52 Perret — Representation of Volcanic Phenomena. 



modern activity, being divided, when the emission is strongly 

 intermittent, into puffs, rings, vortices, etc. 



Stronger explosions form waves of aerial concussion, often 

 destructive, as at Stromboli in 1907, when most of the win- 

 dows on the island were shattered to fragments. At Krakatoa 

 the aerial concussion was of enormous power. When these 

 waves result from explosions, which are very sharp and sudden 

 as well as powerful, they may be visible near their source as 

 " flashing arcs," springing upward and outward from the 

 crater.* 



Subterranean detonation is frequently so powerful as to be 

 heard at enormous -distances. The magnitude of these deep- 

 seated manifestations is sometimes greater than that of the 

 surface eruption, as was the case at Teneriffe in 1909. f 



Catastrophic explosion is that of extraordinary violence, gen- 

 erally involving considerable deformation of the volcano itself, 

 and extending its destructive effects to the surrounding regions 

 (Vesuvius 1906, Pelee 1902, Krakatoa 1883, etc.). 



The " Fragmental " division, on the right, represents those 

 products issuing most directly from the active lava, commenc- 

 ing with that of the most liquid consistency. 



Lava Fountains, with the constant product of their scattering 

 jets — the filamentary lava, or " Pele's hair." Glass foam — the 

 so-called thread-lace scorise, or " limu," and the interesting 

 little lava drops — " Pele's tears." % Section h represents ejecta 

 from a somewhat more viscous quality of lava, the vitreous 

 sand often formed from the surface material in great quantities 

 by the expanding gases, the well-known lapilli and scoriae and 

 the figured projectiles and bombs. In section c we have ejected 

 pumice and that marvelous result of rapidly exploding viscous 

 lava which is the ww^e ardente. 



A digression must here be made in order to point out that 

 the diagram is not, and cannot be, concerned with the origin or 

 cause of the phenomena, nor with the chemical nature of the 

 products. For example, explosion may be due to juvenile 

 gases or to those of purely "phreatic" origin, but this is a 

 matter for ulterior investigation and description — the diagram 

 records the physical phenomenon. Similarly, the water of a 

 mud-flow may be volcanic or wholly meteoric, or it may result 

 from the melting of snow by hot volcanic gases — this is matter 

 for the observer's notes. And — more important still — ejected 

 ash may be due to the trituration of " old material," or it may, 

 on the contrary, consist of finely divided magma from the lower 

 parts of the conduit and, therefore, be a direct ejection of co- 



* This Journal, xxxiv, 329, October, 1912. 



f Volcanological Eeview, No. 1. 



X This Journal, xxxv, 611, June, 1913. 



