930 



SCIENCE. 



[N. S. Vol. XXI. No. 546. 



that the material extruded in a massive condi- 

 tion consisted of fresh lava which was forced 

 outward from deep within the earth. The 

 observed results appear, also, to be consistent 

 with the view that the cooling of the material 

 extruded progressed most rapidly in its peri- 

 pheral portion and that its central portion, 

 particularly after partial truncation had oc- 

 curred, was sufficiently hot to cause explosion 

 accompanied by fragmental-solid discharges.* 

 If these contentions are well founded, it fol- 

 lows that the exterior of the plug — considering 

 the dome and the obelisk as portions of a 

 single extruded mass — should be composed of 

 dense and possibly vitreous rock, and become 

 more and more scoriaceous towards its center. 



The above suggestion in reference to the 

 physical condition of the extruded material 

 differs from the conclusion reached by me in 

 the same connection, in a previous publica- 

 tion,'f which was based largely on the observed 

 granular condition of the rock present in the 

 central portion of a massive-solid extrusion 

 near Pauline Lake, Oregon. | This discrep- 

 ancy seems to indicate that the conditions 

 which modify massive-solid eruptions are 

 more varied than is at present underetood and 

 that the results in any two instances may not 

 be closely similar. 



The theory of volcanic eruptions may in- 



* The above considerations seem to be in har- 

 mony with the results reached by A. Lacroix, in 

 a recently published volume (' La Montaigne Pelee 

 et ses eruptions,' Paris, 1904) . This report has not 

 reached me, but a review by Ernest Howe, Science, 

 April 14, 1905, contains the following statement: 

 " Lacroix denies that it (the dome) is of f rag- 

 mental nature and states that it is, in fact, a 

 homogeneous mass of viscous lava surrounded 

 by an envelope of the same substance cooled and 

 consolidated. * » » The viscous magma on 

 reaching the surface through the throat of the 

 volcano and forming a protuberant mass is 

 quickly surrounded by a solid shell or envelope 

 which protects the still pasty interior from a too 

 rapid cooling." 



t Piussell, Israel C, ' Criteria Relating to Mas- 

 sive-Solid Volcanic Eruption,' in American Journal 

 of Science, Vol. XVII., 1904, p. 264. 



} Mount Newberry, U. S. Geological Survey, 

 Bulletin No. 2.52, 1905, pp. 97, 106-109. 



structively be pressed a step farther: As the 

 cooling and consequent consolidation of a 

 magma forced out of a conduit in a viscous 

 or solid condition, progresses from its periph- 

 ery towards its vertical axis, it follows that 

 in case a volcano becomes dormant the same 

 process will continue in the deeper portions of 

 the conduit, and should eruptions be renewed 

 after a period of rest, the avenues of discharge 

 should be through the central part of the par- 

 tially consolidated material in its throat. In 

 case the energy of the renewed eruptions was 

 not sufficient to fracture and discharge, or the 

 heat adequate to re-fuse the consolidated 

 lining of the conduit, it would remain, and if 

 the process was repeated, the tube would 

 become closed by successive additions to its 

 rigid lining, and final extinction result. 



In the case of Mont Pele, the process just 

 outlined is, perhaps, in progress, and the dense 

 rocks in the rim of the crater, which form 

 Morne Lacroix and Petit Bohhomme, be rep- 

 resentatives of the outer portion of a pre- 

 vious eruption similar to the one of recent 

 occurrence. 



Generalizations. — The various forms which 

 massive-solid volcanic eruptions may be ex- 

 pected to assume should be regulated by sev- 

 eral secondary conditions, a number of which 

 might be in operation during a single period 

 of discharge, or some one dominate all the 

 others. For example: (1) the physical condi- 

 tion of the extruded material may range from 

 plasticity to rigidity, dependent upon the 

 chemical composition of the rising magma, its 

 temperature, rate of cooling, rate of ascent and 

 probably still other conditions; (2) the degree 

 of explosive energy may range from such 

 intensity that the entire summit-portion of the 

 extruded material would be blown away, to 

 such feebleness that but little change would be 

 produced in the constructional form due to 

 extrusion ; (3) differences in the rate of cooling 

 and consequent consolidation may result in the 

 formation of a solid plug, or a plug with a 

 rigid exterior and a still plastic interior. Each 

 of these various conditions would react on 

 many, if not all, of the others, and in conse- 

 quence the variations in the final result. 



