JuxE IG, ]P05.] 



SCIENCE. 



925 



scribed by several observers as consisting of 

 two parts : a ' dome,' and a ' spine ' or ' obelisk.' 

 The former was a dome-shaped elevation de- 

 veloped within the crater, which occupied a 

 large portion and in fact nearly the whole of 

 its interior and overtopped its rim. It was 

 situated directly over the volcano's conduit, 

 and numerous explosions occurred in its sum- 

 mit portion. The latter, i. e., the ' obelisk,' 

 was situated on one side of the dome and rose 

 as a mighty tower to a height of more than a 

 thousand feet above it. While the two struc- 

 tures just referred to have been described as 

 distinct, perhaps in part for convenience in 

 recording observations, there seem to be good 

 reasons, as will be stated below, for consider- 

 ing them as parts of the same massive-solid 

 extrusion or, as termed by some writers, 

 ' cumtilo eruption.' 



In explanation of the upheaval of a mighty 

 spire or obelisk of rigid rock from within the 

 crater of Mont Pele, two hypotheses have been 

 offered. The obelisk has been considered by 

 several geologists as the freshly congealed and 

 rigid summit portion of a column of molten 

 rock or magma, which was forced out of its 

 conduit in a massive-solid condition. A sec- 

 ond hypothesis, advocated by Angelo Heil- 

 prin,* is in brief, that a plug of old lava, 

 formed by the cooling and hardening of a 

 residuum left in the conduit of the volcano 

 after some previous eruption, was forced up- 

 ward and in part extruded from the crater. 



Of these two hypotheses the known facts 

 seem to favor the acceptance of the first men- 

 tioned, for the reason, in part, that the de- 

 cidedly vigorous eruptions of the volcano, espe- 

 cially during the earlier stages of its present 

 period of activity, when vast quantities of 

 fragmental material were ejected, show that 

 the conduit had what may be designated as a 

 free vent. The large size of the column of 

 black, debris-laden steam, estimated at 1,500 

 feet in diameter at the base, which at times 

 rose straight into the air for several thousand 

 feet, is also proof that the conduit was not 



* Heilprin, Angelo, ' The Tower of Pelee,' pub- 

 lished by J. B. Lippincott Co., Philadelphia and 

 London, 1904, size 9 by 12 inches, pp. 1-62 and 22 

 plates. 



seriously restricted in its upper portion, and 

 demonstrates that one large opening and not 

 several small orifices was present. The great 

 amount of dense and seemingly old lava 

 mingled with scoriaceous and fresher appear- 

 ing material which has been discharged, is 

 apparently good evidence that the plug of old 

 lava present in the conduit when tlie recent 

 eruption began, was, in part at least, shattered 

 and the fragments, together perhaps with 

 masses of similar material torn from the walls 

 of the conduit, widely distributed as ejected 

 blocks, lapilli, dust, etc. There are still other 

 considerations which favor the idea that the 

 obelisk was formed by the recent congealing 

 of a rising magma, rather than that it was a 

 solid mass at the beginning of the recent 

 period of activity. Among these considera- 

 tions is the fact that the total vertical meas- 

 ure of the massive-solid extrusion, in case the 

 obelisk had not in part fallen from time to 

 time, would have been some five or six thou- 

 sand feet. The frictional resistance of such a 

 plug, if rigid throughout, would, as it seems, 

 have been greater than even volcanic energy 

 could have overcome, and certainly far greater 

 than is demanded by the hypothesis that a 

 magma was cooled and consolidated in its 

 upper portion as it rose from below. Then, 

 too, as observation seems to indicate, Mont 

 Pele is composed principally of fragmental 

 material ejected during previous eruptions, 

 and the walls of the present conduit may 

 reasonably be assumed to be relatively weak, 

 and in case it had contained a solid ' volcanic 

 neck ' to a depth of five thousand or more feet, 

 the fresh discharges would have found an exit 

 by means of a new or side opening instead of 

 pushing the plug out vertically and causing 

 explosions about its periphery. 



Recorded knowledge concerning volcanoes is 

 still too incomplete to enable one to form a 

 well-substantiated judgment as to the manner 

 in which the reopening of a conduit is accom- 

 plished after a period of quiescence and ap- 

 parent extinction, but the best tentative view 

 in this connection seems to be that a magma 

 in a volcanic conduit requires a long period of 

 time, possibly several thousand years, to lose 

 sufficient heat to admit of a change to a solid 



