This formation is in many parts overlaid, in the usual order of
superposition, by streams of basaltic lava, which near the coast compose
nearly the whole surface. The course which these streams have followed from
their craters, can often be followed by the eye. The town of Angra is
overlooked by a crateriform hill (Mount Brazil), entirely built of thin
strata of fine-grained, harsh, brown-coloured tuff. The upper beds are seen
to overlap the basaltic streams on which the town stands. This hill is
almost identical in structure and composition with numerous crateriformed
hills in the Galapagos Archipelago.

EFFECTS OF STEAM ON THE TRACHYTIC ROCKS.

In the central part of the island there is a spot, where steam is
constantly issuing in jets from the bottom of a small ravine-like hollow,
which has no exit, and which abuts against a range of trachytic mountains.
The steam is emitted from several irregular fissures: it is scentless, soon
blackens iron, and is of much too high temperature to be endured by the
hand. The manner in which the solid trachyte is changed on the borders of
these orifices is curious: first, the base becomes earthy, with red
freckles evidently due to the oxidation of particles of iron; then it
becomes soft; and lastly, even the crystals of glassy feldspar yield to the
dissolving agent. After the mass is converted into clay, the oxide of iron
seems to be entirely removed from some parts, which are left perfectly
white, whilst in other neighbouring parts, which are of the brightest red
colour, it seems to be deposited in greater quantity; some other masses are
marbled with two distinct colours. Portions of the white clay, now that
they are dry, cannot be distinguished by the eye from the finest prepared
chalk; and when placed between the teeth they feel equally soft-grained;
the inhabitants use this substance for white-washing their houses. The
cause of the iron being dissolved in one part, and close by being again