in form and
composition from those just described, and have their typical
representatives in the Auvergne district, though not without their
analogues elsewhere, as in the case of Chimborazo, in South America, one
of the loftiest volcanic mountains in the world.

[Illustration: Fig. 2.--Cotopaxi, a volcano of the Cordilleras of
Quito, still active, and covered by snow down to a level of 14,800 feet.
Below this is a zone of naked rock, succeeded by another of forest
vegetation. Owing to the continuous extrusion of lava from the crater,
the cone is being gradually built up of fresh material, and the crater
is comparatively small in consequence.--(A diagrammatic view after A.
von Humboldt.)]

Taking the Puy de Dome, Petit Suchet, Cliersou, Grand Sarcoui in
Auvergne, and the Mamelon in the Isle of Bourbon as illustrations, we
have in all these cases a group of volcanic hills, dome-shaped and
destitute of craters, the summits being rounded or slightly flattened.
We also observe that the flanks rise more abruptly from their bases, and
contrast in outline with the graceful curve of the crater cones. The
dome-shaped volcanoes are generally composed of felsitic matter, whether
domite, trachyte, or andesite, which has been extruded in a molten or
viscous condition from some orifice or fissure in the earth's crust, and
being piled up and spreading outwards, necessarily assumes such a form
as that of a dome, as has been shown by experiment on a small scale by
Dr. E. Reyer, of Graetz.[5] The contrast between the two forms (those of
the dome and the crater-cone) is exemplified in the case of the Grand
Sarcoui and its neighbours. The former is composed of a species of
trachyte; the latter of ashes and fragmental matter which have been
blown out of their respective vents of eruption into the air, and piled
up and around in a crateriform manner with sides of gradually
diminishing slope outwards, thus giving rise to the characteristic
volcanic curve. The two varieties here referred to, contrasting in form,
composition, and colour of material, can be clearly recognised from the
summit of the Puy de Dome, which rises by a head and shoulders above its
fellows, and thus affords an advantageous standpoint from which to
compare the various forms of this remarkable group of volcanic
mountains.

Cotopaxi (Fig. 2) has been generally supposed to be a dome; but Whymper,
who ascended the mountain in 1880, shows that it is a cone with a
crater,