40 T. A. Jag gar — Initial Stages of the Spine on Pelee. 



chemical and mechanical conditions favor the fusion and segre- 

 gation within, of the more fusible minerals. 



Rifting of the crust may take place by faulting, differential 

 contraction, or by the action of escaping steam, and the molten 

 matter slowly wells up under the pressure of the slumping 

 agglomerate. 



Exposed to the cooler air this viscid silicate mixture solidifies 

 quickly and impedes the upward progress of the more liquid 

 portions below. 



The actual volume of the molten material would increase 

 with many successive eruptions and diminish with cessation of 

 eruption : it would vary with the growth of the cone, and this 

 is what the spines have been observed to do. There can be no 

 doubt that an enormous amount of red-hot material is confined 

 in the cone and the fissures beneath ; that it would remain 

 incandescent for months even without additions is proved by 

 analogy with the banks of hot gravel along the stream 

 courses. These retain their heat for many weeks after an erup- 

 tion; a rain-crust forms above, and the banked-in gravel causes 

 frequent explosions when ground-water makes contact with it. 

 If these banks so retain their heat at a distance of miles from 

 the crater* much more will the temperatures w T ithin the cone 

 be high and long maintained, for there the fragments are hot- 

 test and largest, are accumulated in greatest volume, are fre- 

 quently added to, and are in contact with dust-laden steam and 

 heated gases rising under pressure from unknown depths. 



Even if no spines had appeared, one might ask on a priori 

 grounds, What has become of all the pasty incandescent mate- 

 rial that has fallen back into the crater and is now under 

 pressure? It cannot be supposed to have hardened at once, and 

 it must have been intimately mixed with pulverized rock of 

 varying fusibility. It would seem d} r namically probable, there- 

 fore, that such material would become agglutinated in fluid 

 masses within the agglomerate of the crater fissure, and escape 

 to form irregular protuberances along paths of least resistance. 



