TRUE VOLCANOES. 401 



would be well, in the perfection to which mineralogical 

 science is now brought, to institute investigations for the 

 purpose of discovering whether oligoclase is not contained in 

 these porphyritic trachytes, as at Teneriffe, Popocatepetl and 

 Chimborazo, or else labradorite, as at Etna and Stromboli. 

 Pumice is entirely wanting on the Galapagos, as at Vesuvius, 

 where although it may be present, it is not produced, nor 

 is hornblende anywhere mentioned to have been found 

 in them. ; consequently the trachyte formation of Toluca, 

 Orizaba, and some of the volcanoes of Java, from which Dr. 

 Junghuhn has sent me some well-selected solid pieces of 

 lava for examination by Gustav Rose, does not prevail here. 

 On the largest and most westerly island of the Galapagos 

 group, Albemarle, the cone-mountains are ranged in a line, 

 and consequently OR fissures. Their greatest height, however, 

 reaches only to 4636 feet. The Western Bay, in which the 

 Peak of Narborough, so violently inflamed in 1825, rises in 

 the form of an island, is described by Leopold von Buch 4 as 

 a crater of up-heaval, and compared to Santorino. Many 

 margins of craters on the Galapagos are formed of beds of 

 tufa, which slope off in every direction. It is a very re- 

 markable circumstance, seeming to indicate the simul- 

 taneous operation of some great and wide-spread catas- 

 trophe, that the margins of all the craters are disrupted or 

 entirely destroyed towards the south. A part of what in 

 the older descriptions is called tufa, consists of palagonite 

 beds, exactly similar to those of Iceland and Italy, as Bun- 

 sen has ascertained by an exact analysis of the tufas of 

 Chatham Island. 6 This island, the most easterly of the 

 whole group, and whose situation is fixed by careful astro- 

 nomical observations by Captain Beechey, is, according to 

 my determination of the longitude of the city of Quito 

 (78 3 44' 8'), and according to Acosta's Mapa de la Nueva 

 Granada of 1849, 536 geographical miles distant from the 

 Punta de S. Francisco. 



cylindrical, annular addition, exactly like what I saw on Cotopaxi; 

 " in some parts the ridge is surmounted by a wall or parapet perpen- 

 dicular on both sides." Darwin, Vole. Isl. p. 83. 



4 L. von Buch, p. 376. 



5 Bunsen, in LeonharcCs Jahrb. fur Mineralvgie, 1851, s. 856 ; also 

 in Poggend, Annalcn der Physik, Bd. Ixxxiii, s. 223. 



VOL. V. 2 D 



