54 Perret — Representation of Volcanic Phenomena. 



which is not an inconsistency, as might be supposed, for their 

 material is in constant circulation and therefore truly fluent. 

 One of the most characteristic of volcanic phenomena is the 

 "crater glow" — that searchlight projection of the Java's lumi- 

 nosity "whose reflection by the vapor clouds so closely resembles 

 true flame. It is here included as constituting a precise indi- 

 cation of the presence of active lava in a crater to which a near 

 approach may be inconvenient. Terminal overflow — a com- 

 mon occurrence at some volcanoes, especially during their 

 period of growth, — and the more frequent out-flow from lateral 

 vents, are provided with subdivision into the two main types 

 of lava — the smoothly undulating or ropy " pahoehoe " and 

 the bristling, disunited surface "a— a." Finally we have the 

 phenomenon of viscous extrusion in the form of domes, spines, 

 etc. (Pelee, Bogoslof, etc.) 



"Electric " effects are inevitable whenever a powerful gase- 

 ous emission is accompanied by abundant detritus, the volcanic 

 lightning appearing in linear flashes (Vesuvius 1906) or as 



f lobes of fire (Stromboli 1907) or in stellar form (Pelee 1902.) 

 ^hen the "pino," as a whole, acquires high potential, the 

 silent discharge from pointed objects — St. Elmo's Fire — is fre- 

 quently seen (Vesuvius 1906). 



In the "Seismic" division, volcanic earthquakes are sub- 

 divided — for convenience in the case of representing an erup- 

 tion with a single diagram — into premonitory, concomitant 

 and consecutive. Bradyseismic phenomena involve local ele- 

 vation or depression which may be transient (Vesuvius 1906) 

 or persistent (Usu-san 1910). Lastly, volcanic " Tsunami," or 

 sea waves, may be of great magnitude and destructiveness. 

 (Krakatoa 1883). 



The so-called " Solfataric " phase of volcanism is of greater 

 importance than is generally supposed, especially as regards 

 the primary fumaroles which develop in communication with 

 the interior. These may continue in operation through long 

 periods of external repose, during which time they serve as 

 indicators of the internal conditions. Those of the second- 

 ary type, forming on erupted lava or upon accumulations of 

 elastic ejecta, have, necessarily, a more ephemeral existence 

 although they may, for a time, exhibit the same temperatures 

 and characteristics as those of the primary type. Flame-emit- 

 ting fumeroles have been, perhaps, the least studied of all the 

 phenomena relating to this interesting phase of volcanic action. 



" Degradation " and deformation of the volcanic edifice may 

 occur through demolition, subsidence or collapse, either ter- 

 minal, (Vesuvius 1906) lateral, (Vesuvius, Etna, Santa Maria, 

 etc.) or subsurface (Bogoslof, Krakatoa, etc). By processes of 

 translocation, the materials left or deposited upon the upper 



