VOLCANOES. 91 



varies very much, depending partly upon the heat, partly on the fusi- 

 bility of the material, and partly upon the kind of fusion. In the 

 Hawaiian volcanoes the lava is a melted glass almost as thin as hone}'. 

 In Kilauea this lava is often thrown into the air by the bursting of gas- 

 bubbles, and drawn out into long threads like spun glass, which is car- 

 ried by the winds, and collects in places as a soft, brownish, towy mass, 

 called " Pete's hair." 



Physical Conditions of Lava. — Completely fused lava, when cooled 

 rapidly, forms volcanic slag or volcanic glass (obsidian) ; but if cooled 

 slowly, so that the several minerals of which it is conrposed have time 

 to separate and crystallize, forms stony lava. If it is full of gas-bubbles 

 {rock-froth), and hardens in this condition, it forms vesicular or scori- 

 aceous lava. If the quantity of gas and steam be very great, the whole 

 liquid mass may swell into a rock-froth, which rises to the lip of the 

 crater, and outpours much as porter or ale from a bottle when the cork 

 is drawn. Or the rock-froth may be thrown violently into the air, and, 

 hardening there, may fall again in cindery or scoriaceous masses ; or, 

 thrown with still greater violence, the rock-froth may be broken into 

 fine rock-spray, and fall as volcanic sand and ashes. Ashes, when con- 

 solidated by time and percolating w 7 ater, or when deposited in water 

 form tufa. Thus, there are four physical conditions in which we find 

 lava — viz., stony, glassy, scoriaceous, and tufaceous. 



Again, the liquidity of lava and its character depend much on the 

 kind of fusion. Daubree has shown that all siliceous rocks and glass 

 mixtures, in the presence of superheated water even in small quantities, 

 and under pressure, will become more or less liquid, at temperatures far 

 below that necessary to produce true fusion. At 400° Fahr., such rocks 

 become pasty, at 800° completely liquid. The same change takes 

 place at even lower temperatures if a little alkali be present. To dis- 

 tinguish this liquidity from that of true igneous fusion, which requires 

 a temperature of 2,500° to 3,000°, it has been called aqueo-igneous or 

 hydrothermal fusion. Now, very much lava at the time of eruption is 

 in this condition. Such lava, when the pressure is suddenly removed by 

 breaking up of the floor of the crater, and the contained water suddenly 

 changed into steam, is blown into the finest dust, which is then carried 

 to great height by the out-rushing steam, and falls again as volcanic 

 ashes, which may consolidate into tufa. If the heat be not sufficient to 

 produce complete aqueo-igneous fusion, the lava is outpoured as a kind 

 of rock-broth, consisting of unfused particles in a semifused mass, which 

 concretes into an earthy kind of rock. Or the material may pour out 

 only as hot mud, which concretes into a kind of tufa. In fact, every 

 variety of fusion and semifusion, depending on the degree of heat and the 

 quantity of water, may be traced, from perfect igneous fusion through 

 various grades of aqueo-igneous fusion, to the condition of hot mud. 



