TRUE VOLCANOES. 375 



pagos in the expedition of the Beagle, under Captain Fitzrcy, 

 two of the craters were simultaneously in a state of igneous 

 eruption. On all the islands, streams of a very fluid lava 

 may be seen which have forked off into different channels, 

 and have often run into the sea. Almost all are rich in 

 augite and olivin ; some, which are more of a trachytic 

 character, are said to contain albite* in large crystals. It 

 would be well, in the perfection to which mineralogical sci- 

 ence is now brought, to institute investigations for the pur- 

 pose 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 any where mentioned to have been found in 

 them ; consequently the trachyte formation of Toluca, Ori- 

 zaba, 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 con- 

 sequently on 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 Buchf as a crater 

 of upheaval, 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 remarkable circum- 

 stance, seeming to indicate the simultaneous operation of 

 some great and wide-spread catastrophe, that the margins of 

 all the craters are disrupted or entirely destroyed toward 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 Bunsen has ascertained by an exact 



* Dai-win, Vole. 1st., p. 104, 110-112, and 114. When Darwin says 

 so decidedly that there is no trachyte on the Galapagos, it is because 

 he limits the term tracl^te to the common feldspar, i. e., to orthoclase, 

 or orthoclase and sanidine (glassy feldspar). The enigmatical frag- 

 ments imbaked in the lava of the small and entirely basaltic crater of 

 James Island contain no quartz, although they appear to rest on a 

 Plutonic rock (see above, p. 367 et seq.). Several of the volcanic cone 

 mountains on the Galapagos Islands, have at the orifice a narrow cyl- 

 indrical, annular addition, exactly like what I saw on Cotopaxi ; "in 

 Borne parts the ridge is surmounted by a wall or parapet perpendicular 

 on both sides." Darwin, Vole. 7s/., p. 83. 



t L. von Buch, p. 37G. 



