692 DYNAMICAL GEOLOGY. 



required. About Vesuvius and many other volcanoes incrustations of com- 

 mon salt and other chlorids form during an eruption in places a little distant 

 from the scenes of intensest action ; and these, as well as the muriatic acid, 

 appear to show that sea- water gains access to the lavas ; and, if so, fresh waters 

 also may. The steam may come partly from the depths of the lava, and partly 

 from superficial waters. 



2. Volcanic Phenomena. 



1. Rising and projectile effects of escaping vapors. — The water 

 and other vaporizable substances within the lava are under a press- 

 ure of about 125 pounds to a square inch for every 100 feet of 

 depth. Owing to the heat and their consequent expansion, they 

 slowly rise in the heavy, viscid liquid ; as they rise, they keep ex- 

 panding, until, nearing the surface, they begin to take the form of 

 vapors, and finally break through. 



The bubble or vapor in boiling water has projectile force enough, 

 as it breaks at the surface, to throw up water in jets to a height of 

 two or three inches. Were the resistance greater, as in a more 

 dense and viscid liquid, the bubbles would become larger by addi- 

 tions before they could escape ; the force would therefore be greater 

 and the jets higher. In lavas which have the freest liquidity, as 

 those of Kilauea, the jets are thirty to forty feet high. Conse- 

 quently, a surface of liquid lava, as in the lakes of lava in Kilauea, 

 is covered throughout with jets, like a vat of boiling water, and 

 there is only a muttering noise from the action. It looks like ordi- 

 nary ebullition, only the jets are jets of fiery liquid rock. They 

 rise vertically, and fall back into the pool, or on its sides, before they 

 have cooled. A lake 1000 feet in diameter (at a, fig. 969) was there 

 in brilliant play over its whole surface when visited by the author 

 in 1840 ; and, in more active times, a large part of the area of four 

 square miles has been in this boiling state. 



If the lavas be less liquid, the vapors are kept from escaping, by 

 the resistance, until they have collected in far larger bubbles, and, 

 when such bubbles burst, the projectile force may be enormous ; it 

 carries the fragments far aloft, to descend in a shower of cinders of 

 great extent. 



Such bubbles, rising and bursting, were seen by Spallanzani in the crater of 

 Stromboli, a high cinder-cone in the Mediterranean, north of Sicily. In times 

 of moderate action at Vesuvius, the outbursts of cinders occur every three to 

 ten minutes ; but in a period of eruption they are almost incessant. Accord- 

 ing to Sir Wm. Hamilton, the cinders rose to a height of 10,000 feet at the erup- 

 tion of 1779, — a height indicating a vast projectile force. Occasionally masses 

 of lava are thrown up which descend like huge cannon-balls, having been 

 rounded by the rotation before they had cooled, and rendered compact exter- 



