VULCANISM 379 



hard while the interior is still fluid. The fluid part may then 

 break out at the side or end of the hardened shell and flow away, 

 leaving a hollow crust of solidified lava. On further cooling, the 

 shell contracts and cracks, and often caves in. The hardened sur- 

 face of a lava-flow may be broken by the movement of the fluid 

 lava below, and the solid fragments be displaced and upturned 

 so as to give the surface a jagged appearance. 



Lava takes on various phases as it becomes solid. If it hardens 

 under little pressure, or at the surface, the gases and vapors which 

 it contains may expand so that it is converted into a sort of solidified 

 rock froth, called scoria; or if the pores are very small, pumice. 

 If the lava solidifies quickly without becoming frothy, it usually 

 makes volcanic glass or obsidian. If the lava cools slowly under 

 pressure, the substances of which it is composed usually crystallize 

 into various minerals. The kinds and proportions of the minerals 

 depend chiefly upon the composition of the lava. 



Gases and vapors. The gases and vapors which issue from 

 volcanoes are of many kinds. Among the commoner ones are those 

 of water (H 2 0), carbon dioxide (CO 2 ), chlorine (Cl), hydrochloric 

 acid (HC1), sulphur dioxide (S0 2 ), and hydrogen sulphide (H 2 S); 

 but with these more important ones there are many others. Some 

 of the gases are poisonous, and, as in the case of Pelee, their tem- 

 perature is in some cases so high as to be destructive to life. 



Formation of Cones 



Lava-cones. The lava usually flows away from the vent in streams 

 which solidify before running far. As the lava-streams flow in 

 different directions at different times, the total effect is a low cone 

 formed of tongues of lava radiating from the point of exit. The 

 streams often congeal before they reach much beyond the base of 

 the cone, and not rarely while they are yet on its slope. So far, 

 therefore, as the volcanic cone is formed of lava, it has a radiate 

 structure made up of a succession of congealed lava-streams. In 

 these cases the slopes are low, because the fluidity of the lava pre- 

 vents the development of high gradients. It is, however, the 

 exception rather than the rule, that the cone is made up mainly of 

 lava-streams, though the great Hawaiian volcanoes are of this class. 



