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THE POPULAR SCIENCE MONTHLY 



lava by the action of the escaping steam. Some such miniature vol- 

 canoes, formed on Vesuvius in 1872, were so small that they were car- 

 ried away on boards to be employed as illustrations in the lecture-rooms 



Fig. 5. Concentric Folds on Mass of Cooled Lava. 



of the University of Naples. When very viscous lavas are forced 

 through fissures, they arrange themselves in concentric masses, like 

 that represented in the illustration (Fig. 5), which is from New Zea- 



Fig. 6. Mass of Cooled Lava formed over a Spiracle on the Slopes of Hawaii. 



land. More liquid masses give rise to variously shaped structures, the 

 bottle-shaped heaps of the "petrified fountains" (Fig. 6), or forms illus- 

 trated by the groups (Fig. 7) of small cones from Vesuvius. The con- 



Fig. 7. Group of Small Cones thrown up on the Vesuvian Lava-Current of 1855. 



traction of lava in cooling tends to produce fissures through the mass, 

 breaking it up into prisms. Hence we have the columnar structure 



