140 



DYNAMICAL GEOLOGY. 



of in eruptions. It is possible that there may be some- 

 times feeble flame from the combustion of II or II 2 S, hut 

 probably the so-called flame is nothing else than the 

 ruddy reflection of the glowing liquid in the crater upon 

 the smoke and cloud hanging in the air. 



Formation of Volcanoes and their Structure. It 

 is now generally admitted that volcanic cones are built 

 up mainly by their own eruptions. On. this view, their 

 origin and mode of growth may be briefly described as 

 follows : 1. The increase of heat (by causes which we lit- 



PIG. 71. Section across Hawaii. , Mauna Loa ; A", Manna Kea. 



tie understand) at the focus of the volcano thins the crust 

 in that point, until it gives way, and the melted matter is 

 outpoured on the surface around the opening. 2. With 

 every eruption the accumulated material rises higher and 

 spreads farther ; and thus a conical mound is formed. 

 The shape of this mound will depend on the kind of mat- 

 ter erupted. If it be very liquid lava, it will spread far, 



and the cone will be 

 low in proportion to the 

 base, as in the Hawaiian 

 volcanoes (Fig. 71) ; but 

 if the material be cin- 

 ders, these will pile up 

 into a steep cone (Fig. 

 72). The repeated lay- 

 ers of lava or cinders produce a stratified appearance ; 

 but this must not be confounded with true stratification. 

 3. With every eruption, the eruptive throes split the 

 sides of the cone with radiating cracks, which, filling 

 with liquid and hardening, form radiating rocky ribs 

 called dikes (Fig. 73), and these bind the lava or cinder 



FIG. 72. Section of cinder cone. 



