CHAPTER IX 



VOLCANOES AND IGNEOUS INTRUSIONS 



A volcano may be regarded as an opening in the earth's surface 

 through which various gases and solid or molten rocks are ejected. 

 The materials brought to the surface accumulate around the opening, 

 forming a conical hill or mountain. The rapidity with which volcanic 

 cones are built up is in contrast to the slowness with which other 

 elevations are formed (p. 362), and they are able, consequently, to 

 defy the agents of erosion during the period of rapid growth. Vol- 

 canoes are, moreover, capable of destroying in a very short time re- 

 liefs which erosion would be able to wear down only after centuries 

 of work ; as, for example, when one destroys a large part of its cone in 

 a few hours. However, although conspicuous, the work of volcanism, 

 because of its limited extent is unimportant as compared, on the one 

 hand with the movements which are raising the earth's surface, and 

 on the other with erosion, which is lowering it and is universal in its 

 effects. 



How Volcanoes Begin. — The first step in the development of a 

 volcano is the opening of a passage to the surface. This opening 

 may be caused by the " blowing out " of a portion of the earth's 

 crust with the resulting formation of a funnel. More usually, how- 

 ever, it is a fissure through which gases, ash, and lava are ejected. 

 The opening through which the material is ejected is usually en- 

 larged by explosive action or by melting, and the lava and other 

 ejectamenta tend to form a ring as they fall back to earth, until a 

 hill is built up with a depression or crater in the summit. It is ap- 

 parent, therefore, that the cone is not an essential part of a volcano, 

 but is secondary to the vent, being merely a result of its action, not 

 a cause. 



New Volcanoes. — It is not often that man h?s seen the birth of 

 ;i volcano, hut a few well-known instances may be mentioned. On 

 the- shore of the Bay of Naples, amidst gardens and cottages, 2. vol- 

 cano called Monte Nuovo had its birth in 1538, and in the course of 



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