Ch. XXIX.] HYPOTHESIS OF UPHEAVAL. 501 



and never yet answered are the following : — First, in most calderas, as 

 in Palnia, 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 of the compass from a centre, 

 and yet that no great fractures should have been produced. We should 

 expect to see some open fissures on every side, widening as they approach 

 the caldera. The dikes, it is true, do undoubtedly attest many disloca- 

 tions of the mass, which have taken place at successive and often distant 

 periods. But none of them can have belonged to the supposed period of 

 terminal and paroxysmal upheaval, for, had the caldera existed when they 

 originated, the melted matter now solidified in each dike must, instead of 

 filling a rent, have flowed down into the caldera, tending sc %• to ob- 

 literate the great cavity. 



The second objection is the impossibility of imagining that so vast 

 a series of agglomerates, tuffs, stratified lapilli, and highly scoriaceous 

 lavas could have been poured out within a limited area without soon 

 giving rise to a hill, and eventually to a lofty mountain. Such heavy 

 angular fragments as are seen in the agglomerates, single beds of which 

 are sometimes 200 or 300 feet thick, must when hurled into the air 

 have fallen down again near the vent, and would be arranged in inclined 

 layers dipping outwards from the central axis of eruption. It is in per- 

 fect accordance with this hypothesis that we should behold agglomerates, 

 lapilli, and scoriae predominating in the walls of the Caldera ; whereas 

 in the ravines nearer the sea, where the inclination of the beds has di- 

 minished to 10 and even to 5 degrees, the proportion of stony as com- 

 pared to fragmentary materials is precisely reversed. It is also natural 

 that the dikes should be most numerous where the ejectamenta are to 

 the more solid beds in the proportion of 3 to 1, as at b, fig. 645, p. 496 ; 

 while the dikes are few in number where the stony lavas predominate 

 (as at c, ibid.). Many of the scoriaceous beds at b may be the upper 

 extremities of currents which became stony and compact when they 

 reached c, and flowed over a more level country ; but this suggestion 

 cannot be assented to by the advocates of the upheaval theory, for it 

 assumes the existence of a cone long before the time had arrived for 

 the catastrophe which according to their views gave rise to a conical 

 mountain. 



If, however, we reject the doctrine that the beds were tilted by a 

 movement posterior to the accumulation of all the compact and frag- 

 mentary rocks, how are we to account for the steepness of the dip of 

 some stony lavas high up in the walls of the Caldera ? These masses 

 are occasionally 50 or 100 feet thick, of lenticular shape, as seen in the 

 cliffs from below, and to all appearance parallel to the associated layers 

 of scoriae and lapilli. But unfortunately no one can climb up and de- 

 termine how far the supposed parallelism may be deceptive. The solid 



