500 CALDERA OF PALMA. [Ch. XXIX 



on the exact site of an equally vast accumulation of comparatively mod- 

 ern lavas and scoriae is peculiarly worthy of notice as a general phenome- 

 non observed in very different parts of the globe. It proves that, 

 notwithstanding the fact in the past history of volcanoes that one region 

 after another has been for ages and has then ceased to be the chief theatre 

 of igneous action, still the activity of subterranean heat may often be per- 

 sistent for more than one geological period in the same place, relaxing 

 perhaps in its energies for a while, but then breaking out afresh with an 

 intensity as great as ever. 



We have still to consider the mode of origin of the higher volcanic 

 mass, or the upper series of rocks with which the peculiar form of the 

 Caldera is more intimately connected. The principal question here 

 arising is this, whether the mass was dome-shaped from the beginning, 

 having grown by the superposition of one conical envelope of lava and 

 ashes formed over another, or whether, as Von Buch and his followers 

 imagine, its component materials were first spread out in horizontal or 

 nearly horizontal deposits, and then upheaved at once into a dome-shaped 

 mountain with a caldera in its centre. According to the first hypothesis 

 the cone was built up gradually, and completed with all its beds dipping 

 as now, and traversed by all its dikes, before the Caldera originated. 

 According to the other, the Caldera was the result of the same move- 

 ments which gave a dome-shaped structure to the mass, and which 

 caused the beds to be highly inclined ; in other words, the cone and 

 the Caldera were produced simultaneously. So singularly opposite are 

 these views, that the principal agency introduced by the one theory is 

 upheaval, by the other subsidence. The very name of " Elevation Cra- 

 ters" points to the kind of movement to which one school attributes the 

 origin of a cone and caldera ; whereas the chief agencies appealed to by 

 the other school are gaseous explosions, engulfment, and aqueous denu- 

 dation. 



The favorable reception of the doctrine of upheaval has arisen from 

 the following circumstances. Streams of lava, it is said, which run down 

 a declivity of more than three degrees are never stony ; and, if the slope 

 exceed five or six degrees, they are mere shallow and narrow strings of 

 vesicular or fragmentary slag. Whenever, therefore, we find parallel 

 layers of stony lava, especially if they be of some thickness, high up 

 in the walls of a caldera, we may be sure that they were solidified origi- 

 nally on a very gentle slope ; and if they are now inclined at angles 

 of 10°, 20°, or 30°, not only they, but all the interstratified beds of 

 lapilli, scoria?, tuff, and agglomerate, must have been at first nearly flat, 

 and must have been afterwards lifted up with the solid beds into 

 their present position. It is supposed that such a derangement of the 

 strata could scarcely fail to give rise to a wide opening near the centre 

 of upheaval, and in the case of Palma, the Caldera (which Von Buch 

 called "the hollow axis of the cone") may represent this breach of 

 continuity. 



Among other objections to the elevation-crater theory often advanced 



