The World Before the Deluge

• "Volcanoes," 2nd ed.

II.—Mountain and basaltic crater of La Coupe d’Ayzac, in the Vivarais.

BASALTIC FORMATIONS,

47 existing m 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

Fig. 5.— Theoretical view of a basaltic plateau.

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

Fig. 6. — Basalt in prismatic columns.

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

Fig. 7.— Basaltic Causeway, on the banks of the river Volant, in the Ardeche.

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. 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 isalways 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 Ardeche.

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.

THE WORLD BEFORE THE DELUGE,

50 Grottoes are sometimes formed by basaltic eruptions on land^ followed by their separation into regular columns. The Grotto of Cheeses, at Bertrich-Baden, between Treves 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.

If we consider that in basalt-flows the lower part is compact, and often divided into prismatic columns, while the upper part is porous,.

Fig. 8. — Basaltic cavern of Staffa — exterior.

cellular, scorlaceous, 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

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III.—Extinct volcanoes forming the Puy-de-Dôme Chain.

had flowed upon inclined surfaces it could not have preserved the uniform surface and constant thickness which it generally exhibits.

Volcanic or Lava Formations.

The lava formations comprehend both extinct and active volcanoes. "The term," says Lyell, "has a somewhat vague signification, having been applied to all melted matter observed to fiow^ in streams from volcanic vents. When this matter consolidates in the open air, the upper part is usually scoriaceous, and the mass becomes more and more stony as we descend, or in proportion as it has consolidated more slowly and under greater pressure."*

The formation of extinct volcanoes is represented in France by the volcanoes situated in the ancient provinces of Auvergne, Velay, and the Vivarais, but principally by nearly seventy volcanic cones of various sizes and of the height of from 500 to 1,000 feet, composed of loose scorise, lava, and pozzuolana, arranged upon a granitic table-land, about twelve miles wide, which overlooks the town of Clermont-Ferrand, and which seem to have been produced along a longitudinal fracture in the earth's crust, running in a direction from north to south. It is a range of volcanic hills, the " chain of Piiys^' nearly twenty miles in length, by two in breadth. By its cellular and porous structure, which is also granular and crystalline, the felspathic or pyroxenic lava which flowed from these volcanoes is readily distinguishable from the analogous lavas which belong to the basaltic or trachytic fomiations. Their surface is irregular, and bristles with asperities, formed by heaped-up angular blocks.I

The volcanoes of the chain of Puys^ represented on opposite page (Pl. III.) are so perfectly preserved, their lava is so frequently superposed on sheets of basalt, and presents a composition and texture so distinct, that there is no difficulty in establishing the fact that they are posterior to the basaltic formation, and of very recent age. Nevertheless, they do not appear to belong to the historic Ieruptionsages, for inno thetraditionLower attestsMiocenetheirperiod,eruption.and theirLyellgreatestplacesactivitythese in the Upper Miocene and Pliocene eras. " Extinct quadrupeds of those eras," he says, " belonging to the genera mastodon, rhinoceros, and others, were buried in ashes and beds of alluvial sand and gravel, which owe their presetvation to overspreading sheets of lava."t

• " Elements of Geology," p. 596. "j- Ibid, p. 677.

All volcanic phenomena can be explained by the theory we have already indicated, of fractures in the solid crust of the globe resulting from its cooling. The various phenomena which existing volcanoes present to us are, as Humboldt has said, " the result of every action exercised by the interior of a planet on its external crust."* We

Fig. 9 — Section of a volcano in acti'

designate as volcanoes all conduits which establish a permanent communication between the interior of the earth and its surface — a conduit which gives passage at intervals to eruptions oi lava, and in

Fig. 9 we have represented, in an ideal section, the geological mode of action of volcanic eruptions. The volcanoes on the surface of the globe, known to be in an occasional state of activity, number about

• "Cosmos," vol. i., p. 25. Bolin.

three hundred, and these may be divided into two classes: the isolated or central^ and the lijiear or those volcanoes which belong to a series*

The first are active volcanoes, around which there may be established many secondary active mouths of eruption, always in connection with some principal crater. The second are disposed like the chimneys of furnaces, along fissures extending over considerable distances. Twenty, thirty, and even a greater number of volcanic cones may rise above one such rent in the earth's crust, the direction of which will be indicated by their linear' course. The Peak of Teneriffe is an instance of a central volcano; the long rampart-like chain of the Andes, presents, from the south of Chili to the northwest coast of America, one of the grandest instances of a continental volcanic chain; the remarkable range of volcanoes in the province of Quito belong to the latter class. Darwin relates that on the 19th of March, 1835, the attention of a sentry was called to something like a large star which gradually increased in size till about three o'clock, when it presented a very magnificent spectacle. " By the aid of a glass, dark objects, inconstant succession, were seen in the midst of a great glare of red light, to be thrown up and to fall down. The light was sufficient to cast on the water a long bright reflection — it was the volcano of Osorno in action." Mr. Darwin was afterwards assured that Aconcagua, in Chili, 480 miles to the north, was in action on the same night, and that the great eruption of Coseguina (2,700 miles north of Aconcagua), accompanied by an earthquake felt over 1,000 miles, also occurred within six hours of this same time; and yet Coseguina had been dormant for six-and-twenty years, and Aconcagua most rarely shows any signs of action.t It is also stated by Professor Dove that in the year 1835 the ashes discharged from the mountain of Coseguina were carried 700 miles, and that the roaring noise of the eruption was heard at San Salvador, a distance of 1,000 miles.