The Student’s Elements of Geology

Author: Charles Lyell

Columnar and Globular Structure.

One of the characteristic forms of volcanic rocks, especially of basalt, is the columnar, where large masses are divided into regular prisms, sometimes easily separable, but in other cases adhering firmly together. The columns vary, in the number of angles, from three to twelve; but they have most commonly from five to seven sides. They are often divided transversely, at nearly equal distances, like the joints in a vertebral column, as in the Giant’s Causeway, in Ireland. They vary exceedingly in respect to length and diameter. Dr. MacCulloch mentions some in Skye which are about 400 feet long; others, in Morven, not exceeding an inch. In regard to diameter, those of Ailsa measure nine feet, and those of Morven an inch or less. (MacCulloch System of Geology volume 2 page 137.) They are usually straight, but sometimes curved; and examples of both these occur in the island of Staffa. In a horizontal bed or sheet of trap the columns are vertical; in a vertical dike they are horizontal.

(FIGURE 588. Lava of La Coupe d’Ayzac, near Antraigue, in the Department of Ardeche.)

It being assumed that columnar trap has consolidated from a fluid state, the prisms are said to be always at right angles to the COOLING SURFACES. If these surfaces, therefore, instead of being either perpendicular or horizontal, are curved, the columns ought to be inclined at every angle to the horizon; and there is a beautiful exemplification of this phenomenon in one of the valleys of the Vivarais, a mountainous district in the South of France, where, in the midst of a region of gneiss, a geologist encounters unexpectedly several volcanic cones of loose sand and scoriae. From the crater of one of these cones, called La Coupe d’Ayzac, a stream of lava has descended and occupied the bottom of a narrow valley, except at those points where the river Volant, or the torrents which join it, have cut away portions of the solid lava. Figure 588 represents the remnant of the lava at one of these points. It is clear that the lava once filled the whole valley up to the dotted line d-a; but the river has gradually swept away all below that line, while the tributary torrent has laid open a transverse section; by which we perceive, in the first place, that the lava is composed, as usual in this country, of three parts: the uppermost, at a, being scoriaceous, the second b, presenting irregular prisms; and the third, c, with regular columns, which are vertical on the banks of the Volant, where they rest on a horizontal base of gneiss, but which are inclined at an angle of 45 degrees, at g, and are nearly horizontal at f, their position having been everywhere determined, according to the law before mentioned, by the form of the original valley.

(FIGURE 589. Columnar basalt in the Vincentin. (Fortis.)

In Figure 589, a view is given of some of the inclined and curved columns which present themselves on the sides of the valleys in the hilly region north of Vicenza, in Italy, and at the foot of the higher Alps. (Fortis Mem. sur l’Hist. Nat. de l’Italie tome 1 page 233 plate 7.) Unlike those of the Vivarais, last mentioned, the basalt of this country was evidently submarine, and the present valleys have since been hollowed out by denudation.

(FIGURE 590. Basaltic pillars of the Kasegrotte, Bertrich-Baden, half-way between Treves and Coblentz. Height of grotto, from 7 to 8 feet.)

The columnar structure is by no means peculiar to the trap rocks in which augite abounds; it is also observed in trachyte, and other feldspathic rocks of the igneous class, although in these it is rarely exhibited in such regular polygonal forms. It has been already stated that basaltic columns are often divided by cross-joints. Sometimes each segment, instead of an angular, assumes a spheroidal form, so that a pillar is made up of a pile of balls, usually flattened, as in the Cheese-grotto at Bertrich-Baden, in the Eifel, near the Moselle (Figure 590). The basalt there is part of a small stream of lava, from 30 to 40 feet thick, which has proceeded from one of several volcanic craters, still extant, on the neighbouring heights.

In some masses of decomposing greenstone, basalt, and other trap rocks, the globular structure is so conspicuous that the rock has the appearance of a heap of large cannon balls. According to M. Delesse, the centre of each spheroid has been a centre of crystallisation, around which the different minerals of the rock arranged themselves symmetrically during the process of cooling. But it was also, he says, a centre of contraction, produced by the same cooling, the globular form, therefore, of such spheroids being the combined result of crystallisation and contraction. (Delesse sur les Roches Globuleuses Mem. de la Soc. Geol. de France 2 ser. tome 4.)

(FIGURE 591. Globiform pitchstone. Chiaja di Luna, Isle of Ponza. (Scrope.))

Mr. Scrope gives as an illustration of this structure a resinous trachyte or pitchstone-porphyry in one of the Ponza islands, which rise from the Mediterranean, off the coast of Terracina and Gaeta. The globes vary from a few inches to three feet in diameter, and are of an ellipsoidal form (see Figure 591). The whole rock is in a state of decomposition, "and when the balls," says Mr. Scrope, "have been exposed a short time to the weather, they scale off at a touch into numerous concentric coats, like those of a bulbous root, inclosing a compact nucleus. The laminae of this nucleus have not been so much loosened by decomposition; but the application of a ruder blow will produce a still further exfoliation." (Scrope Geological Transactions second series volume 2 page 205.)


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Chicago: Charles Lyell, "Columnar and Globular Structure.," The Student’s Elements of Geology, ed. Bryant Conant, James and trans. Babington, B. G. (Benjamin Guy), 1794-1866 in The Student’s Elements of Geology Original Sources, accessed October 4, 2023,

MLA: Lyell, Charles. "Columnar and Globular Structure." The Student’s Elements of Geology, edited by Bryant Conant, James, and translated by Babington, B. G. (Benjamin Guy), 1794-1866, in The Student’s Elements of Geology, Original Sources. 4 Oct. 2023.

Harvard: Lyell, C, 'Columnar and Globular Structure.' in The Student’s Elements of Geology, ed. and trans. . cited in , The Student’s Elements of Geology. Original Sources, retrieved 4 October 2023, from