CHAPTER II. 



Fernando Noronha — Precipitous hill of phonolite. Terceira — 

 Trachytic rocks ; their singular decomposition by steam of high tem- 

 perature. Tahiti — Passage from wacke into trap ; singular vol- 

 canic rock with the vesicles half filled with mesotype. Mauritius 

 — Proofs of its recent elevation — Structure of its more ancient 

 mountains ; similarity with St. Jago. St. Paul's Eocks— Not of 

 volcanic origin — their singular mineralogical composition. 



Fernando Noronha. — During our short visit at this 

 and the four following islands, I observed very little 

 worthy of description. Fernando Noronha is situated 

 in the Atlantic Ocean, in lat. 3° 50' S., and 230 miles 

 distant from the coast of South America. It consists 

 of several islets, together nine miles in length by three 

 in breadth. The whole seems to be of volcanic origin ; 

 although there is no appearance of any crater, or of any 

 one central eminence. The most remarkable feature 

 is a hill 1,000 feet high, of which the upper 400 feet 

 consist of a precipitous, singularly shaped pinnacle, 

 formed of columnar phonolite, containing numerous 

 crystals of glassy feldspar, and a few needles of horn- 

 blende. From the highest accessible point of this hill, 

 I could distinguish in different parts of the group 

 several other conical hills, apparently of the same nature. 

 At St. Helena there are similar, great, conical, protu- 

 berant masses of phonolite, nearly 1,000 feet in height, 

 which have been formed by the injection of fluid feld- 

 spathic lava into yielding strata. If this hill has had, 



