SUBMARINE MOUNTAINS 51 



island. No human eye can witness the whole titanic battle, but 

 there is no reason to doubt that the process of upbuilding below 

 sea is like that above sea. 



Above sea we can see that explosions periodically reshape 

 the pile. The hoisting power is supplied by the tension of 

 water vapor imprisoned in old lava or of original gases im- 

 prisoned in the new, hot lava. The ash and so-called tuff made 

 by the explosions are characteristically weak materials, subject 

 to rapidly effective attack by the ocean breakers. For a period 

 of time, therefore, the top of the slowly growing volcanic 

 mountain is kept reduced to a shoal. 



Ultimately, however, a more stable cone with crater or so- 

 called caldera, now defended against the surf by interbedded 

 flows of solidified lava, is developed. Niaufou Island of the 

 central Pacific (Figure 23) represents this stage. Still later, 

 eruptions at new fissures and pipes, opened through the sub- 

 marine mass, ornament the island with many craters, as in the 

 case of the Samoan Upolu, where Robert Louis Stevenson spent 

 his last years (Figure 24). 



The essential structure of the submarine part of each island 

 can be reasonably inferred from that exposed in the subaerial 

 part, where valley-wall and sea-cliff show scores of superposed 

 lava flows with occasional thin beds of volcanic "ash." 



Long after the emerged part of the volcanic mass has been 

 deeply dissected by streams, it is reduced to a state of low relief, 

 and finally, by the unceasing wave-attack, reduced to a shoal. 

 The changes are summarized by the cross-sections of Figure 25. 

 The top section is that of the relatively young, full-grown 

 island; the other three sections portray successive stages of the 

 destruction. The detritus made by torrent and crashing wave 

 is built out to form growing shelves, as at XY and XZ. The 

 bottom cross-section shows the ultimate stage, where the island 

 is completely replaced by a shoal. The erosive attack on hard 



