Ch. XXIX.] AQUEOUS EROSION IN PALMA. 639 



would be swept down in the shape of mud into the Atlantic. Even 

 the hard rounded stones, since they were once angular and are now 

 ground down into pebbles, must have lost more than half their original 

 bulk, and bear witness to large quantities of sedimentary matter con- 

 signed to the bed of the ocean. We saw in the Caldera blocks of 

 huge size thrown down by cascades from the upper precipices during 

 the melting of the snows, a fortnight before our visit, and much de- 

 struction was likewise going on in the lower set of rocks by the same 

 agency. We also learned that a great flood rushed down the Barranco 

 in the spring of 1854, shortly before our arrival, damaging several 

 houses and farms, and I have therefore no doubt that the erosive 

 power even of rain and river water, aided by earthquakes, might in 

 the course of ages empty out a valley as large as the Caldera, although 

 probably not of the same shape. I am disposed to attribute the cir- 

 cular range of cliffs surrounding the Caldera to volcanic action, be- 

 cause they forcibly reminded me of the precipices encircling three 

 sides of the Val del Bove, on Etna ; and because they agree so well 

 with Junghuhn's description of the " old crater-walls " of active vol- 

 canoes in Java, some of which equal or surpass in dimensions even 

 the Caldera of Palma. The latter may have consisted at first of a true 

 crater, enlarged afterwards into a caldera by the partial destruction of 

 a great cone ; but, if so, it has certainly been since modified by denu- 

 dation. Nor can any geologist now define how much of the work has 

 been accomplished by aqueous, and how much by volcanic agency. The 

 phenomenon of a river cutting its channel through a dense mass of 

 ancient alluvium formed during oscillations in the level of the land is 

 not confined to volcanic countries, and I need not dwell here on its 

 interpretation, but refer to what was said in the seventh chapter. 

 (See p. 84.) 



There remains, however, another question of high theoretical 

 interest; namely, whether the denudation was marine or fluviatile. 

 It was stated that the materials of the great cone or assemblage of 

 cones in the north of Palma are of subaerial origin, as proved by the 

 angularity of the fragments of rock in the agglomerates ; but it may 

 be asked, whether, when the Caldera was formed long afterwards, it 

 may not, like the crater of St. Paul's (fig. 702, p. 643), have had a 

 communication with the sea, which may have entered by the great 

 Barranco, and if, after a period of partial submergence, the island may 

 not then have risen again to its original altitude. In such a case the 

 retiring waters might leave behind them a conglomerate, partly of 

 river-pebbles, collected at the points where the torrent successively 

 entered the sea, and partly of stones rounded by the waves. The 

 torrent may have finally cut a deep ravine in the gravel and associated 

 lavas when the land was rising again. Such oscillations of level, 

 amounting to more than 2000 feet, would not be deemed improbable 

 by any geologists, provided they enable us to explain more naturally 

 than by any other causation, the origin of the physical outlines of the 



