_Trachyte_ (derived from τραχυς, rough), having a coarse, cellular
appearance, and a rough and gritty feel, belongs to the class of
volcanic rocks. The eruptions of trachyte seem to have commenced towards
the middle of the Tertiary period, and to have continued up to its
close. The trachytes present considerable analogy in their composition
to the felspathic porphyries, but their mineralogical characters are
different. Their texture is porous; they form a white, grey, black,
sometimes yellowish matrix, in which, as a rule, felspar predominates,
together with disseminated crystals of felspar, some hornblende or
augite, and dark-coloured mica. In its external appearance trachyte is
very variable. It forms the three most elevated mountain ranges of
Central France; the groups of Cantal and Mont Dore, and the chain of the
Velay (Puy-de-Dôme).[19]
[19] For full information in reference to the rocks and geology of
this part of France, the reader is referred to the masterly work
on “The Geology and Extinct Volcanoes of Central France,” by G.
Poulett Scrope, 2nd edition, 1858.
[Illustration: I.--Peak of Sancy in the Mont Dore group, Auvergne.]
The igneous group of Cantal may be described as a series of lofty
summits, ranged around a large cavity, which was at one period probably
a volcanic crater, the circular base of which occupies an area of nearly
fifteen leagues in diameter. The strictly trachytic portion of the group
rises in the centre, and is composed of high mountains, throwing off
spurs, which gradually decrease in height, and terminate in plateaux
more or less inclined. These central mountains attain a height varying
between 4,500 and 5,500 feet above the level of the sea. A scaly or
schistose variety of trachyte, called _phonolite_, or clinkstone (from
the ringing metallic sound it emits when struck with the hammer), with
an unusual proportion of felspar, or, according to Gmelin, composed of
felspar and zeolite, forms the steep trachytic escarpments at the
centre, which enclose the principal valleys; their abrupt peaks giving a
remarkably picturesque appearance to the landscape. In the engraving on
p. 40 (Fig. 4) the slaty, laminated character of the clinkstone is well
represented in one of the phonolitic peaks of the Cantal group. The
group at Mont Dore consists of seven or eight rocky summits, occupying a
circuit of about five leagues in diameter. The massive trachytic rock,
of which this mountainous mass is chiefly formed, has an average
thickness of 1,200 to 2,600 feet; comprehending over that range
prodigious layers of scoriæ, pumiceous conglomerates, and detritus,
interstratified with beds of trachyte and basalt, bearing the signs of
an igneous origin, tufa forming the base; and between them occur layers
of lignite, or imperfectly mineralised woody fibre, the whole being
superimposed on a primitive plateau of about 3,250 feet in height.
Scored and furrowed out by deep valleys, the viscous mass was gradually
upheaved, until in the needle-like Pic de Sancy (PLATE I.), a pyramidal
rock of porphyritic trachyte, which is the loftiest point of Mont Dore,
it attains the height of 6,258 feet. The Pic de Sancy, represented on
page 40 (Fig. 4), gives an excellent idea of the general appearance of
the trachytic mountains of Mont Dore.
Upon the same plateau with Mont Dore, and about seven miles north of its
last slopes, the trachytic formation is repeated in four rounded
domes--those of Puy-de-Dôme, Sarcouï, Clierzou, and Le Grand Suchet. The
Puy-de-Dôme, one of the most remarkable volcanic domes in Auvergne,
presents another fine and very striking example of an eruptive trachytic
rock. The rock here assumes a peculiar mineral character, which has
caused the name of _domite_ to be given to it.
The chain of the Velay forms a zone, composed of independent plateaux
and peaks, which forms upon the horizon a long and strangely
intersected ridge. The bareness of the mountains, their forms--pointed
or rounded, sometimes terminating in scarped plateaux--give to the whole
landscape an appearance at once picturesque and characteristic. The peak
of Le Mezen, which rises 5,820 feet above the sea, forms the culminating
point of the chain. The phonolites of which it consists have been
erupted from fissures which present themselves at a great number of
points, ranging from north-north-west to south-south-east.
On the banks of the Rhine and in Hungary the trachytic formation
presents itself in features identical with those which indicate it in
France. In America it is principally represented by some immense cones,
superposed in the chain of the Andes; the colossal Chimborazo being one
of those trachytic cones.
[Illustration: II.--Mountain and basaltic crater of La Coupe d’Ayzac, in
the Vivarais.]
[Illustration: Fig. 5.--Theoretical view of a basaltic plateau.]
[Illustration: Fig. 6.--Basalt in prismatic columns.]