Perret — Representation of Volcanic Phenomena. 53 



eval lava.* But it is often exceedingly difficult to distinguish 

 between the two varieties and, as regards the diagram proper, 

 an emission of ash is to be represented as such, and ulterior 

 information given in writing. 



As for the nuee ardente, its complex constitution of explod- 

 ing lava, avalanche form, and cauliflower ash-cloud development, 

 makes it somewhat difficult of emplacement, but this has here 

 been done on the basis of its fundamental nature, viz., that of 

 a direct, explosive-fragmental product. This rare phenomenon 

 should not be confounded with the hot ash avalanches men- 

 tioned below. 



A study of the chart will also show that, in some cases, 

 a certain subdivision will not need to be indicated on the dia- 

 gram, as it will be understood from the other phenomena. For 

 example, in the case of strong ejection of bombs and scorias 

 directly from the active lava — especially at a volcano of the 

 liquid lava type — it would be superfluous to indicate the 

 " crater-glow." 



Another point requiring explanation, and which some may 

 be disposed to criticise, is the omission of a number of forma- 

 tions resulting from the various activities. Avalanches and 

 mud-flows will form "breches" and "conglomerates," cones 

 may be erected by fragmental ejecta, etc., but all these involve 

 the time element and are not, strictly speaking, contained 

 within the category of the phenomena and immediate products 

 of eruption, and should not, therefore, be made to encumber 

 the diagram. 



We now return to the diagram key, at the left hand " Frag- 

 mental " division. The products here are non-luminous, or 

 only partially so, according to the conditions of their ejection 

 and the temperature of the gases, and consist of debris pro- 

 jected in rapid jets or majestically unfolding volutes, or quickly 

 darting spear-head formations. These, in case of a powerful 

 emission, will form the ash "j?mo," from which descend the 

 showers of ash. When the explosive power is sufficient, angu- 

 lar blocks and bowlders are added to the ejected debris. The 

 presence of water, either volcanic or meteoric, may result in 

 the concentration of the air-borne ash into pea-like spheres — 

 the pisolites, — or, if in great quantity, precipitate the ash 

 cloud in veritable showers of mud. 



The "Effusive" division begins with inter-crateric flows of 

 lava which, with the ejecta, constitute the usual process of 

 crater filling. Lava lakes also are included in this division, 



* See Johnston Lavis : " The Mechanism of Volcanic Action," Geological 

 Magazine, October, 1909, p. 441. See also paper by the present writer in 

 this Journal, xxxiv, 405, November, 1912, written in ignorance of having been 

 anticipated by Dr. Johnston Lavis in this matter of direct ash production. 



