140 



EARTH FEATURES AND THEIR MEANING 



tinually paring down the earth's surface and disclosing its buried 

 structures. How much more convincing than any results of 



FIG. 146. Volcanic plug exposed by natural dissection of a 

 volcanic cone in Colorado (U. S. G. S.). 



artificial excavation, as evidence of the internal structure of a 

 volcano, is the monument represented in Fig. 146, since here the 



( , lava plug stands in relief like a 



gigantic thumb still surrounded by 

 a remnant of cinder deposits. Such 

 exposed chimneys of former volcanoes 

 are found in many regions, and have 

 become known as volcanic necks, 

 pipes, or plugs. 



Not infrequently the beds of tuff 

 composing the flanks of the volcano, 

 upon dissection by the same process, 

 bring to light walls of cooled lava 

 standing in relief (Fig. 147) the 

 filling of the fissure which gave outlet 

 i'iu. 147. A dike cutting beds of to the flanks of the mountain at the 



tun in a partly dissected volcano 4-' f J.T~ j.* cij. j r 



of southwestern Colorado (after eruption. Study of CX- 



Howe, u. S. G. s.). posed dikes formed in connection 



with recent eruptions of Vesuvius 



has shown that in many instances they are still hollow, the lava 

 having drained from them before complete consolidation. 



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