RISE OF MOLTEN ROCK TO THE EARTH'S SURFACE 123 



toward the chimney of the volcano at all points within the crater 

 rim, and outward or away from it at all points outside (Fig. 119). 

 If the experiment is carried so far that at its termination sand 

 slides down the crater walls into the chimney below, the inward 

 dipping layers will be truncated, or even removed entirely, as 

 shown in Fig. 119 6. 



The profile lines of cinder cones. The shapes of cinder cones 

 are notably different from those of lava mountains. While the 



FIG. 120. Diagram to show the contrast between a lava dome and a cinder cone. 

 A A A, cinder cone ; BabC, lava dome ; DE, line of low cinder cones above a fissure 

 (after Thoroddsen). 



latter are domes, the mountains constructed of cinder are conical 

 and have curves of profile that are concave upward instead of 

 convex (Fig. 120). In the earlier stages of its growth the cinder 

 cone has a crater which in proportion to the height of the moun- 

 tain is relatively broad (Fig. 99, p. 104). 



Speaking broadly, the diameter of the crater is a measure of 

 the violence of the explosions within the chimney. A single series 

 of short and violent explosive 

 eruptions builds a low and 

 broad cinder cone. A long- 

 continued succession of moder- 

 ately violent explosions, on the 

 other hand, builds a high cone 

 with crater diameter small if 

 compared with the mountain's 

 altitude, and the profile afforded 



is a remarkably beautiful Sweep- FIG. 121. Mayon volcano on the island 



ing curve (Fig. 121). Toward 

 the summit of such a cone the 



of Luzon, P.I. A remarkably perfect 

 high cinder cone. 



loose materials of which it is composed are at as steep an angle 

 as they can lie, the so-called angle of repose of the material; 

 whereas lower down the flatter slopes have been determined by the 

 distribution of the cinder during its fall from the air. When one 



