Plate I: Volcanoes & their products. 1. General View of Vesuvius in 1757.

Plate II: Volcanoes. 1. Eruptions of Vesuvius in 1754.

Plate III: Volcanoes. 1. Lava emanating from the flanks of Vesuvius following the 1754 eruption.

Plate IV: Volcanoes. 1. Summit of Vesuvius. 2. Other summit of Vesuvius during a small eruption.

Plate V: Solfatara. View of the Solfatara or Sulfur Mine near Pozzuoli in the Kingdom of Naples. Alum work sites. 2. Boiling spring that appears as if on fire.

Plate VI: Giant's Causeway. 1. Giant's Causeway in County Antrim in Ireland. AAA. Articulations in the shape of an old crown. BBB. Other articulations on which contiguous ones fit. CCC. Articulations that are convex on both sides.

Plate VII: Basalt of Auvergne. 1. Face of a mound [2] composed entirely of articulated prisms, [3] on which the old castle of La Tour d'Auvergne was situated. View of the natural pavement that covers a large flat area where the markets [4] of this small town are held. [5]

For a long time, the county of Antrim in the kingdom of Ireland was taken as the only place in possession of one of the most curious and magnificent monuments of Natural History. Basalt in prisms composed of regular articulations such as are described in the article Giant’s Causeway had been found only in this district. [6] In 1763, M. Desmarest, Inspector of manufactures of the généralité of Limoges, informed about what had been published on this subject, discovered in Auvergne the same kind of stone, also in regular prisms, and with the same curious details that were admired as a unique phenomenon in the Giant’s Causeway . He first established the perfect analogy of the two phenomena by assimilating all the features of resemblance that he could draw from observations. [7] This important point having been recognized, he set himself to the consideration of other equally interesting objects. In the varied clusters of Basalt that Auvergne displays on all sides, he had the advantage of studying these features. This information made possible his resolution [8] of the singular forms that Basalt takes, and moreover the unqualified determination of the nature and origin of this stone over which the nomenclators had put forward only vague and often contradictory statements. [9]

By a series of observations that he continued in 1764 and 1766, [10] he has made certain that the assemblages of prisms constituted part of the flows of melted matter issued from several volcanoes, whose mouths, as well as the products of the various eruptions they formerly experienced, are located in the vicinity of Mont d'Or. [11] He saw these prisms placed quite constantly toward the extremity of all these flows on beds of cinders and scoria, which usually rested on top of intact masses of granite. And he had the satisfaction at length to follow these same flows up a slope that led him without any break from the prismatic masses to the volcanoes' mouths where these flows took their origin.

It therefore results from his researches, 1. that Basalt in prisms, articulated or non-articulated, is a substance that has been melted in the hearth [12] of volcanoes, and has issued forth during time of eruptions; 2. that this substance is an infallible indication of the existence of former, extinct volcanoes; 3. that the regular shapes of Basalt are a consequence of the uniform contraction this material has undergone in fusion as it has cooled and congealed, in contracting around several centers of activity.

M. Desmarest was led to this last conclusion through examination of several facts that cannot be set out in this summary, and that must be seen in an extended work, where he reports on his discoveries. [13] This presentation will be limited to offering only some main observations regarding the theory of prismatic form, and especially that of articulations.

This prismatic shape, to which the English authors who have described the Giant’s Causeway appear to have been attached a bit too much in preference to all others, is not the only form that Basalt takes, nor the only one meriting the attention of Naturalists. [14]

Basalt is often seen in the shapes of balls, [15] or in ellipsoids, composed of concentric layers quite distinct from one another. In certain parts of these same flows displaying prisms, one finds considerable quantities of these balls accumulated one on top of another. Ball-shaped Basalt is noticed just as frequently as in prismatic shapes, with this particular feature, that the balls or ellipsoids are located amidst powdery materials which seem to have enveloped precisely each of these rounded bodies, or which present a smooth surface on all sides.

But very often when segments of the matter of melted Basalt disposed to form in round shapes is not quite completely insulated in envelopes of cinders and scoria, and has run into several other segments in the open, this has resulted in varied points of contact, such that the round masses have become, in some parts of their surface, or even at all parts, faceted bodies, more or less regular. The points of contact have sometimes produced planar faces on the two matching sections that have been flattened; in other cases the surface of a ball has undergone a concavity which matches and takes in a convexity in the neighboring ball. [16] All these different cases are combined in several different ways. There is no great distance between all these forms and the articulations of even the oddest prisms.

The balls and the faceted bodies are sometimes cleaved by planes running from the circumference to the center, thus separating the different parts which are found sometimes dislodged from each other, sometimes still attached.

Basalt is also found in large tables accumulated in bundles [17] arranged in every direction; these tables touch one another in very smooth and glossy surfaces; they are ordinarily five to six feet in length by two or three feet in width, and three to four inches in thickness. When balls are found in the vicinity of these tables, they have left impressions of their convexity in hollows on parts of these tables.

Finally, Basalt appears in large, irregular trapezoidal sections distributed amidst different materials which compose the flows issued from Volcanoes.

From a consideration of all these singular shapes, M. Desmarest has assimilated the dispostion of the prisms found in all sorts of situations. There are structures of vertical prisms, and these are the most common; certain prismatic masses are horizontal, some others display prisms in positions diagonally inclined to the horizon; some are seen that are parallel among themselves and of the same size throughout; others are arranged around a common center, like radii of a circle, forming pointed pyramidal parts. From reflective study of all these combined dispositions, M. Desmarest believes he is justified in concluding that the axis of the prisms is always oriented to the small dimension of a mass composed of an assemblage of prisms, such that the bases are part of the largest surfaces of these masses. Thus in supposing a certain quantity of melted Basalt material, flowing and extending over a nearly horizontal surface, the solid crust which it will form following its cooling being a very large surface and of little thickness, the prisms into which it will divide through the effect of regular contraction will be vertical, and their bases will constitute part of the large surfaces where cooling has taken place. [18]

If it is assumed that the material flows in such a way as to form a solid like a wall, [19] the prisms will pass through the thickness of the wall and will be horizontal, and their bases will cover the two opposite faces like stones in regular masonry. If the material has gathered in spherical form, the prisms will be arranged like spokes radiating outward. The two plates and their figures show a large portion of these interesting phenomena. The first represents a front of an assemblage of prisms with articulations, located at the end of a flow terminating in a mound where the old castle of the Tour d'Auvergne was situated. The second shows a mass of Basalt seen above the castle of Pereneire ( Pera nera ), across from the village of Saint-Sandoux; this drawing shows the interesting breakdown of the section of a spherical arrangement. The lower and lateral part reveals several bundles of very fine prisms without articulations, oriented on different planes, but all tending like spokes to the center of the round structure. The upper part shows countless concave and convex articulations, the ends of the truncated prisms that are in view. These two drawings have been placed here deliberately adjacent to the view of the Giant’s Causeway , along with this summary, to serve as a sequel to the details found in the article Giant’s Causeway , acquaintance with which is assumed in what precedes. [20]

Plate VIII: Rock of Pereneire in Auvergne. 1. Rock of Pereneire, near Saint-Sandoux, in Auvergne; it is composed of an assemblage of prisms whose general structure tends toward the spherical. [21]

1. This article—the legend for Plate VII, and in effect for Plate VIII as well—is not attributed in the Encyclopédie , but there is no doubt that it was written by Nicolas Desmarest (whose actions and ideas are described here in the third person). Desmarest eventually made a fuller, formal presentation on this subject to the Paris Academy of Sciences, in 1771, which was published in three parts, the first two in the Academy’s Mémoires for that year (pp. 705‒775) and the third part in the volume for 1773 (pp. 599‒670). In the expanded account published in the 1771 Mémoires he explicitly asserted his authorship of this plate legend (at p. 746, note h).

The present article represents the first printed declaration of the volcanic origin of prismatic basalt. Desmarest made this discovery in 1763 (as stated in the article) during his first journey into Auvergne, which was almost certainly made primarily for reasons connected with his official duties in examining industrial practices (notably including papermaking and cheese manufacture). He initially presented an oral account of the discovery to the Paris Academy of Sciences on 3 July 1765 (Archives de l’Académie des Sciences, Procès-Verbaux des Séances, vol. 84, 1765, fol. 292v). The text of Desmarest’s 1765 account has not been found; but there does survive the Academy commissioners’ report on this presentation, by Mathieu Tillet, Bernard de Jussieu, and L.-J.-M. Daubenton, 23 July 1766 (PV vol. 85, fol. 238r‒245r; also in the pochette de séance for that date). See Kenneth L. Taylor, “Nicolas Desmarest and Geology in the Eighteenth Century,” in Toward a History of Geology , ed. Cecil J. Schneer (Cambridge, MA and London: MIT Press, 1969), pp. 339‒356; and “New Light on Geological Mapping in Auvergne during the Eighteenth Century,” Revue d’Histoire des Sciences , 1994, vol. 47, pp. 129‒136; both articles appear also in Taylor, The Earth Sciences in the Enlightenment: Studies on the Early Development of Geology (Aldershot, UK; Burlington, VT, USA: Ashgate Publishing, 2008). See also Pascal Richet, “Nicolas Desmarest et l’origine volcanique des basaltes,” Travaux de Comité Français d’Histoire de la Géologie , 3e série, tome 17 (2003), pp. 81‒98.

2. Mound: butte could also be translated as hillock , or butte .

3. Prismatic stones were said to be articulated (as distinct from being monolithic or unbroken) when they were constituted by distinct parts or sections, as shown plainly in Plate VII. Articulations are conspicuous also in basalt columns displayed in the neighboring fold-out Plate VI, depicting the Giant’s Causeway, or Pavé des Géans . (Plate VI clearly was pirated from one of the two famous “prospects” engraved by François Vivarès, from paintings done about 1740 by Susanna Drury. Cf. Martyn Anglesea and John Preston, “‘A Philosophical Landscape’: Susanna Drury and the Giant’s Causeway,” Art History , vol. 3, 1980, pp. 252‒273.)

4. Foires , translated here as markets , might instead possibly be rendered as fairs .

5. This two-sentence caption accompanies Plate VII. The legend follows. The engraving is identified as having been done by Robert Bénard (1736‒1777), from a drawing by Jean-Jacques de Boissieu (1736‒1810). It is now known that Boissieu’s drawing dated from 1766. Cf. Marie-Félicie Perez and Madeleine Pinault, “Three New Drawings by Jean-Jacques de Boissieu,” Master Drawings , vol. 23/24, 1985/1986, pp. 389‒395, 475‒477.

6. The Encyclopédie article Pavé des géans has been attributed to Paul-Henri Thiry, Baron d’Holbach.

7. Desmarest had never seen the Giant’s Causeway, nor would he ever do so. Such direct observations as he was in a position to make, for the purpose of comparing the phenomena of the Giant’s Causeway with those of Auvergne, would at this time have been limited to comparative examination of specimens of prisms brought back from the Giant’s Causeway to Paris (by the academician Étienne Mignot de Montigny) with Auvergne specimens collected by Desmarest. And such comparisons do figure in the 1765 and 1771 reports Desmarest made to the Academy, although they are not mentioned explicitly in this article.

In this connection, there is another, more curious omission in this article. Desmarest made no mention here of the paintings by Susanna Drury, or their engravings by F. Vivarès (whereas in his 1771 memoir Drury’s name is invoked at least three times—although as M . Drury!). But to judge from the 1766 Tillet-Jussieu-Daubenton report, in 1765 Desmarest had specifically compared the landscape shown in the Drury engravings with the scenery around Mont d’Or (as he was to do in his longer memoirs), implying that these pictures indicated significant similarities in the terrain of County Antrim and around Mont d’Or. That is, in his formal scientific reports to the Academy in 1765 and 1771 (but not in this plate legend—at least, not explicitly) Desmarest was prepared to rely on comparison of others’ pictorial representations of landscapes in Antrim with his own observational experiences in Auvergne, as one element in an observation-based analysis. Thus he implicitly included evidence conveyed in such images as a legitimate part of the process of scientific observation.

8. Resolution: dénouement could also be translated as clarification .

9. The term nomenclateurs , which could also be translated as classifiers , had been applied disparagingly by Buffon to systematists, most notably his Swedish rival Linnaeus. Desmarest mentioned no particular classifiers here, but in his lengthy report to the Academy in 1771 he named a few names: especially J. G. Wallerius and his translator the Baron d’Holbach, as well as J. H. Pott and E. Mendes da Costa, all of whom he treated with respect even while criticizing their efforts to classify basalts. It is apparent that he meant to demarcate investigatory naturalists working in the field (among whom he would count himself) from those whose classificatory studies focused on specimens examined mainly or exclusively in the cabinet.

10. In summer 1765 Desmarest departed on a long journey to Italy, in the company of the Duke Louis-Alexandre de La Rochefoucauld d’Enville (1743‒1792). They returned to France in May 1766. See Kenneth L. Taylor, “Nicolas Desmarest and Italian Geology,” in Rocks, Fossils and History , ed. Gaetano Giglia, Carlo Maccagni, and Nicoletta Morello (Florence: Festina Lente, 1995), pp. 95‒109; also in Taylor, Earth Sciences in the Enlightenment . Incidentally, another of the Duke’s companions on this journey was the artist J.-J. de Boissieu (see note 5 above).

11. The volcanic character of much of the Auvergne terrain had been established just a few years earlier, during a journey undertaken in 1751 by the naturalist Jean-Étienne Guettard (1715‒1786) and his companion Guillaume-Chrétien de Lamoignon de Malesherbes (1721‒1794). Guettard published an account of this in the Mémoires of the Paris Academy of Sciences for 1752 (published 1756). See François Ellenberger, “Précisions nouvelles sur la découverte des volcans de France: Guettard, ses prédécesseurs, ses émules clermontois,” Histoire et Nature , nos. 12/13 (1978), pp. 3‒42; also Kenneth L. Taylor, “Geological Travellers in Auvergne, 1751‒1800,” in Four Centuries of Geological Travel: The Search for Knowledge on Foot, Bicycle, Sledge and Camel , ed. Patrick N. Wyse Jackson, Geological Society Special Publication no. 287 (London: The Geological Society of London, 2007), pp. 73‒96.

12. Foyer translated as hearth . An alternative choice could be seat . At this time (1760s) most naturalists continued to hold the long-standing view that volcanic action is a comparatively superficial and limited phenomenon, taking place not very far below the surface. Pre-existing materials were thought to be altered by volcanic heat, which might be generated by one of several sorts of exhaustible processes (such as combustion of subterranean coal, or fermentation of certain sorts of minerals). Volcanic processes were generally thought of as destructive, not generative. See Kenneth L. Taylor, “Before Volcanoes Became Ordinary,” in History of Geoscience: Celebrating 50 Years of INHIGEO , ed. Wolf Mayer, et al ., Geological Society of London Special Publication 442 (2017), pp. 117‒126.

13. At this point (1768) Desmarest had not yet presented the thorough account of his Auvergne investigations to which he refers; he was to do that in 1771 (see note 1 above).

14. This article specified no particular English authors. Descriptions of the Giant’s Causeway began to be published in the Royal Society’s Philosophical Transactions during the 1690s. But Desmarest might have had in mind Richard Pococke, whose description appeared in the Philosophical Transactions in 1748. There was also extensive discussion of the Giant’s Causeway in Emanuel Mendes da Costa, A Natural History of Fossils (London: Printed for L. Davis and C. Reymers, 1757), especially pp. 252‒261. Both Pococke and da Costa are mentioned by name in Desmarest’s 1771 memoir, with several pages of commentary on da Costa’s work.

Da Costa stated (p. 252), in a passage quite similar to the opening two sentences of this article by Desmarest: “This curious and extraordinary species of stone has as yet been discovered only in one place of the known globe, that is, in the country of Antrim, in the north of the kingdom of Ireland, where that stupendous and amazing production of nature, called the Giants causeway, consists of an assemblage of several thousand columns of this stone.” Da Costa considered (p. 262) that columns of articulated basalt constituted a different species of stone than that seen in pillars of monolithic or non-articulated basalt, such as had been described by Agricola and Gesner at Stolpen, near Dresden. It was a distinction recognized in the Baron d’Holbach’s Encyclopédie article Pavé des géans . In his later, extended papers on Auvergne basalts published in the Mémoires of the Paris Academy of Sciences for 1771 and 1773, Desmarest recognized this same distinction, although he also proceeded to discriminate between different types of basalt on other, lithological grounds as well (his main concern apparently being the differences among basaltic rocks formed, as he believed, from the action of volcanic heat on varied sorts of primary materials).

15. Balls: boules . By the 1830s some British geologists were using the expression nodular basalt as the equivalent of the French basalte en boules (e.g., W. H. Sykes, in London and Edinburgh Philosophical Magazine and Journal of Science , vol. 1, 1832, p. 305; and Transactions of the Geological Society of London , 2 ^nd ser., vol. 4, 1836, pp. 409‒432, at 417). So one might perhaps consider translating boules as nodules ; however, this is avoided here as anachronistic, particularly since the term nodule was already in use for a different purpose, in reference to relatively small, round or irregular lumps or masses of minerals (e.g., E. Mendes da Costa, A Natural History of Fossils , 1757; A. F. Cronstedt, Essay Towards a System of Mineralogy , 1770). Two somewhat later studies of basaltes en boules observed in Auvergne during the 1780s, were published by naturalists Guillaume Delarbre and Alexandre-Charles Besson, in Observations sur la physique , vol. 31 (1787), pp. 133‒153.

16. Matches and takes in: admet et reçoit . Evidently Desmarest was concerned to explain the varied forms of the surfaces joining articulations—sometimes flat, but often curved—as the natural results of a uniform process.

17. Bundles: paquets ; this could also be translated as parcels .

18. Desmarest stated in effect that basalt columns take form with their axes perpendicular to the surfaces where the rate of cooling is greatest.

19. One circumstance in which igneous matter could flow in this fashion would be when forced into vertical fissures. The resulting emplacement of congealed magma in wall-like form is what geologists would in due course call a dike (probably borrowing from mining terminology).

20. Pavé des Géans (article by d’Holbach, as already mentioned above, note 6). In his 1771 memoir Desmarest preferred using the name Chaussée des Géans (although the Academy’s permanent secretary, Grandjean de Fouchy, stuck with pavé des géans in his commentary on Desmarest’s paper in the annual Histoire ). The Academy’s commissioners reporting in 1766 on Desmarest’s initial 1765 memoir (note 1 above) wrote of the Chaussée des Géants .

21. Like Plate VII, Plate VIII was engraved by Bénard from a drawing by de Boissieu. This drawing, like the other one, was made in 1766.