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THE POPULAR SCIENCE MONTHLY. 



streams, b, which have been ejected from the crater or from fissures, 

 and flowed down the sides of the cone ; and, 3. Masses of lava, c, 

 filling up cracks in the cone, called "dikes." Most volcanic moun- 

 tains are built up of these three kinds of material, but with varying 

 arrangement and proportions. One kind very often predominates 



Fig. G. Natural Section of a Volcanic Cone in the Island of Vulcano. , crater ; b b, lava- 

 streams ; c, dikes which have clearly formed the ducts, through which the lava has risen to 

 the crater ; d cl, stratified volcanic scoria* ; e, talus of fallen materials. 



almost to the exclusion of the others. The materials falling through 

 the air upon the surface of the mountain assume a stratified arrange- 

 ment, in which the finer matters are sorted out and carried to a 

 greater distance than the others, the same as when the deposit is made 

 from water. When materials of a different character are thrown out 

 by different eruptions, the distinctions are very plainly marked. 



An opportunity was given for observing the formation of a volcanic 

 cone through all its stages in the case of the eruption of Monte Nuovo, 

 or New Mountain, on the shores of the Bay of Naples, in 1538 (Fig. 



1C-. 



Fig. 7. Monte Nuovo (440 Feet high), on the Shores of the Bat of Naples. 



7). After the neighborhood had been affected by earthquakes for 

 more than two years, on the 29th of September, at eight o'clock in 

 the morning, to be exact, water, first cold, afterward becoming tepid, 

 was observed to issue from a depression which was noticed on the site 

 of the future hill. Four hours afterward the ground was seen to swell 



