ce of fossils in any of the rocks of Teneriffe, that the
alternating trachytes and trachytic conglomerates originated beneath the
sea. If this opinion be correct, and it is at least very probable,
geologists may still speculate on two modes in which the mass of the
island acquired its present form and elevation above the sea. 1st, The
advocates of Von Buch's crater-of-elevation hypothesis may imagine that
a succession of horizontally superimposed beds were upheaved by a sudden
movement, and tilted so as to dip in all directions outwards from the
centre of a new dome-shaped eminence, in the middle of which a large
opening or bowl-shaped cavity was produced. 2dly, Or according to the
theory which to me appears preferable, a submarine hill in the form of a
flattened dome may have gradually accumulated, partly below the waters
and partly above by the continued outpourings of sheets of lava and the
ejection of ashes from a central orifice. In this case the dikes would
represent the fissures, which were filled during successive eruptions,
and the original inclination of the beds may have been increased by the
distension and upheaval of the mass during reiterated convulsions,
acting most forcibly at or near the channel of discharge, which would
become partially sealed up with lava from time to time, and then be
burst open again during eruptions. At length the whole island may have
been raised bodily out of the sea by a gradual upward movement.

Whatever theory we adopt, we must always explain the abrupt termination
of the dikes and layers of trachyte and basalt in the steep walls of the
escarpments surrounding the great crater by supposing the removal of
part of the materials once prolonged farther inward towards the centre.
If, according to the elevation-crater hypothesis, a series of sheets of
lava and ashes originally spread over a level and even surface have been
violently broken and uplifted, why do not the opposite walls of the
chasm correspond in such a manner as to imply by their present outline
that they were formerly united? It is evident that the precipices on
opposite sides of the crateriform hollow would not fit if brought
together, there being no projecting masses in one wall to enter into
indentations in the other, as would happen with the sides of many
mineral veins, trap-dikes, and faults, could we extract the intrusive
matter now separating them, and reunite the rocks which have been
fractured and disjoined.