Masterpieces of Science 



jected into the cracks, forming, when cold, huge 

 rocky ribs known as dykes. In other cases, the 

 chimney gets choked up by a plug of hard lava, 

 and new vents may then be opened on the side of 

 the cone. Fig. 53 is an ideal section of a volcano, 

 showing the dykes of lava running through the 

 stratified deposits, and also showing two minor 

 cones a b, thrown up at points where the volcanic 

 matter has been able to force its way to the sur- 

 face. Mount Etna is remarkable for having its 



FIG. 53. Diagrammatic Section of Volcano, with Dykes 

 and Minor Cones 



flanks studded with parasitic cones, some of 

 which are of considerable size, one being upward 

 of nine hundred feet in height. 



After a volcano has long been silent and the 

 large crater has been more or less filled, partly by 

 ejected materials which had fallen back into 

 the cavity during the last eruption, and partly 

 by matter washed in by rain, renewal of activity 

 through the old channel may give rise to the 

 formation of a new cone seated within the old 

 crateral hollow. Great changes may indeed be 

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