VOLCANOES AND IGNEOUS INTRUSIONS 



315 





X 



As long as a volcano remains active, the ravages of rain and 

 torrents are repaired by the material ejected, but when it becomes 

 extinct the work of 



denudation contin- f \^ 



ues uninterruptedly. 

 The rate of erosion 

 varies greatly, de- /I 



pending upon the 

 nature and structure 

 of the materials and 

 upon the climate. 

 Cones composed of ^fe*^>/j 

 cinders are 

 to endure a 

 time than 



coarse 



likely 



longer 



Fig. 310. — Rocks, St. Paul Island; a volcanic cone 

 dissected by the waves' until the crater has been reached, 

 forming a harbor. 



those of dust. They 



are more porous and therefore absorb the rain falling on them to so 

 large a degree that little water is left for erosion. Even before a 

 volcano becomes extinct, deep V-shaped valleys are cut into its sides. 

 We find also that the dust and ash are in layers, and that sometimes 



black beds (Fig. 311) 

 composed of disin- 

 tegrated ash and 

 humus, varying from 

 a few inches to several 

 fe°et in thickness, are 

 interbedded with the 

 ash. These black 

 beds are ancient soils 

 and prove that in the 

 past the volcano ex- 

 perienced many years 

 of inactivity, which 

 were followed by 

 eruptions. 



After prolonged 

 erosion it often hap- 

 pens that long, wall-like bodies of hardened lava, called dikes (p. 

 324), are exposed. These dikes were formed during eruptions, when 

 the force of the explosions or the pressure of the column of lava in 



Fig. 311. — A ravine (baranca) in the side of the vol- 

 cano Toluca, Mexico. The light-colored deposit is vol- 

 canic ash; the dark bands are ancient soils which prove 

 long periods of quiet after periods of activity. 



