222 The Realm of Nature CHAP. 



ground water ( 313) creeps down by capillarity deep into 

 the heated layers under the crust, there combining chemi- 

 cally with the rock under pressure, but always ready to 

 resume the form of steam if the pressure is relaxed. 



294. Volcanic Materials. In addition to water- vapour 

 volcanoes throw out other gases in great abundance. Hydro- 

 gen and oxygen, resulting from the dissociation of water at 

 high temperature ( 7 1, 220), combine as they rush out, pro- 

 ducing violent explosions and great flames. These flames, 

 together with the reflection of glowing liquid rock on the 

 overhanging vapour, gave to volcanoes the popular name of 

 burning mountains. Sulphurous acid, sulphuretted hydrogen, 

 nitrogen, carbonic acid, hydrochloric acid, and the vapour 

 of boracic acid, also occur very frequently, being produced 

 by the chemical action of heat and water-vapour on minerals 

 in the volcano. Lava, or molten rock, is the most im- 

 portant of all volcanic products. Welling over the cup-like 

 hollow at the summit it flows down the sides of the mountain 

 in white-hot streams, which gradually solidify on the outside, 

 and advance like a glacier of slow- moving viscous rock, 

 ultimately hardening into crystalline igneous rocks, such as 

 basalt and trachyte. Pumice is a sponge-like glassy rock 

 which forms over the surface of certain lavas, being frothed 

 up by the vapours which are continuously given off. Scoria 

 are the rough cindery upper portions of very viscous lavas 

 formed in the same way. During eruption immense quan- 

 tities of these crusts of lava, together with stones torn from 

 the throat of the volcano, are thrown out. The finer grained 

 loose materials are known as dust or volcanic sand. A light 

 gray powder, known from its appearance as ash, is the 

 solidified spray of molten rock similarly thrown into the air 

 by the explosion of escaping vapours. 



295. Volcanic Mountains. Wherever a crack or 

 fissure of the Earth's crust allows volcanic activity to assert 

 itself the material driven out from below accumulates and 

 solidifies on the foundation of the surface rocks, which are 

 usually sedimentary, and a cone or mountain of accumula- 

 tion (contrast 303, 329) is thus piled up. If the lava is 

 very fluid and escapes from a long fissure it may flood 



