634: CALDERA OF PALMA. [Ch. XXIX. 



the same place, relaxing perhaps 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 fol- 

 lowers imagine, its component materials were first spread out in hori- 

 zontal 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 movements 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 intro- 

 duced by the one theory is upheaval, by the other the fall of matter 

 from the air. The very name of " Elevation Craters " 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 are 

 gaseous explosions, engulfment, and aqueous denudation. 



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 thick- 

 ness, high up in the walls of a caldera, we may be sure that they were 

 solidified originally on a very gentle slope ; and if they are now in- 

 clined at angles of 10°, 20°, or 30°, not only they, but all the inter- 

 stratified beds of lapilli, scorise, 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 Yon Buch called " the hollow axis of the cone ") may 

 represent this breach of continuity. 



Among other objections to the elevation-crater theory often- ad- 

 vanced and never yet answered are the following: First, in most 

 calderas, as in Palma, the rim of the great cavity and the circular 

 range of precipices surrounding it remain entire and unbroken on 

 three sides, whereas it is difficult to conceive that a series of volcanic 

 strata 2000 or 3000 feet thick could have once extended over an area 

 six or seven miles in its shortest diameter and then have been upraised 

 bodily, so that the beds should dip at steep angles towards all points 



