calcareous matter into a firm conglomerate. To show how exceedingly firm
this recent conglomerate is, I may mention, that I endeavoured with a heavy
geological hammer to knock out a thick bolt of iron, which was embedded a
little above low-water mark, but was quite unable to succeed.


CHAPTER II.--FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.

FERNANDO NORONHA.
Precipitous hill of phonolite.

TERCEIRA.
Trachytic rocks: their singular decomposition by steam of high temperature.

TAHITI.
Passage from wacke into trap; singular volcanic 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 ROCKS.
Not of volcanic origin.
Their singular mineralogical composition.


FERNANDO NORONHA.

During our short visit at this and the four following islands, I observed