1794, one branch of 

 which passed through Torre del Greco in a sheet 1130 feet broud 

 and 15 feet thick, and extended 360 feet into the sea. The cele- 

 brated lava flow from Etna in 1669, issuing on the flank of the 

 mountain, ran for fifteen miles, passing over part of Catania, and 

 entered the sea in a mass 1800 feet broad and 40 feet thick. In 

 Hawaii, some lava-flows are 25 miles long; in Iceland, streams from 

 Skaptar Jokul have run for 60 miles ; in Greenland, they are longer 

 still, though even those are far surpassed by the great lava sheets 

 of the west of i^orth America. 



Temperature. The temperature of the lava is very variable. In 

 the Vesuvian lavas the heat is above the melting point of silver, but 

 does not always melt copper. A stream from Etna in 1766 in a 



