234 



THE ERUPTIOX OF VESUVIUS IX APRIL, 1906. 



and the distribution of the material that buried Pompeii indicates an 

 eruption of this character. 



Fall of ash. — This includes all fine products of ejection. In tlie 

 Strombolian ejections these products are portions of the new magma, 

 thrown out in a more or less fluid condition. They are distorted 

 drops of glass or scoria of minute dimensions, broken and Avorn by 

 friction. This tyj^e has played only a small part at the beginning 

 of an eruption, and the deposits are covered by the products of the 

 Vulcanian explosions, which are composed of fragments of solid 

 rock, broken b}' the explosions, and have a complex origin and com- 

 jjosition. The coarse, ash-like, fine sand which fell at Naples in the 

 night of April 4, was black, and was composed of minute fragments 

 rich in glass, the result of the destruction of the small interior cone. 

 It was therefore formed of the new magma thrown out by Strom- 

 bolian explosion a short time before the main activity. But the fine 

 dust of the following days resulted from the trituration of old prod- 

 ucts of eruption and was formed in the process of emptying the 

 crater by Vulcanian explosions. This material, consisting at first of 

 large blocks, lapilli. and small fragments, became gradually finer 

 through the repeated trituration received by falling back into the 

 crater to be reejected. 



The rocks thus reduced to fine dust were of varied nature and had 

 undergone metamorphism and decomposition. The composition has 

 not remained constant like that of the Strombolian ejections, being 

 more like that of the scoria of Ottajano than that of the new magma. 

 The following analysis of material collected at the edge of the crater 

 illustrates this point : 



SiO, 48.00 



ALO3 10. 10 



Fe,03 3.35 



FeO 4.00 



MgO 6..^3 



CaO 11.3.0 



Na„0 3.04 



K,0 5. 20 



TiO, 1. 02 



I^Os Trace 



CI .49 



Fire loss .2.5 



100. 2!) 



The fine ash, through transportation by wind, undergoes a classifi- 

 cation according to size of grain and densit3\ It ranges in color 

 from grayish white to rose. The latter tint results from oxida- 

 tion of the ferruginous minerals, and can be produced after the fall. 

 The same fact was noticeable on Mont Pelee. The ash, at first 

 a dazzling white, l^ecame on the following day a reddish tint that 

 deepened rapidly. The white color of the Vesuvian ash, formed 

 from dark rocks, is explained by the fineness of the particles. The 

 slopes covered by this white dust present an aspect of snow fields. 

 The surface of this mobile material was first modeled by the wind 

 like sand dunes; then it began to yield to gravity and absorbed 



