SUBMARINE MOUNTAINS 53 



lected under a tight cover made by the superficial beds of 

 frozen lava. As this gas accumulates, its tension may increase 

 to the point of overcoming the combined weight and strength 

 of the imprisoning cover. Normally the tension is relieved by 

 escape of the gas through cracks, causing moderate explosions 

 but no great change in the island topography. On the other 

 hand, the famous evisceration of Krakatoa Island of the East 

 Indies illustrates the fact that with terrifying suddenness a 

 deep-sea volcano may be modified on a grand scale. Dr. B. G. 

 Escher has drawn the multiple diagram of Figure 26, showing 

 how the original and broad Krakatoa Island (at top of the 

 series of stereograms) was long ago reduced by evisceration, 

 perhaps accompanied by subsidence, to three small islands. 

 The successive stereograms below are intended to illustrate 

 how eruptional additions were made to the dry land, and how 

 in 1883 the main island was once more torn out by what was 

 probably the most violent explosion recorded in human his- 

 tory. Fortunately for colonizing man, such murderous episodes, 

 eviscerations of islands, are rare, though doubtless in prehis- 

 toric time, through the geological ages, a good many high 

 islands have been so transformed into "basal wrecks" or even 

 great pits in the sea floor. 



Special conditions favor landsliding on a large scale, the 

 second kind of sudden change interrupting the normal evolu- 

 tion of island topography. The constructional slopes of the vol- 

 canic mountains are comparatively steep, and gravity is always 

 at work on the slopes. And, secondly, beneath those slones 

 there are outward-dipping, weak beds along which the slipping 

 may take place. Lubricants of the kind include water-soaked 

 tuffs and volcanic ash. We know, too, that the older layers of 

 lava and ash of a great cone are subject to invasion by out- 

 wardly inclined sheets of younger, liquid lava, serving as a 

 different kind of lubricant. Perhaps in some instances violent 



