Column 1: Name of the volcano.
Column 2: Total height in toises.
Column 3: Height of the cone covered with ashes.
Column 4: Proportion of the cone to the total height.
Vesuvius : 606 : 200 : 1/3.
Peak of Teneriffe : 1904 : 84 : 1/22.
Pichincha : 2490 : 240 : 1/10.
This table seems to indicate, what we shall have an opportunity of
proving more amply hereafter, that the peak of Teneriffe belongs to
that group of great volcanoes, which, like Etna and Antisana, have
had more copious eruptions from their sides than from their
summits. Thus the crater at the extremity of the Piton, which is
called the Caldera, is extremely small. Its diminutive size struck
M. de Borda, and other travellers, who took little interest in
geological investigations.
As to the nature of the rocks which compose the soil of Teneriffe,
we must first distinguish between productions of the present
volcano, and the range of basaltic mountains which surround the
Peak, and which do not rise more than five or six hundred toises
above the level of the ocean. Here, as well as in Italy, Mexico,
and the Cordilleras of Quito, the rocks of trap-formation* are at a
distance from the recent currents of lava (* The trap-formation
includes the basalts, green-stone (grunstein), the trappean
porphyries, the phonolites or porphyrschiefer, etc.); everything
shows that these two classes of substances, though they owe their
origin to similar phenomena, date from very different periods. It
is important to geology not to confound the modern currents of
lava, the heaps of basalt, green-stone, and phonolite, dispersed
over the primitive and secondary formations, with those porphyroid
masses having bases of compact feldspar,* which perhaps have never
been perfectly liquified, but which do not less belong to the
domain of volcanoes.
