IGNEOUS AGENCIES. 



141 



layers into a stronger mass. 4. When the cone grows 

 very high, eruptions will take place through these fis- 



FIG. 73. Dikes in Etna. 



sures, as well as from the top crater, and thus will be 

 formed secondary cones or monticles. 5. If any of these 

 monticles cease to erupt, they will be covered up by ejec- 

 tions from the main crater or other secondary craters. 

 All these facts are shown in Fig. 74. 6. From time to 



FIG. 74. Ideal section of a volcano, ss, original surface ; mm, monticles ; m'm', 

 extinct monticles ; cr, cr, original stratified crust. 



