TRUE VOLCANOES. 331 



strata of volcanic tufa and loam, sometimes covered by more 

 recent basalt. Fernando de Noronha, lat. 3 50' S. and 2 

 27 X to the east of Pernambuco ; a group of very small isl- 

 ands ; phonolitic rocks containing hornblende no crater, 

 but vein-fissures filled with trachytic and basaltic amygda- 

 loid, penetrating white tufa layers.* The island of Ascen- 

 sion, highest summit 2868 feet ; basaltic lavas with more 

 glassy feldspar than olivin sprinkled through them, and well- 

 bounded streams traceable up to the eruptive cone of tra- 

 chyte. The latter rock of light colors, often broken up like 

 tufa, predominates in the interior and southeast of the island. 

 The masses of scoriae thrown out from Green Mountain in- 

 close immersed angular fragments! containing syenite and 

 granite, which remind one of the lavas of Jorullo. To the 

 westward of Green Mountain there is a large open crater. 

 Volcanic bombs, partly hollow, of as much as ten inches in 

 diameter, lie scattered about in innumerable quantities, to- 

 gether with large masses of obsidian. St. Helena : the whole 

 island volcanic, the beds of lava in the interior rather felds- 

 pathic ; basaltic toward the coast, penetrated by innumera- 

 ble, dikes as at Flagstaff Hill. Between Diana Peak and 

 Nestlodge, in the central series of mountains, are the curved 

 and crescentic shaped fragments of a wider, destroyed crater 

 full of scoriae and cellular lava (" the mere wreck of one 

 great crater is left"). The beds of lava are not limited, and 

 consequently can not be traced as true streams of small 

 breadth. Tristan da Cunha (lat, 37 3 X S., long. 11 26 7 

 W.), discovered as early as 1506 by the Portuguese ; a small 

 circular island of six miles in diameter, in the centre of which 

 a conical mountain is situated, described by Captain Denham 

 as about 8300 feet in height, and composed of volcanic rock 

 (Dr. Petermann's Geogr. MittheiL, 1855, No. iii., s. 84). To 

 the southeast, but in 53 S. lat., lies the equally volcanic 

 Thompson's Island ; and between the two, in the same direc- 

 tion, Gough Island,^also called Diego Alvarez. Deception 



* Darwin, Folcanic Islands, 1844, p. 23, and Lieutenant Lee, Cruise 

 of the United States Brig Dolpttn, 1854,. p. 80. 



f See the admirable description of Ascension in Darwin's Volcanic 

 Islands, p. 40 and 41. 



J Darwin, p. 84 and 92, with regard to "the -great hollow space, or 

 valley southward of the central curved ridge, across which the half of 

 the crater must once have extended. It is interesting to trace the 

 steps by which the structure of a volcanic district becomes obscured 

 and finally obliterated." (See also Scale, Geognosy of the Island of 

 St. Helena, p. 28.) 



