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Brongniart, Alexandre

Brongniart, Alexandre

(b. Paris, France, 5 February 1770; d. Paris, 7 October 1847)

geology.

Brongniart was the son of the distinguished Parisian architect Alexandre-Théodore Brongniart (1739–1813) and Anne-Louise Degremont. He studied at the École des Mines and later at the École de Médecine, and for a time acted as assistant to his uncle Antoine-Louis Brongniart (1742–1804), who was then professor of chemistry at the Jardin des Plantes. After serving as aide-pharmacien in the French forces in the Pyrenees, Brongniart returned to Paris; in 1794 he was appointed ingénieur des mines, and in 1797 became professor of natural history at the École Centrale des Quatre-Nations. In 1818 he was appointed ingénieur en chef des mines, and in 1822 he succeeded R. J. Haüy as professor of mineralogy at the Muséum d’Histoire Naturelle. Brongniart was elected a member of the Académie des Sciences in 1815. Most of his life was spent in Paris, in teaching, research, and administration. He is said to have been exceptionally helpful and generous to his students, for whom he opened his collections every Sunday; and he attracted distinguished gatherings of scientists at his evening salons.

Brongniart traveled widely in western Europe and published geological papers on areas ranging from Sweden to Italy. As a young man, immediately after the Revolution, he visited England to learn the techniques of the ceramics industry, and in 1800 he was appointed director of the Sèvres porcelain factory, a post he held until his death. The problems of ceramic technology occupied his attention increasingly toward the end of his life, and his last major work was the two-volume Traité des arts céramiques (1844). He married Cécile, daughter of the statesman-scientist Charles-Étienne Coquebert de Montbret; their only son was the botanist and paleobotanist Adolphe-Théodore Brongniart.

Brongniart’s earliest scientific papers (the first was published in 1791) were on various zoological and mineralogical subjects. In the former he was strongly influenced by Georges Cuvier, who was almost exactly his contemporary. For example, his “Essai d’une classification naturelle des reptiles” (1800) emphasized the prime importance of careful comparative anatomy, and on that basis he divided the class Reptilia into four groups. He recognized, however, that one group, the batrachians, was significantly different from all the others, especially in the reproductive organs, and that this distinction was far more important than the more striking difference between the limbless snakes and the rest. In 1804 Pierre Latreille elevated the batrachians into a separate class, the amphibians; Brongniart’s grouping of the true reptiles—into chelonians, saurians, and ophidians—has been retained, in essence, in modern systematics.

In 1807 Brongniart published Traité élémentaire de minéralogie, a work commissioned as a textbook for his and Haüy’s courses at the Faculté des Sciences and the Muséum d’Histoire Naturelle. He adopted a simple scheme of classification based mainly on physical properties, but he also made extensive use of Haüy’s crystallographic work. Like other mineralogists at this time, he could not easily distinguish some fine-grained rocks from true simple minerals; but he classed clay and basalt, for example, as fausses espèces, recognizing that they were mélangées although they were too fine-grained to be analyzed. He emphasized the importance of studying the modes of occurrence of minerals as well as their properties, but firmly avoided any discussion of their origins as being too speculative. The sole exception to this was a review of divergent opinions on the aqueous or igneous origin of basalt, but even in this he avoided expressing his own opinion.

Brongniart’s early studies in zoology and mineralogy coalesced in the geological work that made him famous throughout the scientific world. Cuvier had already begun his series of spectacular reconstructions of extinct mammals from the Paris region; these fossils clearly belonged to several distinct periods, but he needed a reliable clue to their relative ages. He and Brongniart therefore collaborated in surveying the region and determining the order of the strata in which the fossils had been found. About 1804 they began a series of traverses of the region, and on 11 April 1808 they read their “Essai sur la géographie mineralogiquc des environs de Paris” before the Institute. The paper was first published in June 1808; a greatly enlarged version, accompanied by a large colored geological map and several horizontal sections, appeared in 1811. With characteristic modesty Brongniart allowed his name to appear after Cuvier’s on this memoir, although the geological work seems to have been largely Brongniart’s.

Nine “formations” (distinctive rock units) were recognized in the initial version of the work (the details were amended and amplified later, but the main conclusions were unchanged). The oldest was the Chalk, which underlay a series of later strata that had been deposited in succession, in a kind of gulf or embayment. There was no transition between the lowest, an unfossiliferous clay (Argile plastique), and the underlying Chalk; and at one locality a conglomerate showed that the Chalk must have been lithified before the clay was deposited, thus implying a long interval of time. The next formation, the Calcaire grossier, a series of limestones, was remarkable in that several subordinate beds could be distinguished over a very wide area, invariably in the same order, by their distinctive assemblages of fossils: “This is a mark of recognition which up to the present time has not deceived us.” Marine mollusks were the most abundant fossils (they were described and classified by Lamarck), but they were totally distinct from those of the Chalk. Slow deposition in a calm sea was inferred from the regular stratification and the excellent preservation of the fossils. These limestones passed laterally into an unfossiliferous Calcaire silicieux. Both these formations were succeeded by the Formation gypseuse, a series of marls and beds of gypsum; among the latter were those quarried for plaster at Montmartre, from which Cuvier was obtaining many of his most remarkable fossil vertebrates—mammals, birds, and reptiles that were unknown not merely in species but even in genus. Rare shells of freshwater genera, and a total absence of marine shells, confirmed the probable freshwater origin of the series; but the following Sables et Grès marins, with abundant marine fossils, indicated a return to marine conditions. Yet after an unfossiliferous sandstone (Grès sans coquilliers), a Terrain d’eau douce with freshwater shells demonstrated that the conditions had changed yet again. This was the highest, and therefore the youngest, of the regularly stratified deposits. But after all of these, and after the excavation of valleys, a superficial Limon d’aterrissement had been deposited; and in the valleys these deposits had yielded a fauna of extinct species of elephants, antelopes, and so forth. Although very modern in comparison with all the other deposits, this last formation clearly originated before historic times.

The significance of Brongniart’s stratigraphy of the Paris “basin” was quickly recognized. The general nature of stratified sedimentary rocks and the importance of observing their order of superposition were commonplaces in geology before his time. The highest, and therefore most recent, stratified deposit that could be recognized over a wide area was the Chalk, however; only “superficial” deposits, assumed to be relatively recent in origin, were thought to overlie it. Brongniart’s work proved that above the Chalk was a complex series of stratified rocks, many of them evidently formed by very slow deposition. By implication, therefore, the time that must have elapsed since the end of the Chalk period was greatly extended. This extension of geological time was the first important effect of the stratigraphy.

Second, the strata showed an alternation between marine and freshwater conditions, countering the earlier assumption that all stratified rocks had been deposited in a gradually shrinking ocean. So important was this conclusion that Brongniart devoted a separate memoir, “Sur les terrains qui paraissent avoir été formés sous l’eau douce” (1810), to arguing it in detail, demonstrating the close analogies between living freshwater mollusks and the corresponding fossils. In this memoir he also described similar freshwater deposits from far outside the Paris region, even into central France (Cantal), arguing that such extensive deposits were not improbable when compared with the Great Lakes of the present day.

This alternation of marine and freshwater conditions implied a broadly cyclic, or at least a repetitive, character for this part of geological time. With his usual caution, Brongniart refrained from speculating on the causes of these changes; but his stratigraphical conclusions were certainly influential in molding Cuvier’s geological theory, as first set out in the Discours préliminaire to the Recherches sur les ossemens fossiles (1812). Here Cuvier used the rather sharp breaks between the successive marine and freshwater formations in support of his hypothesis of sudden changes of sea level as a cause of the extinction of terrestrial faunas; but the recurrence of broadly similar marine and freshwater shells in the successive formations proved that these sudden “revolutions” must have been local in their effects.

The third important feature of Brongniart’s stratigraphy was his use of fossils for the detailed correlation of strata. Previously it had been normal practice to use the lithology, physical position, and fossil content of a formation, with varying relative emphases, as criteria for recognizing it in widely separated areas. But Brongniart’s work demonstrated the value of precisely collected and identified fossils as criteria for tracing a detailed series of strata, which might differ little in either lithology or physical position, across an extensive area. It was the precision with which the method was applied that was original.

A similar detailed use of fossils had been made some years previously by William Smith, who worked on the Jurassic strata around Bath and subsequently over a wide area of England. Smith’s work was known to English geologists, but its validity could not be assessed by the scientific community as a whole until, some years later than Brongniart, he eventually published his geological map (1815) and the illustrations of the characteristic fossils on which it was based (1816–1819). Although Smith’s work had strict priority, Brongniart’s independent discovery of the value of fossils as a tool for stratigraphy was the first to be published, and therefore had the greater influence on the direction of geological research. Brongniart’s method was rapidly and successfully applied in other areas, not only to the recognition of similar strata younger than the Chalk but also to the analogous problems of the older strata.

Brongniart himself played an important part in this development. By 1822 he had traveled widely enough to be able to describe strata, equivalent to those of the Paris region, from many different parts of Europe: these descriptions were inserted in the Description géologique des environs de Paris (a new edition of the Géographie minéralogique). But this extension of Tertiary stratigraphy over a wider geographical area posed the fundamental problem of geological facies. Rocks of the same age could not be expected to have the same lithological characters if they were deposited under different conditions in different areas. Even within the Paris region Brongniart had recognized lateral changes in lithology; on a wider scale the changes became more general. It was this that led him to stress the primacy of fossil evidence over that of lithology as a criterion for age, wherever the two sources of evidence were found to conflict. Thus he recognized that the London Clay of southern England must be the approximate equivalent in age of the Calcaire grossier of Paris, although they were totally different in appearance: they occurred in equivalent positions in the succession and, more important, they had similar fossils.

Brongniart defended this methodology in a special memoir, “Sur les caractères zoologiques des formations” (1821), in which its validity was argued with reference to the strata equivalent to the Chalk. His most striking example was his discovery of fossils identical to those of the Chalk (more precisely, of the Greensand) in a hard black limestone outcropping more than 2,000 meters above sea level on a mountain in the Savoy Alps. This countered the earlier belief that strata formed at a single period should be relatively constant in both lithology and physical position; but Brongniart described a series of other occurrences of rocks with similar fossils, which bridged the vast difference between this black Alpine rock and the more usual appearance and position of the Chalk.

The Mémoire sur les terrains de sédiment supérieurs (1823) described a similar variety of occurrence, in rocks containing fossils identical to those of the Tertiary strata around Paris. Like the “Chalk” rocks, these varied greatly in lithology, some being associated with volcanic rocks; they might be flat-lying or highly folded; and they might outcrop at any altitude from sea level to the summits of the Alps. In spite of this variety, Brongniart maintained, it was far more satisfactory to accept them all as being of the same age than to assign the Alpine occurrences to a much earlier epoch simply on the grounds of their position. Although such differences of altitude seemed vast by human standards, he argued, they amounted to no more than a millimeter on a two-meter model globe, and should be kept in proportion. Nevertheless, although Brongniart himself drew no such conclusion, his demonstration of such spectacular elevation of relatively recent strata later acted as powerful evidence for a greatly expanded time scale for the earth’s history in the hands of those who, like Charles Lyell, believed that the elevation had occurred slowly and gradually.

Brongniart’s empirical demonstration of the primacy of fossils as criteria for correlation was justified by a theoretical argument that owed much to Cuvier’s catastrophism. Brongniart stressed the relative uniformity of faunas in different areas at the present time, and argued that whereas a whole fauna might be suddenly and drastically extinguished, leaving few survivors, it would take a long time for a new fauna to replace it; each major period should therefore be characterized by a distinctive set of fossils, although a few might be common to more than one period. Brongniart recognized the possible effects of climatic differences on the faunas of a single period and the need to discount “derived” fossils, but he concluded that fossil evidence must be given greater weight than lithology or physical position in determining the relative age of a deposit: only superposition évidente could have priority over fossils.

In the oldest “Transition” (broadly, Lower Paleozoic) strata then known, great interest was aroused by the fossil trilobites, a class totally unknown in more recent strata, which nevertheless seemed remarkably “highly organized” for such an ancient period of earth history. In the Histoire naturelle des crustacés fossiles (1822) Brongniart published the first full-length study of the trilobites (in the same volume, A.-G. Desmarest made a similar study of the fossil remains of true crustaceans). He classified a wide variety of species from many parts of Europe and even from America, and attempted to group them according to their relative age. For the latter objective he had insufficient evidence from superposition and was misled, for example, by the undisturbed and “young-looking” strata in Scandinavia; but his systematic work on trilobites was an important contribution to the later unraveling of Paleozoic stratigraphy.

Among the distinctive rocks that were commonly thought to antedate even these earliest fossils were the coarse-grained crystalline rocks granite and gabbro (the evidence for an intrusive origin for granite was well-known but generally regarded as atypical; most lists of strata still showed granite as the earliest rock of all). It was therefore an important conclusion when Brongniart showed, in the “Gisement des ophiolites” (1821), that in the Apennines some gabbros and similar rocks actually overlay normal limestones and detrital rocks, and were not the oldest rocks in the area. Brongniart had no fossil evidence of the age of the sediments but thought the limestones resembled those of the Jura (i.e., of Mesozoic age). This implied that at least one crystalline rock analogous to granite could have been formed much later than the earliest fossils. This conclusion served to emphasize still further the fallibility of distinctive rock types as criteria of particular periods of earth history.

Throughout his life Brongniart remained reluctant to speculate on the causes of the phenomena he described. This is best illustrated by his contribution to one of the most difficult geological puzzles of the period, the phenomena of erratic blocks and striated pavements. These features, which in the 1840s were recognized as the effects of slowly moving ice sheets, were in the 1820s generally attributed to some sudden “diluvial” action in the geologically recent past. In his paper “Blocs des roches des terrains de transport” (1828), Brongniart gave careful descriptions of erratic blocks and striated pavements, and also the first detailed account (with a map) of the sinuous ridges or eskers that cross the low-lying glaciated areas of Sweden. He pointed out that all these features indicated some extremely powerful transporting process acting from the north and able to move large blocks of rock even across the Baltic; but he offered this as a simple conclusion from his observations and refused to speculate further on the nature of the remarkable force involved.

Brongniart’s last major geological work, the Tableau des terrains qui composent l’écorce du globe (1829), was the culmination of his life’s work on the ordered classification and interpretation of rocks. It had a disappointing reception, however, and exerted little influence on the further development of geology. Yet in some ways it was an attempt to tackle problems that are still important in modern geology. Having recognized that the rocks formed at one period might be widely different in appearance, he was concerned to establish a system of nomenclature for the periods of earth history, which would not involve misleading references to particular rock types: in other words, he was trying to distinguish time units from rock units. But this led him to propose a cumbersome and largely novel nomenclature that was difficult to remember and therefore failed to win general acceptance. Moreover, although he expressed the intention of avoiding theorizing about causes, he implicitly accepted the theories of others. Thus, his first and main division of time was between a période jovienne (Recent) and a période saturnienne (all earlier time); the sedimentary rocks of the latter were divided into Terrains clysmiens (“diluvial” or glacial deposits) and Terrains izemiens (all other sediments). This implied a distinction between past and present and a unique role for the most recent “revolution” (the glacial period), which were far more questionable than he seemed to realize. On the other hand, he made a clear distinction between the Terrains stratifiés ou Neptuniens, which invariably occurred in the same order, and the Terrains massifs ou Typhoniens (broadly, igneous rocks), which might be intercalated at any point in the series. He also emphasized that the stratigraphical succession would have to be pieced together from many sections in different areas, correlated as well as possible with each other; and he urged the desirability of fixing type sections for the definition of stratal terms (“une suite de terrains admis comme type ou module”). In such ways he recognized the problems of stratigraphical geology more perceptively than most of his contemporaries. Although his classification had little effect on their thinking, he provided valuable lists of characteristic fossils for each terrain from the most recent back to the Terrains hemilysiens (pre-Carboniferous); and these were widely used in the rapidly developing stratigraphical research of the following decade.

In retrospect, Brongniart’s influence on nineteenth-century geology might have been greater if he had been less cautious about theorizing. In spite of that limitation, however, the influence of his early work on the Paris region, and its later extension to the whole of western Europe, is difficult to overestimate. For this careful stratigraphical work provided the principal model on which much of the exceptionally productive geological research of the period 1810–1840 was based; and thus it lay at the root of the greatest achievement of early nineteenth-century geology, the elucidation of the main outlines of the history of the earth and of life on earth.

BIBLIOGRAPHY

Brongniart’s principal writings are “Essai d’une classification naturelle des reptiles,” in Bulletin de la Société Philomathique, 2 (1800), 81–82, 89–91, and Mémoires de l’Institut de France, 1 (1806), 587–637; Traité élémentaire de minéralogie, 2 vols. (Paris, 1807); “Essai sur la géographie minéralogique des environs de Paris,” written with Georges Cuvier, in Journal des mines, 23 , no. 138 (June 1808), 421–458; Annales du Muséum d’Histoire Naturelle, 11 (1808), 293–326; Mémoires de l’Institut Impérial de France, année 1810 (1811), 1–274, also issued separately (Paris, 1811); and in Cuvier’s Recherches sur les ossemens fossiles, 1 (Paris, 1812); “Sur les terrains qui paraissent avoir été formés sous l’eau douce,” in Annales du Muséum d’Histoire Naturelle, 15 (1810), 357–405; “Eau (minéralogie et géognosie),” in Dictionnaire des sciences naturelles, 14 (1819), 1–62; “Sur le gisement des ophiolites (roches à base de serpentine), des euphotides, etc., dans quelques parties des Apennins,” in Annales des mines, 6 (1821), 177–238; “Sur les caractères zoologiques des formations, avec application de ces caractères à la détermination de quelques terrains de craie,” ibid., 537–572; Description géologique des environs de Paris, written with Georges Cuvier, new ed. with additions by Brongniart (Paris, 1822); Histoire naturelle des crustacés fossiles, sous les rapports zoologiques et géologiques, written with A.-G. Desmarest (Paris, 1822); Mémoire sur les terrains de sédiment supérieurs calcaro-trappéens du Vicentin, et sur quelques terrains d’Italie, de France, d’Allemagne, etc., qui peuvent se rapporter à la même époque (Paris, 1823); Classification et caractères minéralogiques des roches homogènes et hétérogènes (Paris, 1827); “Notice sur les blocs des roches des terrains de transport en Suède,” in Annales des sciences naturelles, 14 (1828), 5–22; Tableau des terrains qui composent l’écorce du globe, ou Essai sur la structure de la partie connue de la terre (Paris, 1829), also published as “Théorie de la structure de l’écorce du globe,” in Dictionnaire des sciences naturelles, 54 (1829), 1–256; and Des volcans et des terrains volcaniques (Paris, 1829), also published as “Volcans,” in Dictionnaire des sciences naturelles, 54 (1829), 334–446.

The most important biography is Louis de Launay, Une grande famille de savants. Les Brongniart (Paris, 1940). This is chiefly devoted to Alexandre Brongniart, and makes use of MS sources. It includes an extensive, though incomplete, list of Brongniart’s published works.

M. J. S. Rudwick

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Brongniart, Alexandre

Alexandre Brongniart (älĕksäN´drə brôNyär´), 1770–1847, French geologist, mineralogist, and chemist. As director of the Sèvres porcelain factory from 1800, he was responsible for its international fame. He was professor of mineralogy at the Museum of Natural History in Paris from 1822–47. Brongniart established basic principles of ceramic chemistry in his Traité des arts céramiques et des poteries (1844). With George Cuvier he wrote Essai sur la geographie mineralogique des environs de Paris (1811), in which a system of stratigraphy was developed that relied on the use of fossils for the precise dating of strata. He was also the first to develop a systematic study of trilobites and a system for the classification of reptiles.

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Brongniart, Alexandre

Brongniart, Alexandre (1770–1847) Brongniart was professor of mineralogy in the Paris Museum of Natural History, and collaborated with Cuvier in his work on the mapping of the Paris basin. He demonstrated that lower taxonomic orders of animals were found lower in the stratigraphic column, claiming this gave evidence of progression in the succession of life.

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Brongniart, Alexandre

Brongniart, Alexandre (1770–1847)Professor of mineralogy at the Muséum d'Histoire Naturelle, Paris, Brongniart collaborated with Cuvier in his work on the mapping of the Paris basin and demonstrated that lower taxonomic orders of animals were found lower in the stratigraphic column, and hence gave evidence of progression in the succession of life.

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"Brongniart, Alexandre." A Dictionary of Ecology. . Encyclopedia.com. 20 Jul. 2017 <http://www.encyclopedia.com>.

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