234 THE ERUPTION OF KRAKATOA IN 1883 



The loss suffered by the pitchstone during fusion is nil, while the ob- 

 sidian loses from i, to 5 or 6 per cent of its weight. There is no 

 doubt that the pumice ejected in such enormous quantities was formed 

 by the expansion of the volatile gases contained in the obsidian. The 

 condition in which the water and gases existed in the rock is a problem 

 worthy of study. 



The pumice contains the same crystals as the obsidian, and also 

 little pieces of a black glass that have been shown to be identical with 

 the pitchstone already referred to. Mr. Verbeek has calculated that 

 the pumice and dust form 95 per cent of all the materials ejected. 

 Some of the smaller pieces of pumice were thrown to a great height, 

 and were carried by the wind some distance from the scene of eruption ; 

 the greater quantity, however, fell close to the volcano, covering the 

 sea, in places, to such a depth as to effectually hinder the progress of 

 vessels. 



As the larger pieces of pumice fell back into the crater, they came 

 in collision with others that were being thrown out, and in this way the 

 pieces were broken, the finer particles forming the volcanic dust. The 

 dust fell in great quantities in the vicinity, and samples have also been 

 collected from many localities, some as far as a thousand miles from 

 the island. Its exceeding fineness is shown by the fact that it pene- 

 trated everywhere, no box or watch being sufficiently tight to exclude 

 it. The particles composing the dust have been separated and ana- 

 lyzed, and their optical properties investigated. The appearance and 

 composition of the dust varies considerably ; that found in the vicinity 

 is heavier, darker in color, and more basic in composition than that 

 gathered at a greater distance from Krakatoa. This difference is due 

 to the \\Qd,Yy particles of magnetite pyroxene, and feldspar it contains. 



The quantity of dust thrown into the air is not known, but it was 

 probably much less than that formed at some previous eruptions. The 

 work of trituration, however, continued about three months, and the 

 last paroxysmal explosions of Krakatoa were altogether of exceptional 

 violence and carried the finer particles high into the rarer regions of the 

 atmosphere. From the data supplied by the vessels on which the dust 

 fell, it is found to have covered an area of 1,100,000 square miles. 

 From the depth of the material it is calculated that the quantity of dust 

 was equal in bulk to 3 J cubic miles, while the entire mass of ejected 

 matter equalled about 4^ cubic miles. These figures, however, are 

 estimates and probably only approximate the actual quantities. 



