This rises above the level of the sea
almost as high as the city of Quito, and the summit of Mount
Lebanon.
The greater the quantity of matter that has issued from the crater
of a mountain, the more elevated is its cone of ashes in proportion
to the perpendicular height of the volcano itself. Nothing is more
striking, under this point of view, than the difference of
structure between Vesuvius, the peak of Teneriffe, and Pichincha. I
have chosen this last volcano in preference, because its summit*
enters scarcely within the limit of the perpetual snows. (* I have
measured the summit of Pichincha, that is the small mountain
covered with ashes above the Llano del Vulcan, to the north of Alto
de Chuquira. This mountain has not, however, the regular form of a
cone. As to Vesuvius, I have indicated the mean height of the
Sugar-loaf, on account of the great difference between the two
edges of the crater.) The cone of Cotopaxi, the form of which is
the most elegant and most regular known, is 540 toises in height;
but it is impossible to decide whether the whole of this mass is
covered with ashes.
TABLE 3: VOLCANOES:
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.
