124 



EARTH FEATURES AND THEIR MEANING 



Mont Dore Province of central 

 France (after Scrope). 



makes the ascent of such a mountain, he encounters continually 

 steeper grades, with the most difficult slope just below the crest. 



The composite cone. The life 

 histories of volcanoes are generally 

 so varied that lava domes and the 

 pure types of cinder cones are less 

 FIG. 122. A series of breached common than volcanoes in which 



cinder cones where the place of , . , , -, 



eruption has migrated along the paroxysmal eruptions have alternated 

 underlying fissure. The Puys with explosions, and where, therefore, 

 Noir, Solas, and La Vache in tiie the s t ruc t u re of the mountain repre- 

 sents a composite of lava and cinder. 

 Such composite cones possess a skele- 

 ton of solid rock upon which have been built up alternate sloping 

 layers of cinder and lava. In most respects such cones stand in 

 an intermediate position be- 

 tween lava domes and cinder 

 cones. 



Regarded as a retaining wall 

 for the lava which mounts in 

 the chimney, the cinder cone 

 is obviously the weakest of 

 all. Should lava rise in a 

 cinder cone without an ex- 

 plosion occurring, the cone is 

 at once broken through upon 

 one side by the outwelling 

 of the lava near the base. 

 Thus arises the characteristic 

 breached cone of horseshoe 

 form (Fig. 122). 



Quite in contrast with the 

 weak cinder cone is the lava 

 dome with its rock walls and ^ 



, . FIG. 123. The bocca or mouth upon the 



"Clatively flat slopes. Con- inner cone of Mount Vesuvius from which 

 sidered as a retaining wall for flowed the lava stream of 1872. This 



lava it is much the strongest lava stream appears in the fore g round 



with its characteristic " ropy " surface. 



type of volcanic mountain, 



and it is likely that the hydrostatic pressure of the lava within 



the crater would seldom suffice to rupture the walls, were it not 



