nature."

_Recent formation of oolitic travertin in Lancerote._--Throughout a
considerable part of Lancerote, the old lavas are covered by a thin
stratum of limestone, from an inch to two feet in thickness. It is of a
hard stalactitic nature, sometimes oolitic, like the Jura limestone, and
contains fragments of lava and terrestrial shells, chiefly helices and
spiral bulimi. It sometimes rises to the height of 800 feet above the
level of the sea. Von Buch imagines that this remarkable superstratum
has been produced by the furious northwest storms, which in winter drive
the spray of the sea in clouds over the whole island; from whence
calcareous particles may be deposited stalactitically. Mr. Darwin
informs me that he found a limestone in St. Helena, the harder parts of
which correspond precisely to the stone of Lancerote. He attributes the
origin of this rock in St. Helena not to the spray of the sea, but to
drifting by violent winds of the finer particles of shells from the
sea-beach. Some parts of this drift are subsequently dissolved by
atmospheric moisture, and redeposited, so as to convert calcareous sand
into oolite.

_Recent eruption in Lancerote._--From the year 1736 to 1815, when Von
Buch visited Lancerote, there had been no eruption; but, in August,
1824, a crater opened near the port of Rescif, and formed by its
ejections, in the space of twenty-four hours, a considerable hill.
Violent earthquakes preceded and accompanied this eruption.[605]

_Teneriffe._--The Peak of Teneriffe is about 12,000 feet high, and
stands, says Von Buch, like a tower encircled by its fosse and bastion.
The bastion consists, like the semicircular escarpment of Somma turned
towards Vesuvius, of precipitous cliffs, composed of trachyte, basalt,
coarse conglomerates, and tuffs, traversed by volcanic dikes, mostly
vertical, and of basalt. These cliffs vary in height from 1000 to 1800
feet, and are supposed by Von Buch to have been heaved up into their
present position by a force exerted from below, in accordance with the
theory proposed by the same author for the origin of the cones of
Vesuvius and Etna. According to the observations of M. Deville in
1839[606], the trachytes are often granitoid in their aspect, and
contain instead of glassy felspar the allied mineral called oligoclase,
which had been previously considered as characteristic of more ancient
igneous rocks. The same traveller supposes, although he found no
limestone or tra