June 16, 1P05.] 



SCIENCE. 



929 



is a complete gradation. The range in the 

 physical . condition of the extruded lava is 

 from fluidity approaching that of water, to 

 rigidity such as cold lavas ordinarily present. 



The constructional shapes resulting from 

 the extrusion of lava ranging in consistency 

 from a highly viscous to a rigid condition, 

 should form a sequence from irregular tume- 

 factions and bottle-shaped towers, to angular 

 crags with nearlj^ vertical sides. A sequence 

 of variations should also be produced owing 

 to the amount of material extruded, its rate 

 of ascent, etc. The constructional topographic 

 forms should present destructional modifica- 

 tions, in case explosions occur, or portions of 

 the extruded material are dislodged. 



The results produced bj^ massive-solid erup- 

 tions, as may be judged if all the modifying 

 conditions are considered, are so varied that 

 it is not practicable at present to follow each 

 modification in detail. The place of the Pele 

 obelisk in the general sequence, however, may 

 be readily indicated. 



Under the liyiDothesis that the Pele 

 obelisk was composed of fresh lava, which 

 congealed and became rigid, at least in part, 

 as it rose through its conduit, and was pro- 

 truded high in the air, let the reader endeavor 

 to construct in fancy the topographic form 

 that would have resulted, had there been no 

 explosions and no loss owing to dislodgment. 

 Under these assumptions the outer portion of 

 the extruded mass would cool more rapidly 

 than its central portion, and the surface might 

 become rigid while the core was yet plastic. 



a 6 c 



Fio. 1. Ideal diagrams ilhistrating the shapes 

 of massive-solid volcanic extrusions. 



The extruded material at an assumed stage in 

 the eruption would have some such form as 

 is outlined in Fig. 1, a. At such a stage the 

 highly heated core of the extruded column 

 might serve as a continuation of the conduit 



and an escape of steam possibly with explosive 

 violence and accompanied by fragmental dis- 

 charges, occur in the pseudo-crater at its 

 summit. 



Providing explosions occurred as the dome 

 rose, it might be more or less shattered. 

 Vigorous explosions might remove the sum- 

 mit-portion of the ascending mass as fast as 

 it was forced out of its conduit, and the fact 

 that a massive-solid extrusion had occurred be 

 masked so as to make it appear that only a 

 fragmental-solid eruption was in progress or 

 had taken place. It should be borne in mind 

 that efl:"usive, fragmental-solid and massive- 

 solid eruptions with all of their varied accom- 

 paniments are but phases of a single process. 



Vigorous explosions during the ascent of a 

 column similar to the one suggested in Fig. 

 1, a, might cause the removal of a part of the 

 material composing it, leaving portions of its 

 solid exterior standing as angular crags and 

 tower-like spines as shown in Fig. 1, h. This 

 diagram, as the reader will recognize, is in- 

 tended to indicate in outline — all suggestions 

 as to accumulation of debris, and other sec- 

 ondary results being eliminated — the condition 

 of the massive extrusion in the crater of Mont 

 Pele at the time the obelisk was most prom- 

 inent. During the continuance of the condi- 

 tions indicated in the diagram, vigorous ex- 

 plosions might occur in the truncated summit 

 of the ' dome ' while portions of its shattered 

 periphery formed conspicuous eminences. Ex- 

 plosions of this nature, as has been observed, 

 occurred in the summit of the ' dome ' of 

 Mont Pele; and, as it is reasonable to assume, 

 the final fall of the obelisk was due chiefly to 

 the shocks caused by exi:)losions at its base. 

 The general shape that the extruded material 

 would present after being truncated by ex- 

 plosions, is roughly indicated by Fig. 1, c. 



The considerations just presented seem to 

 make it reasonable to conclude as already 

 stated, that the ' dome ' and the ' obelisk ' of 

 ]\font Pele were parts of the same massive 

 protrusion and that the various shapes as- 

 sumed from time to time were due mainly to 

 the shattering and in part the dispersion of 

 the rising mass by steam explosions ; also, 



