248 THE ERUPTIOK OF VESU^TUS IN APRIL, 1906. 



explosions of material completely solidified, has accumulated breccias 

 identical in structure with those of rhyolites, trachytes, and acid 

 andesites. It has, then, reproduced in a few days the secular history 

 of Mont Pelee, and brought another argument in support of the 

 idea mentioned at the beginning of this pajDcr — that the form of 

 dynamism is essentially a function of the physical condition of the 

 magma at the beginning of the eruption, and the recent eruption has 

 further shown how easily the form of d^vnamism changes with this 

 essential condition. 



In conclusion, this eruption throws a definite light upon that 

 which destroyed Pompeii in the 3'ear T9, and the explosive part of it 

 is undoubtedly the reproduction of that described by Plin}^ The 

 disaster at Ottajano and San Giuseppe is, then, the repetition on a 

 small scale of the destruction of Pompeii, but with this difference — 

 that the j^umice of 79 is constituted very probably not of ancient 

 debris like the scoriae of 1906, but of new magma. 



