506 EXTENT AND NATUEE OF [Ch. XXIX. 



origin of the physical outlines of the country. As to the fact that no 

 marine shells have yet been discovered in the conglomerate, sufficient 

 search has not yet been made for them to entitle us to found an argu- 

 ment on such negative evidence. At the same time I confess, that, 

 having found sea-shells and bryozoa abundantly in certain elevated 

 marine conglomerates in the Grand Canary, before I visited Palma, and 

 being unable to meet with any in the Barranco de las Angustias, I re- 

 garded the old gravel when I was on the spot as of fluviatile origin. 

 Such inferences are always doubtful in the absence of more positive data, 

 and the intervention of the sea will unquestionably account for some 

 phenomena in the configuration of the Caldera and Barranco more 

 naturally than river action. For example, we have the lofty cliff e, fig. 

 p. 504, already mentioned, and c,/, map, p. 494, extending four or five 

 miles from the Caldera to the sea on the right bank of the Barrranco, 

 and no cliff of corresponding height or structure on the other bank, 

 where for miles towards the southeast there is the platform f, fig. p. 504, 

 supporting several minor volcanic cones. The sea might be supposed to 

 leave just such a cliff as e, after cutting away a portion of the southwest- 

 ern extremity of the old dome-shaped mountain in the north of Palma, 

 whereas a torrent or river would leave a cliff of similar structure and 

 nearly equal height on both banks. As to the fact of the old con- 

 glomerate ascending an inclined plane, i, I, k, p. 497, from the sea-level 

 to an elevation of about 1500 feet, near the entrance of the Caldera, this 

 is by no means conclusive in favor of fluviatile action, although some ele- 

 vated patches of the same may in truth belong to an old river-bed ; but 

 in South America gravel-beds of marine origin have a similar upward 

 slope, when followed inland, and the cause of such an arrangement has 

 been explained in a satisfactory manner by Mr. Darwin.* 



Another argument in favor of marine denudation may be derived 

 from that peculiar feature in the configuration of Palma, before alluded 

 to, called the pass of the Cumbrecito (e, fig. 646, p. 497), forming a 

 notch in the uppermost line of precipices surrounding the Caldera. 

 This break divides the mountain called Alejanado, d, fig. p. 497, from 

 the eastern wall c, /, and cuts quite through the upper formation ; yet 

 the range of precipice /, e, on the eastern side of the Caldera is con- 

 tinued uninterruptedly, and retains its full height of 1500 or 2000 feet 

 above its base, to the southward of the Cumbrecito, or from e towards a, 

 map, fig. 642, p. 494. In this prolongation of the cliff for half a mile 

 southward beds of volcanic matter and dikes are seen, as in the walls of 

 the Caldera. 



The indentation forming the pass of the Cumbrecito, e, p. 497, has 

 more the appearance of an old channel, such as a current of water may 

 have excavated, than of a rent or a chasm caused by a fault. In case or 

 a fault the lower formation would not be persistent and uninterrupted 

 across the Cumbrecito, constituting the watershed ; but would have 

 sunk down and have been replaced by the upper basaltic rocks. If 

 * Geolog. Observ.,- South America, p. 48. 



