94 St- Helena. paet l 



not unfrequently within volcanic craters, and their 

 formation seems to be due to the sinking down of a 

 level sheet of hardened lava, the edges of which re- 

 main (like the ice round a pool, from which the water 

 has been drained) adhering to the sides. 1 



In some parts, the ridge is surmounted by a wall 

 or parapet, perpendicular on both sides. Xear Diana's 

 Peak this wall is extremely narrow. At the Galapagos 

 Archipelago I observed parapets, having a quite similar 

 structure and appearance, surmounting several of the 

 craters ; one, which I more particularly examined, 

 was composed of glossy, red scoriae firmly connected 

 together; being externally perpendicular, and extend- 

 ing round nearly the whole circumference of the crater, 

 it rendered it almost inaccessible. The Peak of Tene- 

 riffe and Cotopaxi, according to Humboldt, are similarly 

 constructed ; he states 2 that c at their summits a cir- 

 cular wall surrounds the crater, which wall, at a 

 distance, has the appearance of a small cylinder placed 

 on a truncated cone. On Cotopaxi 3 this peculiar 

 structure is visible to the naked eye at more than 

 2,000 toises' distance ; and no person has ever reached 

 its crater. On the Peak of Teneriffe, the parapet is so 

 high, that it would be impossible to reach the cald&ra, 

 if on the eastern side there did not exist a breach.' 

 The origin of these circular parapets is probably due 

 to the heat or vapours from the crater, penetrating 

 and hardening the sides to a nearly equal depth, and 

 afterwards to the mountain being slowly acted on by 

 the weather, which would leave the hardened part, 



1 A most remarkable instance of this structure is described in 

 Ellis's ' Polynesian Kesearches' (second edit.), where an admirable 

 drawing is given of the successive ledges or terraces, on the borders 

 of the immense crater at Hawaii, in the Sandwich Islands. 



2 'Personal Narrative,' vol. i. p. 171. 



8 Humboldt's 'Picturesque Atlas,' folio, pi. 10. 



