Gh. XXIX.] AQUEOUS EROSION IN PALMA. Qoy 



the strata, whether solid or incoherent, have been tilted, exclusively to 

 one terminal catastrophe. The whole development of subterranean 

 force is represented as the last incident in every series of volcanic 

 operations, the closing scene of the drama; and the sudden and par- 

 oxysmal nature of the catastrophe is inferred from the absence of all 

 signs of successive and intermittent action so characteristic of the ante- 

 cedent volcanic phenomena. 



I have alluded to an opinion entertained by some able geologists, 

 that no lava can acquire any degree of solidity if it flows down a 

 declivity of more than three degrees. This doctrine I have, I think, 

 proved in my memoir on Mount Etna * to be entirely erroneous. I 

 have there shown, from observations made by me in 1857, that mod- 

 ern lavas, several of them of known date, have formed continuous 

 beds of compact stone on slopes of 15, 36, and 38 degrees, and, in the 

 case of the lava of 1852, more than 40 degrees. The thickness of 

 these tabular layers varies from 1-J foot to 26 feet ; and their planes of 

 stratification are parallel to those of the overlying and underlying 

 scoriae which form part of the same currents. 



There are some lavas northeast of Fuencaliente, at the southern 

 extremity of Palma, so modern as to be still black and uncovered 

 with vegetation, which descend slopes of no less than 22 degrees, and 

 yet contain large masses of compact stone, formed chiefly on the sides 

 of tunnel-shaped cavities, 15 or 20 feet deep, in which one layer has 

 solidified within another on the walls of these channels, while in the 

 central part the lava seems to have remained fluid so as to run out 

 of the tunnel, leaving an arched cavity, the roof of which has in most 

 cases fallen in. The strength of the enveloping crust of scoriae at the 

 lower end of a lava-current in which one of these tunnels existed, may 

 have been sufficient to arrest the progress of the stream for hours or 

 days, and during that time solidification may have occurred under 

 great hydrostatic pressure. 



Before taking leave of Palma, we have yet to consider another dis- 

 tinct point, namely, what amount of denudation has taken place in the 

 Caldera and its environs. Assuming that the great cavity or some 

 part of it may have originated in the truncation of a cone in the man- 

 ner before suggested, to what extent has its shape been subsequently 

 enlarged or modified by aqueous erosion? It will be remembered 

 that a conglomerate of well-rounded pebbles, no less than 800 feet 

 thick, was spoken of as visible in the great Barranco (see description 

 of section, pp. 630-632). That conspicuous deposit, 3 or 4 miles in 

 length, was evidently derived from the destruction of rocks like those 

 in the Caldera, for the present torrent brings down annually similar 

 stones of every size, some very large, and rounds them by attrition in 

 its channel. By what changes in the configuration of the island after 

 the old volcano and its Caldera were formed was so vast a thickness of 



* Phil. Trans. 1858. 



