1849.] LYELL ON THE STRUCTURE OF VOLCANOS. 213 



would now be seen breaking through the walls of the crater, and 

 widening as they approached the empty central space*. Instead of 

 any such open rents being visible in the walls of the Caldera of Palma, 

 and in analogous crateriform cavities, we invariably find dikes or up- 

 filled fissures, in which, as well as in tortuous veins, often forming a 

 reticulated mass, the melted matter was clearly consolidated before 

 the boundary cliffs were formed. The origin therefore of all such 

 rents, numerous as they are, was wholly antecedent in date to the 

 whole movement assumed as the cause of the elevation-crater. I 

 have also in every edition of my works uniformly contended, in com- 

 mon with Messrs. Scrope, C. Prevost, and others, that in mountains 

 like Etna, Mont Dor, and the Cantal, we must look to that area 

 where we now find the greatest thickness of lava and fragmentary 

 ejections as the chief and permanent source of the alternating lavas, 

 tuffs, scoriae, and conglomerates composing the volcanic cone. The 

 increase of the cone, so far as it consists of such superimposed igneous 

 products, I compared to the exogenous growth of a tree, and in Etna 

 and some other volcanos a series of superimposed sloping beds has 

 been piled up successively to a thickness of more than 4000 feet. 

 We may call the injection of lava, and the distension and upheaval 

 caused by the hydrostatic action of imprisoned vapours, to which 

 M. de Beaumont has justly attributed much greater importance than 

 I had previously conceded, or even than Mr. Scrope had assumed, 

 the endogenous growth of the mountain. The intensity of this last- 

 mentioned mode of increase is much greater in the more central than 

 in the marginal parts of a volcano. For this reason v/e perceive near 

 the margin or base of the cone that the lava and l^eds of scoriae, as 

 they gradually thin out, become intersected by fewer and fewer dikes, 

 until these at length entirely cease to appear. Not only the number, 

 but the size or width also of such dikes may often be seen to augment 

 as we approach nearer and nearer to the central axis of the cone. 

 Other generalizations on the origin and growth of cones and craters 

 I shall defer to the sequel, as they will be best explained when I am 

 commenting on the structure and probable mode of formation of par- 

 ticular volcanos. 



Palma. 



To one of the most remarkable of these, the island of Palma, I 

 shall first allude. Von Buch has given us a graphic picture of what 

 seems to be the most splendid and perfect example yet discovered of 

 a huge and deep cavity, surrounded on all sides by a circular range 

 of precipices which are 4000 feet in height, the beds dipping out- 

 wards in all directions from the centre of the void space, which is 

 about six geographical miles in diameter. The sloping beds consist 

 chiefly of basalt alternating with conglomerates, composed m part of 

 rolled masses of similar basalt. Here, therefore, we seem to have 

 evidence of the subaqueous origin of a portion at least of the volcanic 

 accumulation, while the highest part of the cone may have raised 



* Principles of Geology, cU. 24. Mem. de la Soc. Geol. de France, tome ii. p. 91 . 



