HEAT. 



267 



unknown volcanoes, high up in the mountains. The baked rocks, besides giving their red 

 tints to the country, resist erosion, as Mr. Allen states (1874), and so protect the hills 

 from denudation, and become prominent features of the region. 



Volcanoes. 



1. General Characteristics. 



An active volcano, as ordinarily understood, is a mountain or hill more 

 or less conical in shape, having at or near its top a cavity called a crater ; 

 and, within the crater, a vent or source of liquid rock and hot vapors, whence 

 proceed at times ejections of lava in streams, or else in projected fragments. 

 It is fundamentally a vent of vapors and liquid rock, which, by its projectile 

 action has been and is still surrounding itself by an elevation of more or less 



226. 



4^^^- _^- _ — 





Mount Shasta as seen from the south. Height, 14,401 feet. From a photogruph by Walking. 



conical form and pericentric structure. The ejected materials descend around 

 the vent, and by this pericentric work build up the rising volcanic cone. 

 The liquid rock and its cooled streams are the lava of the volcano, and 

 the loftily projected lava-fragments, cooled as they fall, which may be for 

 years the only ejected material, are the cinders (lapilli of the Italians), or 

 volcanic ashes when fine, or volcanic scoria when made up of light cellular 

 pieces. 



A view of Vesuvius as it appeared in July, 1834, is given in Figure 225. 

 The main body of the mountain is made of lava streams, with some layers 



