OBITUARY OF H. ARNAUD

by A. de Grossouvre

     We have just lost one of our oldest colleagues, Hilaire Arnaud, a lawyer in Angoulême. He had been a member of our Society for fifty years: it was at the meeting of 2 November 1857 that he was proclaimed a member, with Coquand and our revered and illustrious colleague, Mr Albert Gaudry, as his patrons. A long friendship, which had succeeded to relations initially brought about by our common love for geology, authorizes me to bring him here a last homage and to recall the scientific works which must perpetuate his memory among us.
 

     Arnaud was born in Angoulême on September 10, 1827: his father was a soldier of the Great Army of 1812 and he charmed his childhood with the story of our glories and our setbacks. After brilliant school studies, he attended the Faculty of Law in Poitiers and there his taste for the natural sciences began to reveal itself; the leisure time he could spare was devoted to the study of botany and medicine.
     Later, as a lawyer in Angoulême, then as a substitute for the Imperial Prosecutor of Cognac, he made his first geological explorations under the direction of Coquand, who was then in charge of the execution of the geological map of the Charente. His vocation for geology was definitively fixed when our society came to hold one of its annual meetings in Angoulême in 1857. The main object of this meeting was to examine in the field the subdivisions established by Coquand for the Chalk of the South-West, subdivisions which have now become classic and have been adopted by the majority of geologists, both abroad and in France. Coquand was elected president and M. Gaudry secretary. We see in the report that the programme of the races was fixed in part on Arnaud's indications and that in the meeting of 11 September he was presented as a member at the same time as another of our colleagues and also one of our deans, M. Ch. Boreau, linked with him by a friendship that only death could interrupt. Driven by a zeal no less ardent than Arnaud's, he often accompanied him on his excursions and, with this memory, we regret not finding in our Bulletins the results of his activity in the field.
     Arnaud, a magistrate first in Cognac, then in Périgueux, lived in the middle of the chalk country. He made numerous and fruitful explorations there, beginning to gather material for a collection that he would enrich until his death and gathering observations that he would later put to use. His choice was fixed: from now on he would confine himself to the study of the Cretaceous of Aquitaine, his little homeland; he would concentrate his research there, dissecting the subsoil, isolating the various layers and carefully collecting in each of them the remains of the ancient beings that were buried there. At the same time, he followed with interest everything that was being written about the Chalk in France and abroad and made it the starting point for important comparisons with the region to which he had devoted himself.

     In 1862, Arnaud's first work appeared in our Bulletin, a note on the Chalk of the Dordogne in which he tried to apply Coquand's classification in this department, with a view to verifying whether the divisions created by this scholar had a character of generality that justified their adoption. From the very first pages of this work, two of Arnaud's main guiding ideas appear: on the one hand, the correlative modifications of the faunas and of the mineralogical composition of the deposits which contain them, and on the other, their gradual extinction and their progressive renewal.
 

     It should not be forgotten that these two principles, the first of which derives directly from the observation of actual facts, now universally accepted and regarded by all as fundamental truths, were at that time completely disregarded and that contrary propositions served as a starting point for geological speculation. It was believed that everywhere deposits of the same age had the same constitution and the same fauna; it was thought that the limits of stages corresponded to a complete renewal of the faunas. We must therefore admire Arnaud's sagacity, the independence of his scientific mind and the boldness with which he was able to free himself from the errors of the official research school in order to bring to light truths that only triumphed definitively long afterwards.
 

     I believe I must reproduce here some of the passages in which he highlights these very important principles.
 

     In his note on the Chalk of the Dordogne [1862, B. S. G. F., (a), XIX, p. 476], he points out that " la réapparition des grandes Caprines dans le banc supérieur des Ichthyosarcolithes, malgré leur absence dans les couches intercalées entre ce banc et le banc inférieur (grès et argiles tégulines), montre que ces assises intermédiaires, quoique nettement différenciées des Calcaires à Ichthyosarcolithes par le caractère minéralogique, ne constituent qu'un accident local, et que ces Rudistes, au développement desquels un sol marécageux et l'agitation des sables littoraux opposaient un obstacle temporaire, ont trouvé à faible distance des eaux paisibles au sein desquelles, et certainement dans une formation calcarifère contemporaine, se sont perpétuées leurs espèces, jusqu'au moment où elles ont été rappelées, dans la partie aujourd'hui apparente du bassin, par un abaissement du sol et un changement dans la direction des courants ; leur persistance, malgré ce changement, en montre à un autre point de vue la faible importance".

     He insisted on the dangers of seeking clear-cut divisions "where nature has only proceeded by the gradual extinction and successive renewal of faunas", phenomena attested to, he said, by these layers of passage "where the remains of two generations have been buried, one at its dawn, the other at its decline".

     A few years later, he was no less explicit on these same questions and in his "Observations géographiques sur la Craie du Sud-Ouest [1869, B.S.G.F., (3), XXVII, p. 18] he writes: "under the influence of mineralogical variations, species emigrate, they seek environments suitable for their preservation and prolong on points more or less distant from their origin the existence they seemed to have lost: often the return of the mineralogical conditions within which they had originated recalls them, mixed and altered by time, to a level higher than the one they had left".

     What depth of vision these lines reveal! Not only do we see the theory of facies clearly defined, but we also find in it the seeds of many notions which, developed later, will contribute greatly to the progress of science.
 

     Arnaud shows us, for example, the influence of currents on the migration of species and, when he speaks of the reappearance of species altered by time, does he not awaken in us the idea of mutations, that is to say, variations of the same type over time, an idea developed the same year by Waagen in his famous memoir on the series of forms of Ammonites subradiatus. Is it not this notion which, applied to the palaeontological study of coralligenous groups, has enabled us to resolve so happily the difficulties presented by the classification of coral levels in the Jurassic and Rudist levels in the Cretaceous? How many sterile and resounding discussions would have been avoided if these principles had been better appreciated by Arnaud's contemporaries, for these are not truths that escaped by chance from his pen in the course of writing, since we find them formulated in all of his works; therefore, should we not regret that he was unable to devote all of his time, all of his strength and all of his activity to geological studies?
 

     The political events having obliged him to leave the magistracy, he returned in 1870 to settle in Angoulême, in his father's house, and was registered again at the bar of this city. The profound honesty of his character, his legal knowledge, the authority of his word, soon gained him the confidence of his fellow citizens: he was soon one of the most sought-after lawyers and he obtained the honours of the Bâtonnat.
 

     Despite his professional activities, he did not neglect geology and it was around this time that he inaugurated what he called his profile campaign. Armed with permits, which the railway companies had graciously placed at his disposal, he explored, trench by trench, all the Orleans, Charentes and State lines that crossed the region. He carefully recorded the detailed cross-section of each one, noting layer by layer the fossils that inhabited them and deducing from them the position they should occupy in his classification. He did the same work on the cliffs of the Gironde and continued it as the new routes were completed. The results of this painstaking research appeared in a series of notes beginning in 1873 and continuing until 1892, when the line from Angoulême to Marmande was finished.

     The numerous observations collected during his excursions finally led him to coordinate all of them in a masterly memoir on the Cretaceous terrain of the South-West, published in 1857 in the Memoirs of our Society.
 

     The starting point of this work consists of a series of tables of synchronism which give, for a series of numerous localities taken from the North-East to the South-West, that is to say from the Charente-Inférieure to the Lot, the detailed section of the Cretaceous layers. The text summarises this set of data; it summarises the characteristics used to define with greater precision the levels created by Coquand and to subdivide them into zones, each of which has a special facies and a particular fauna.
 

     All the observations relating to the strength of the layers and their nature are brought together in a very suggestive table which, at first sight, shows the changes they undergo throughout the region studied. This diagram is probably the first, or certainly one of the first, to have been drawn up in this order of ideas: today there are hardly any memoirs where we do not come across similar ones. But it should not be forgotten that at the time Arnaud published his, the principle of lateral variation of facies was a daring novelty. We thus see Arnaud here reaffirming the ideas he had previously put forward again, remarking that "nowhere did the divisions he had drawn correspond to an absolute extinction and complete renewal of organic life...; that, more or less weakened, the palaeontological link nevertheless persists throughout the entire series of Cretaceous periods.
 

     Arnaud had no less clearly foreseen the greatness of the erosion phenomena and, at a time when people liked to consider the outcrops of the layers as representing the shores of ancient seas, he wrote: "He added: "the Jurassic gulfs that scallop the perimeter of the basin represent, not the promontories that emerged at the time of the deposition of the Chalk, but the bare axes of the later levels".
Arnaud's memoir ends with a palaeontological section in which a series of tables give a detailed inventory for the most important genera, Cephalopoda, Ostracea, Rudistae, Brachiopoda and Echinoderms, of the species collected in the South-Western Chalk with the indication of the areas where they were encountered. The description of a number of new species shows that Arnaud was no less perceptive in palaeontology than in stratigraphy.
 

     As a further conclusion of his observations, Arnaud reconstructs the history of the events that led to the correlative modifications observed in the nature of the deposits of the different seas, and in the organisms that inhabited them: emersion and subsidence movements to which correspond more or less fine sediments, sands, clays, marls, limestones, and a population appropriate to these conditions of deposition. This is how we see the appearance of banks of Rudists at the top of the Lower Chalk, the Middle Chalk and the Upper Chalk, while others interspersed in the middle of these last two subdivisions indicate accidents that occurred during these periods, interrupting the regular progression of sedimentary phenomena.

     The divisions that Arnaud had just traced with such sagacity in the Chalk of the South-West, were again specified by him in the summary that he presented at the end of the Extraordinary Meeting of our Society, in 1887; he briefly gives the essential characters of the various strata and distinguishes each of them by a letter of the alphabet, as Quenstedt had done for the Jurassic. The stratigraphic scale thus established has become classic and serves as a term of comparison for the classification of Cretaceous layers in other regions: today, when we want to specify the position of one of them in the sedimentary series, it is usually to one of the layers defined by Arnaud that we refer.
 

     After the publication of this important memoir, Arnaud successively published the profiles he had recorded on the various railway lines of the South-West.
 

      On several occasions he intervened in the ardent discussions which took place within our Society on the parallelism of the Northern Chalk and the Southern Chalk and which brought together Hébert, Arnaud, Peron and M. Toucas.
 

     Hébert, to whom his official position gave great authority, and with him many geologists professed that the terrains of the same age present everywhere the same mineralogical constitution and that the limits of stage correspond to a complete renewal of the faunas; they refused to admit that the same facies can be found in layers of different ages, errors against which Arnaud had protested, from his first scientific publications: Today, the principles that he has always upheld have definitively triumphed and, if we can be astonished by anything, it is that it has taken such a long time to have truths accepted that seem so obvious to us. And it must be recognised that they are not always interpreted in a sound manner.
 

     Hébert therefore considered all the Hippurite limestones of southern France as belonging to a single zone which formed the upper part of the Turonian stage, the base of which was constituted by the Touraine Chalk, so that, for him, a gap corresponding to this horizon existed throughout northern Europe. He also believed that all the Chalk of Aquitaine was only a magnificent development of the Villedieu Chalk, whose general characteristics it presented from the base to the highest layers.
 

     Arnaud rightly replied that, if there is a link of continuity between the various layers of the Upper Chalk of the South-West, this is not enough to demonstrate the unity and indivisibility of this whole.
 

     The question was also of a palaeontological nature, because the return of the same facies leads to the recurrence of analogous faunas, which the geologists of that time, in the absence of meticulous studies, considered to be identical: palaeontologists did not know how to recognise the distinctive characters of these recurrent types "altered by time", as Arnaud used to say, and the knowledge of the mutations of the Rudists, due to the fine research of Messrs. Toncas is of very recent date.
 

     The important memoir by A. Peron on the Echinid limestones of Rennes-les-Bains, published in 1877, was the starting point of the long discussions to which the problem of the parallelism of the Cretaceous layers of the North and South of France gave rise. The principles he invoked are precisely those that Arnaud posed some twenty years earlier, in his first note published in our Bulletin. "The necessity where we believe we are, says A. Peron, to consider as absolutely synchronous the deposits where certain characteristic species are found, can sometimes lead to real inconsistencies. In the case of corals and Rudists especially, I believe it is necessary to use this means of limiting and parallelizing geological horizons with caution. Polypier and Rudist reefs certainly needed certain biological conditions to develop, which did not always occur simultaneously at all points in the same basin and a fortiori in different basins. These favourable environmental conditions could also have been reproduced on a few points at intervals and given rise to the alternations that we see; finally, they may not have occurred at all. And he concludes: "the absence in a series of layers of a level of Rudists or Polypers that exists elsewhere does not necessarily imply a sedimentary interruption. These reefs are truly accidental deposits and it is perfectly obvious that they could not have existed everywhere at once.

     Arnaud intervened in the discussion: he fought, it is true, against part of the conclusions of A. Peron, continuing to believe that there was a gap between the Turonian and the Senonian in the Northern Chalk, which corresponded to the absence of the Hippurite layers, and that the Senonian was the only one of its kind. Peron, continuing to believe that there was a gap between the Turonian and the Senonian in the Northern Chalk which corresponded to the absence of the Hippurite layers, and that the Senonian of the Anglo-Parisian basin had another gap resulting from the absence of the Villedieu Chalk, but the table of synchronicity which he presented in 1878, although tainted by these errors, is much closer to the conclusions currently adopted than anything published at that time.
 

     A few years later, in his 1883 memoir on the division of the Turonian and Senonian in France, he partly cleared himself of these errors. He insisted on the point that "the divisions adopted in geology, more or less general, more or less extensive, necessarily cease, at one point or another, to be graspable: at these points, the deposits succeeded one another in an uninterrupted manner, the species became extinct and were replaced one by one without marking any distinction between the layers they link". He points out "these unexpected returns, these astonishing vertical evolutions, the number of which increases every day as a result of the multiplied research of geologists". "The persistence of mineralogical characteristics is consistent with the sequence of faunas and the longevity of species to attest to the uniformity of depositional conditions. He also concludes that "to require the identity of faunas and rocks in order to establish the synchronicity of the layers of different basins would be to pursue a chimera".
 

     The synchronic table that concludes this memoir, and which relates to the various beds of the Turonian and Lower Senonian, is truly remarkable and deviates very little from the parallelisms now accepted.
 

     Arnaud does not include the gaps admitted by Hébert, referring to the opinion of geologists who attribute the differences reported from one region to another, not to gaps, but to transformations resulting from the difference in environments.
 

     Arnaud does not mention the Cretaceous strata of the Corbières and Provence either, for it was only later that he rallied to the conclusions of A. Peron and M. Toncas. At that time, he refused to admit the parallelism of the Micrasters layers of the South-West and the South and placed those of Rennes-les-Bains and La Cadière in the Turonian.
 

     Arnaud's error in this respect resulted from the generality of extension that he attributed to the disturbing events that came on various occasions to interrupt the regular course of sedimentary phenomena, to what we currently call transgressions and regressions. This error was all the more excusable because even today this principle still has supporters and is invoked as the basis for a methodical classification of the layers.

     On two occasions, in 1885 and 1888, the Geological Society, to show the esteem in which it held his work, elected him as Vice-President and, in 1887, it decided to hold its extraordinary meeting in the Charente-Inférieure and in the Dordogne; Arnaud was appointed President for this session. Sensitive to the honour the Society had done him by choosing as the field of his studies the region to which he had devoted himself, he considered this choice as the most enviable reward that could be attributed to him and it was a real pleasure for him to lead the numerous colleagues who accompanied him on the grounds he had travelled so often.
 

     The marriage of his daughter to M. Dr. Lavielle of Dax, had the consequence of calling him every year to this town: he was thus able to study the Chalk of Angoumé and Tercis and, in 1886, he published a note in which he fixed the exact position of the various levels that he distinguished there.
 

     Arnaud took advantage of all his free time to leave the city and travel the countryside in search of interesting deposits. Gifted with an extraordinary perspicacity, he knew how to discover the rarest and most beautiful samples. "Fossils jump out at you", Coquand used to tell him. In this way, valuable palaeontological treasures accumulated in his collection. He was happy to do the honours to those who came to visit him and he always did it with the most exquisite kindness. He freely communicated his collections to all French and foreign scholars who wished to study them.
 

     The palaeontological works of our time also contain many new types established according to the samples of his collection. He himself was not uninterested in this type of research and in palaeontology as in stratigraphy he showed a penetrating eye. He described several new species of Brachiopods and Rudists, but he was especially fond of Echinids, which are numerous and well preserved in the Chalk of the Southwest; the studies he published on them place him among the most competent echinologists of his time.
 

     Old age was for a long time merciful to the tireless worker and, until a few years ago, still an intrepid walker, he enjoyed finding a companion for his runs.
 

     The day came when the lawyer had to leave the Palace and the geologist had to give up his explorations; the Christian prepared himself for death: it came upon him on November 1st last.
 

     The memory of Arnaud will remain among us as that of a scholar of high value who, from the beginning of his career, knew how to escape from the erroneous doctrines which reigned in science at the time. We must recognise in him a precursor, for he was one of the first among those who brought to light, in spite of strong opposition, the fundamental principles of stratigraphy.

bibliography of H. ARNAUD's scientific works
 

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