52 



THE POPULAR SCIENCE MONTHLY. 



up by alternate ejections of fluid lava and fragmentary materials. The 

 slopes of their sides are subject to a wide range of variation, corre- 

 sponding with the varying character and degree of liquidity of these 

 materials. The sides of the cones are liable to be rent asunder and 

 traversed with fissures, through which liquid lava forces its way and 



Fk;. it. Outlines of the Summit of Vesuvius during the Eruption of 1767. 



gives rise to new subsidiary cones, or series of cones, along the lines 

 of the clefts. These cones are called parasitic cones, and frequently 

 attain considerable dimensions, some of those on the flanks of Etna 

 being nearly eight hundred feet high. A typical example of a group 

 of such cones is given in the Island of Ischia, where several parasitic 



