226 THE ERUPTION OF KRAKATOA IN 1883 



fusible than those that contain a smaller proportion, and it is well 

 known that among the minerals some, as the zeolites, are fusible, 

 while others, containing no water, are practically infusible. 



The primary cause of a volcanic outbreak is the heat that is present 

 below the surface of the earth, but it is the slow, very slow, percolation 

 of a small quantity of water through the rock that is supposed to aid 

 in the fusion. During this process gases are produced that gradually 

 increase in pressure, until at last force enough is generated to overcome 

 the weight above, and an eruption is the result. It has been suggested 

 that a sudden inrush of the sea through a fissure to the molten lava 

 would give rise to an evolution of steam sufficient to cause an eruption, 

 but Professor Judd claims that the effect upon the lava would .be to 

 form a crust of chilled material under the water, which would confine 

 the volatile products generated below, and only when the pressure of 

 these gases became greater than the load above would an outbreak 

 occur. 



Most of the material ejected from a volcano falls close to the vent, 

 thus forming a cone that continues to increase in height, until an erup- 

 tion of exceptional violence entirely removes the upper portion, leaving 

 a ring or crater that Darwin has aptly called the "basal wreck." 

 Within this crater a new cone may be built up, and the former outline 

 of the mountain be restored, to be once more destroyed in its turn. 



In the Strait of Sunda, between the islands of Java and Sumatra, 

 are portions of an old crater ring, which, rising above the water, form 

 the islands of Krakatoa, Verlaten, Lang, and Polish Hat. One of the 

 grandest lines of volcanic activity on the earth runs through these 

 islands, and is continued eastward through the islands of Bali, Sum- • 

 bawa, Flores, and Timor. There is evidence that a second fissure 

 crosses this main one at nearly right angles, and at the intersection of 

 these two lines KJrakatoa is situated. Notwithstanding the important 

 position it occupied, until recently this little island received but a 

 small share of the attention that geologists have given to volcanoes in 

 general. But this is not to be wondered at when it is remembered that 

 Java contains " no fewer than forty-nine volcanic mountains, some of 

 which rise to a height of 12,000 feet above the sea level," and a few are 

 in a state of almost constant activity. In recent geological times 

 Krakatoa, though now insignificant, rivaled these mountains in size, 

 for if the conclusions that geologists have drawn are correct, the present 



