Basaltic eruptions seem to have occurred during the Secondary and
Tertiary periods. Basalt, according to Dr. Daubeny,[20] in its more
strict sense, “is composed of an intimate mixture of augite with a
zeolitic mineral, which appears to have been formed out of labradorite
(felspar of Labrador), by the addition of water--the presence of water
being in all _zeolites_ the cause of that bubbling-up under the
blow-pipe to which they owe their appellation.” M. Delesse and other
mineralogists are of opinion that the idea of augite being the
prevailing mineral in basalt, must be abandoned; and that although its
presence gives the rock its distinctive character, as compared with
trachytic and most other trap rocks, still the principal element in
their composition is felspar. Basalt, a lava consisting essentially of
augite, labradorite (or nepheline) and magnetic iron-ore is the rock
which predominates in this formation. It contains a smaller quantity of
silica than the trachyte, and a larger proportion of lime and magnesia.
Hence, independent of the iron in its composition, it is heavier in
proportion, as it contains more or less silica. Some varieties of basalt
contain very large quantities of olivine, a mineral of an olive-green
colour, with a chemical composition differing but slightly from
serpentine. Both basalts and trachyte contain more soda and less silica
in their composition than granites; some of the basalts are highly
fusible, the alkaline matter and lime in their composition acting as a
flux to the silica. There are examples of basalt existing in
well-defined flows, which still adhere to craters visible at the present
day, and with regard to the igneous origin of which there can be no
doubt. One of the most striking examples of a basaltic cone is furnished
by the mountain or crater of La Coupe d’Ayzac, in the Vivarais, in the
south of France. PLATE II., on the opposite page, gives an accurate
representation of this curious basaltic flow. The remnants of the stream
of liquefied basalt which once flowed down the flank of the hill may
still be seen adhering in vast masses to the granite rocks on both sides
of a narrow valley where the river Volant has cut across the lava and
left a pavement or causeway, forming an assemblage of upright prismatic
columns, fitted together with geometrical symmetry; the whole resting on
a base of gneiss. Basaltic eruptions sometimes form a plateau, as
represented in Fig. 5, where the process of formation is shown
theoretically and in a manner which renders further explanation
unnecessary. Many of these basaltic table-lands form plateaux of very
considerable extent and thickness; others form fragments of the same,
more or less dislocated; others, again, present themselves in isolated
knolls, far removed from similar formations. In short, basaltic rocks
present themselves in veins or dykes, more or less, in most countries,
of which Central France and the banks of the Rhine offer many striking
examples. These veins present very evident proofs that the matter has
been introduced from below, and in a manner which could only result from
injection from the interior to the exterior of the earth. Such are the
proofs presented by the basaltic veins of Villeneuve-de-Berg, which
terminate in slender filaments, sometimes bifurcated, which gradually
lose themselves in the rock which they traverse. In several parts of the
north of Ireland, chalk-formations with flints are traversed by basaltic
dykes, the chalk being converted into granular marble near the basalt,
the change sometimes extending eight or ten feet from the wall of the
dyke, and being greatest near the surface of contact. In the Island of
Rathlin, the walls of basalt traverse the chalk in three veins or dykes;
the central one a foot thick, that on the right twenty feet, and on the
left thirty-three feet thick, and all, according to Buckland and
Conybeare, within the breadth of ninety feet.
[20] “Volcanoes,” 2nd ed.
[Illustration: Fig. 7.--Basaltic Causeway, on the banks of the river
Volant, in the Ardèche.]
One of the most striking characteristics of basalt is the prismatic and
columnar structure which it often assumes; the lava being homogeneous
and of very fine grain, the laws which determine the direction of the
fissures or divisional planes consolidated from a molten to a solid
state, become here very manifest--these are always at right angles to
the surfaces of the rock through which the heat of the fused mass
escaped. The basaltic rocks have been at all times remarkable for this
picturesque arrangement of their parts. They usually present columns of
regular prisms, having generally six, often five, and sometimes four,
seven, or even three sides, whose disposition is always perpendicular to
the cooling surfaces. These are often divided transversely, as in Fig.
6, at nearly equal distances, like the joints of a wall, composed of
regularly arranged, equal-sided pieces adhering together, and frequently
extending over a more or less considerable space. The name of Giant’s
Causeway has been given, from time immemorial, to these curious columnar
structures of basalt. In France, in the Vivarais and in the Velay, there
are many such basaltic causeways. That of which Fig. 7 is a sketch lies
on the banks of the river Volant, where it flows into the Ardèche.
Ireland has always been celebrated for its Giant’s Causeway, which
extends over the whole of the northern part of Antrim, covering all the
pre-existing strata of Chalk, Greensand, and Permian formations; the
prismatic columns extend for miles along the cliffs, projecting into the
sea at the point specially designated the Giant’s Causeway.
These columnar formations vary considerably in length and diameter.
McCulloch mentions some in Skye, which “are about four hundred feet
high; others in Morven not exceeding an inch (vol. ii. p. 137). In
diameter those of Ailsa Craig measure nine feet, and those of Morven an
inch or less.” Fingal’s Cave, in the Isle of Staffa, is renowned among
basaltic rocks, although it was scarcely known on the mainland a century
ago, when Sir Joseph Banks heard of it accidentally, and was the first
to visit and describe it. Fingal’s Cave has been hollowed out, by the
sea, through a gallery of immense prismatic columns of trap, which are
continually beaten by the waves. The columns are usually upright, but
sometimes they are curved and slightly inclined. Fig. 8 is a view of the
basaltic grotto of Staffa.
Grottoes are sometimes formed by basaltic eruptions on land, followed by
their separation into regular columns. The Grotto of Cheeses, at
Bertrich-Baden, between Trèves and Coblentz, is a remarkable example of
this kind, being so called because its columns are formed of round, and
usually flattened, stones placed one above the other in such a manner as
to resemble a pile of cheeses.
[Illustration: Fig. 8.--Basaltic cavern of Staffa--exterior.]
If we consider that in basalt-flows the lower part is compact, and often
divided into prismatic columns, while the upper part is porous,
cellular, scoriaceous, and irregularly divided--that the points of
separation on which they rest are small beds presenting fragments of the
porous stony concretions known under the name of _Lapilli_--that the
lower portions of these masses present a multitude of points which
penetrate the rocks on which they repose, thereby denoting that some
fluid matter had moulded itself into its crevices--that the neighbouring
rocks are often calcined to a considerable thickness, and the included
vegetable remains carbonised--no doubt can exist as to the igneous
origin of basaltic rocks. When it reached the surface through certain
openings, the fluid basalt spread itself, flowing, as it were, over the
horizontal surface of the ground; for if it had flowed upon inclined
surfaces it could not have preserved the uniform surface and constant
thickness which it generally exhibits.
[Illustration: III.--Extinct volcanoes forming the Puy-de-Dôme Chain.]