246 THE ERUPTION OP VESUVIUS IN APRIL, 1906. 



comparable in their effects to trachytic and granitic magmas. The 

 instability of leucite in the presence of f umaroles, gives a better com- 

 prehension of the reasons why this mineral is so rare in plutonic 

 rocks, and for its constant transformation (into orthoclase or into 

 orthoclase and nephelite) in some granular rocks (syenites with 

 pseudoleucite) where it can crystallize under special conditions, but 

 can not maintain itself. 



Conclusions. 



Knowing the different phenomena that have successively taken 

 place in the eruption of 190(), it is in order to consider the place occu- 

 pied by this eruption among the preceding eruptions of Vesuvius. 

 This investigation is facilitated l)y an interesting note by Mercalli on 

 the succession of eruptive phenomena of this volcano. 



Precise observations date from the great eruption of lOSl, and 

 since that time the activity has been almost continuous. The nine 

 eruptions noted in the course of the fifteen centuries that followed the 

 Phinian outburst furnish us with but meager information. 



The eruptions are divisible into two groups — those exclusively 

 explosive and those in which explosions have been accompanied by 

 ejections of lava. 



The eruptions exclusively explosive occur ordinarily after periods 

 of repose. They begin by small Vulcanian explosions, followed soon 

 by a Strombolian explosion accompanied by violent detonations, and 

 terminate by Vulcanian explosions more or less violent, launching 

 into space a great quantity of fine ash. The eruptions of A. D. 79, 

 of 472 (in which ash is reported to have been transported as far as 

 Constantinople), those of 1649 to 1660, of May, September, and 

 December, 1900, and of March and April, 1903, are examples. 



The eruptions that have produced lavas are also divisible into two 

 groups — those in which lava starts from the flanks of the cone 

 (lateral eruptions) and those in which the point of outflow is exterior 

 to the cone (eccentric eruptions). 



The lateral eruptions are most frequent, and they begin almost 

 always in the same manner. Strombolian explosions fill the crater 

 of the preceding eruption, building there a small terminal cone ; the 

 melted magma mounts high, fills the space between the cone and the 

 edges of the old crater, and often pours over upon the slopes of the 

 main cone ; f umaroles appear above and a fissure is formed ; explo- 

 sions destroy the terminal cone ; earth movements shake the moun- 

 tain, and the Strombolian explosions diminish or cease ; a fissure opens 

 high in the cone, lava flows from it for a day or two; then another 

 opening appears at a lower level. 



