viii Contents. 



CHAPTER ILL 



ASCENSION. 



Basaltic lavas — Numerous craters truncated on the same side — 

 Singular structure of volcanic bombs — Aeriform explosions — 

 Ejected granitic fragments — Trachytic rocks — Singular veins — 

 Jasper, its manner of formation — Concretions in pumiceous tuff 

 — Calcareous deposits and frondescent incrustations on the coast 

 — Remarkable laminated beds, alternating with, and passing 

 into obsidian— Origin of obsidian — Lamination of volcanic 

 rocks . page 40 



CHAPTER IV, 



ST. HELENA. 



Lavas of the feldspathic, basaltic, and submarine series — Section of 

 Flagstaff Hill and of the Barn— Dikes— Turk's Cap and Pros- 

 perous Bays — Basaltic ring — Central crateriform ridge, with an 

 internal ledge and a parapet — Cones of phonolite — Superficial 

 beds of calcareous sandstone — Extinct land-shells — Beds of 

 detritus — Elevation of the land — Denudation — Craters of 

 elevation 83 



CHAPTER Y. 



GALAPAGOS AECHIPELAGO. 



Chatham Island — Craters composed of a peculiar kind of tuff — 

 Small basaltic craters, with hollows at their bases — Albemarle 

 Island ; fluid lavas, their composition — Craters of tuff ; inclina- 

 tion of their exterior diverging strata, and structure of their 

 interior converging strata — James Island, segment of a small 

 basaltic crater : fluidity and composition of its lava streams, and 

 of its ejected fragments — Concluding remarks on the craters 

 of tuff, and on the breached condition of their southern sides 

 — Alineralogical composition of the rocks of the archipelago 

 — Elevation of the land — Direction of the fissures of erup- 

 tion 1 10 



