Ch. XXIX.] AQUEOUS EROSION IN PALMA. 503 



For this reason tlie defenders of the upheaval hypothesis are consistent 

 with themselves in assigning tlie whole movement by which the strata, 

 whether solid or incoherent, have been tilted, exclusively to one terminal 

 catastrophe. The whole development of subterranean force is repre- 

 sented as the last incident in every series of volcanic operations, the 

 closing scene of the drama ; and the sudden and paroxysmal nature of 

 the catastrophe is inferred from the absence of all signs of successive 

 and intermittent action so characteristic of the antecedent volcanic phe- 

 nomena. 



I have alhided 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 believe to be erroneous. The 

 lava which has flowed from the cone of Llarena near Port Orotava, in 

 Teneriffe, is very colunmar in parts, and yet has descended a slope of six 

 degrees. Another stream of recent aspect near the town of El Passo, in 

 Palma, has a general inclination of ten degrees, and is remarkable for 

 the depth and extent of the large basin-shaped hollows, 20, 30, and 35 

 feet deep, seen everywhere on its surface. Whenever another lava-current 

 shall flow down over this one, although its average inclination will be the 

 same, it must fill up all these inequalities, and in doing so must give 

 rise to masses of compact and solid rock 20 or 30 feet thick, resting upon 

 and encircled by vesicular lava. Other lavas northeast of Fuencaliente 

 at the southern extremity of Palma, so modern as to be still black and 

 uncovered with vegetation, 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 solidi- 

 fied 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 suf- 

 ficient 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 j)lace 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 sec- 

 tion, pp. 497, 498). 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 



