1849.] LYELL ON THE STRUCTURE OF VOLCANOS. 209 



same time alluded to its analogy to the single passage leading into 

 the lagoons of many annular coral islands or Atolls. 



Although I then distinctly announced this theory in regard to such 

 narrow ravines, the idea had not occurred to me that the same de- 

 nuding power of the waves and tides, which were thus appealed to as 

 adequate to remove the rocks once filling such deep gorges, must of 

 necessity have exerted a like action on the walls of the craters them- 

 selves. I well knew from the excellent description published by Von 

 Buch of Palma, an island to which I specially alluded, that the long 

 and deep chasm called Baranco de las Angustias, which alone breaks 

 the continuity of the rocks euclosing the caldera, and was bordered 

 on both sides by steep cliffs, was not less than between six and seven 

 miles in length, being at its upper extremity 2000 feet or more in 

 depth. I ought therefore, in consistency, to have inferred, that the 

 same ocean, which I supposed to have stood successively at various 

 levels, and in the course of ages to have ground down and carried 

 away so vast a volume of rock, from this channel, must during the 

 same long period have excavated a part of the hollow once occupied 

 by similar and equally destructible materials. 



By referring to the annexed map, fig. 2, from Capt. Vidal's ' Survey 

 of Palma,' the reader will observe that the sea cliif at Point Juan 

 Graje, 780 feet high, now forming the coast at the entrance of the 

 great ravine, is continuous with an inland cliff which bounds the same 

 ravine on its north-western side. No one will dispute that the pre- 

 cipice at the base of which the waves are now beating, owes its origin 

 to the undermining power of the sea. It is natural therefore to attri- 

 bute the extension of the same cliff to the former action of the waves 

 exerted at a time when the relative levels of the island and the ocean 

 were different from what they are now. 



Of late, after fully reconsidering the subject, I have come to the 

 conclusion that the origin of a great part of the Caldera of Palma was 

 probably due to denudation, and that the same holds true of other 

 analogous cavities, such as are seen in Teneriffe, and many volcanic 

 islands, so well described by M. von Buch in his classical work on 

 the Canaries. Santorin in particular, which has been selected as 

 furnishing the best type of a crater of elevation, owes, I believe, the 

 chief part of the extension of its circular gulf to denudation, the whole 

 crater together with the surrounding rocky islands having subsided 

 bodily since the denudation, so as to be now half submerged in the 

 waters of the Mediterranean. 



Before I proceed to treat more in detail of this and other volcanos, 

 I shall offer a few preliminary remarks, to prepare the geologist for 

 the reception of the views about to be proposed. In the first place 

 it is admitted, that many of the volcanos, in which these large crateri- 

 form hollows exist, have been formed wholly or in part beneath the 

 level of the sea ; 2ndly, the quantity of solid rock assumed by me to 

 have been worn down and carried away through a narrow channel by 

 the waves and currents (as the islands emerged) is by no means great, 

 when contrasted with the masses removed from many elliptical areas, 

 which have been called valleys of elevation, such as the Weal den, or 



