926 



SCIENCE. 



[X. S. Vol. XXI. Xo. 54C. 



condition to a considerable depth. When 

 pressure is renewed at the base or basal por- 

 tion of a conduit after a period of rest, pre- 

 sumably the still hot material contained in it 

 would aiford the path of least resistance for 

 the ascent of material forced upward from 

 deep within the earth, and the former avenue 

 of discharge would be reopened. As will be 

 shown later, there are reasons for assuming 

 that the magma in the conduit of a dormant 

 volcano solidifies progressively from its pe- 

 riphery towards its center, and that previous 

 to complete consolidation there is a tube of 

 rigid lava present, enclosing a vertical core 

 of plastic or liquid rock. The summit por- 

 tion of the material in the conduit of a dor- 

 mant volcano, however, is always rigid and 

 must be fractured and the fragments produced 

 ejected, or else heat ascending from a depth, 

 perhaps conveyed by gases and vapors, leads to 

 a sufficient increase in temperature to cause 

 the previously solid lava forming the summit 

 of the plug to become partially or wholly vis- 

 cous. Should a condition of inactivity con- 

 tinue sufficiently long to permit the magma in 

 a volcano's conduit to become solidified to a 

 considerable depth,- say several thousand feet, 

 it is reasonable to assume that a truly extinct 

 condition would be reached and that an in- 

 crease of pressure at a depth would lead to 

 the formation of fissures and the opening of 

 a new conduit. 



The speculations just indulged in seem to 

 favor the hypothesis that the obelisk of Pele 

 was composed of fresh lava in distinction 

 from lava formed by the secular cooling of a 

 magma left in the conduit at the close of some 

 former eruption. On the other hand, the 

 change of a fresh magma, rising from a depth, 

 to a solid condition as it nears the summit of 

 its conduit, implies a very rapid rate of 

 change from a plastic to a rigid condition. 

 In this connection it is to be remembered that 

 during a certain period of eighteen days the 

 growth of the obelisk of Pele was at the rate 

 of about forty-one feet per day. The sugges- 

 tion here presents itself that the rate of down- 

 ward progressing change from a plastic to a 

 rigid condition might not have been the same 

 as the rate of bodily ascent. Most probably 



each process was variable. The portion of 

 the obelisk extruded during a certain period 

 of time may have required a longer period of 

 time for its consolidation. 



Under the assumption that the obelisk of 

 Pele was composed of fresh lava, two chief 

 methods or processes have been suggested by 

 which a change from a plastic to a rigid con- 

 dition of the material extruded was brought 

 about. One of these explanations and the one 

 I have previously favored, ascribes the loss of 

 heat from the magma in the summit portion 

 of a volcano's conduit, mainly to conduction 

 through its confining walls, aided, and per- 

 haps dominated by, the cooling influence of 

 descending percolating water. Assuming that 

 this is the main process involved or the one as 

 may be said in control, it follows that the 

 magma adjacent to the walls of the conduit 

 in its upper portion would change to a solid 

 condition previous to the central part ; that is, 

 in a given horizontal section consolidation 

 would progress from the circumference toward 

 the center. Coupled with this process and as 

 it seems an accompaniment of any method of 

 cooling after the protrusion of solid material 

 has begun, is the loss of heat from the ex- 

 posed summit-portion of the column by radia- 

 tion, escape of steam and gases through fis- 

 sures, the cooling influences of rain, etc. 



Another process by which the cooling of a 

 magma in the summit-portion of a volcano's 

 conduit may be brought about, has recently 

 been suggested by A. C. Lane* and G. K. 

 Gilbert,t and is based on the principle that 

 vapors and gases on expanding withdraw en- 

 ergy from surrounding media, the assump- 

 tion being that a rising magma near the con- 

 dition of consolidation and containing oc- 

 cluded vapors and gases, would experience a 

 decrease of pressure as it rose; and in conse- 

 quence the vapors and gases would expand and 

 withdraw heat from the walls of the contain- 

 ing vesicles and promote their solidification. 

 The application of this principle to rising 

 magmas seems to be legitimate, but implies 

 that material is removed from the summit of 

 the ascending column; if the material dis- 



* Science, December 11, 1903. 



t Science, June 17, 1904. 



