tinct. It must be supposed, however, that at a former period volcanic
action was vastly more energetic than at present; for, except at the
Grand Canary, Gomera, Forta Ventura, and Lancerote, where various
non-volcanic rocks are found, these islands appear to have been built up
from their foundations of eruptive materials. The highest point in the
Azores is the Peak of Pico, which rises to a height of 7016 feet above
the ocean. But this great elevation is surpassed by that of the Peak of
Teneriffe (or Pic de Teyde) in the Canaries, which attains to an
elevation of 12,225 feet, as determined by Professor Piazzi Smyth.[2]

This great volcanic cone, rising from the ocean, its summit shrouded in
snow, and often protruding above the clouds, must be an object of
uncommon beauty and interest when seen from the deck of a ship. (Fig.
4.) The central cone, formed of trachyte, pumice, obsidian, and ashes,
rises out of a vast caldron of older basaltic rocks with precipitous
inner walls--much as the cone of Vesuvius rises from within the
partially encircling walls of Somma. (Fig. 5.) From the summit issue
forth sulphurous vapours, but no flame.

Piazzi Smyth, who during a prolonged visit to this mountain in 1856 made
a careful survey of its form and structure, shows that the great cone is
surrounded by an outer ring of basalt enclosing two _foci_ of eruption,
the lavas from which have broken through the ring of the outer crater
on the western side, and have poured down the mountain. At the top of
the peak its once active crater is filled up, and we find a convex
surface ("The Plain of Rambleta") surmounted towards its eastern end by
a diminutive cone, 500 feet high, called "Humboldt's Ash Cone." The
slope of the great cone of Teneriffe ranges from 28 deg. to 38 deg.; and below a
level of 7000 feet the general slope of the whole mountain down to the
water's edge varies from 10 deg. to 12 deg. from the horizontal. The great cone
is penetrated by numerous basaltic dykes.

The Cape de Verde Islands, which contain beds of limestone along with
volcanic matter, possess in the island of Fuego an active volcano,
rising to a height of 7000 feet above the surface of the ocean. The
central cone, like that of Teneriffe, rises from within an outer crater,
formed of basalt alternating with beds of agglomerate, and traversed by
numerous dykes of lava. This has been broken down on one side like that
of Somma; and over its flanks are scattered numero