XII 



The Crust of the Earth 223 



extensive tracts of land with nearly level sheets. Such lava 

 floods now occur very rarely, although they were common in 

 past ages. Volcanoes are usually connected with their 

 subterranean lava-stores by a comparatively narrow pipe, in 

 which the lava wells up and overflows on all sides. A very 

 hot and fluid lava forms a hill of gentle slope ; a cooler or 

 viscous lava, which solidifies before it flows far, builds 

 a steeper mound. In either case the centre is formed by a 

 trumpet-shaped hollow called the crater, the rim of which 

 is raised by each successive outflow. In some instances 

 cones are built up round the orifice of a volcano before the 

 flow of lava commences, and are composed of volcanic 

 ashes, pumice, and broken stones, etc., the ejection of 

 which is the prelude to an eruption. When compacted by 

 the pressure of its own weight, and cemented together by 

 the chemical action of rain, such a deposit forms the rock 

 known as volcanic tuff. When fluid lava rises in the pipe 

 of a tuff cone the pressure it exerts frequently bursts an 

 opening in the side, through which a stream escapes. 

 When the force of the eruption is small and the walls of 

 the cone strong, the ascending lava may cool down in 

 the funnel and seal the volcano by solidifying. The most 

 common form of volcanic mountain is of composite struc- 

 ture, being built up of alternate layers of tuff and flows of 

 lava. Such a cone grows slowly, and, as represented in 

 Fig. 43, is the outcome of several periods of activity and 

 quiescence. The explosions which herald a new eruption 

 shake the mountain, and cracking the walls allow tongues 

 of lava to penetrate in all directions from the central shaft. 

 These sometimes force a way to the exterior and form small 

 cones on its slopes, from which streams of lava flow. 

 Sometimes they harden as dykes or walls in the fissures into 

 which they were injected. The cone cac is represented as 

 formed by a late outflow of lava, and occupies the middle 

 of an old crater which had become plugged up and was 

 then partially destroyed by an explosion. 



296. Volcanic Eruptions. Volcanoes are often classed 

 as active, dormant, and extinct. Stromboli, in the Mediter- 

 ranean, is the type of a continuously and moderately active 



