er; in all
cases the basalt is the newer of the two varieties of rock, and this is
generally the case throughout the region here described.

(_e._) _Volcanoes of Mont Dore._--This mountain lies to the north of
that of Cantal, and somewhat resembles it in general structure and
configuration. Like Cantal, it is destitute of any distinct crater; all
that is left of the central focus of eruption being the solidified
matter which filled the throat of the original volcano, and which forms
a rocky mass of lava, rising in its highest point, the Pic de Saucy, to
an elevation (as given by Ramond) of 6258 feet above the level of the
sea, thus exceeding that of the Plomb du Cantal by 128 feet. Its figure
will be best understood by supposing seven or eight rocky summits
grouped together within a circle of about a mile in diameter, from
whence, as from the apex of an irregular and flattened cone, all the
sides slope more or less rapidly downwards, until their inclination is
gradually lost in the plain around. This dome-shaped mass has been
deeply eroded on opposite sides by the valleys of the Dordogne and
Chambon; while it is further furrowed by numerous minor streams.[8]

The great beds of volcanic rock, disposed as above stated, consist of
prodigious layers of scoriae, pumice-stones, and detritus, alternating
with beds of trachyte and basalt, which often descend in uninterrupted
currents till they reach the granite platform, and then spread
themselves for miles around. The sheets of basalt are found to stretch
to greater distances than those of trachyte, and have flowed as far as
15 or 20 miles from their orifices of eruption; while in some cases, on
the east and north sides, they have extended as far as 25 or 30 miles
from the central height. On the other hand, a radius of about ten miles
from the centre would probably include all the streams of trachyte;--so
much greater has been the viscosity of the basalt over the latter rock.
Some portions of these great sheets of lava, cut off by river valleys or
eroded areas from the main mass of which they once formed a part, are
found forming isolated terraces and plateaux either on the granitic
platform, or resting on the fresh-water strata of the valley of the
Allier, while in a northern direction they overspread a large portion of
the granitic plateau from which rise the Puy de Dome and associated
volcanic mountains. Still more remarkable are the cases in which these
lava-streams have