us cones of scoriae,
the most recent dating from the years 1785 and 1799.[3]

[Illustration: Fig. 5.--View of the summit of the Peak of Teneriffe
(12,225 feet) and of the secondary crater, or outer ring of basaltic
sheets which surrounds its base; seen from the east.--(After Leopold von
Buch.)]

The volcanoes of Lancerote have a remarkably linear arrangement from
west to east across the island. They are not yet extinct; for an
eruption in 1730 destroyed a large number of villages, and covered with
lava the most fertile tracts in the island, which at the time of Leopold
von Buch's visit lay waste and destitute of herbage.[4] In the island of
Palma there is one large central crater, the Caldera de Palma, three
leagues in diameter, the walls of which conform closely to the margin
of the coast. Von Buch calls this crater "une merveille de la nature,"
for it distinguishes this isle from all the others, and renders it one
of the most interesting and remarkable amongst the volcanic islands of
the ocean. The outer walls are formed of basaltic sheets, and towards
the south this great natural theatre is connected with the ocean by a
long straight valley, called the "Barranco de los Dolores," along whose
sides the structure of the mountain is deeply laid open to view. The
outer flanks of the crater are furrowed by a great number of smaller
barrancos radiating outward from the rim of the caldera. Von Buch
regards the barrancos as having been formed during the upheaval of the
island, according to his theory of the formation of such mountains (the
elevation-theory); but unfortunately for his views, these ravines widen
outwards from the centre, or at least do not become narrower in that
direction, as would be the case were the elevation-theory sound. The
maps which accompany Von Buch's work are remarkably good, and were
partly constructed by himself.

(_c._) _Volcanic Islands in the Atlantic south of the Equator._--The
island of Ascension, formed entirely of volcanic matter, rises from a
depth of 2000 fathoms in the very centre of the Atlantic. As described
by Darwin, the central and more elevated portions are formed of
trachytic matter, with obsidian and laminated ash beds. Amongst these
are found ejected masses of unchanged granite, fragments of which have
been torn from the central pipe during periods of activity, and would
seem to indicate a granitic floor, or at least an original floor upon
which more recent deposits may have