When modern geology blossomed, right after the turmoil of the French Revolution and during the Napoleonic Era, it was clear that there existed an order of deposition of sedimentary strata, visible in outcrops throughout Europe, crossing national borders. Their study would have shed light on the history of the planet. This order was expressed by the succession of Primary, Transitional, Secondary, and Tertiary rocks, topped by Recent strata. For some time, the thick cover of Tertiary formations were simply subdivided into “older” and “younger”, characterised by fossils that up-section became progressively more similar to the modern fauna. First dissected by the French geologist Alexandre Brongniart, then mastered by the Swiss Amy Boué, Tertiary stratigraphy became central to Charles Lyell’s Principles of Geology, particularly the third volume, published in 1833.
Following by the French conchologist Gerard Paul Deshayes, expert of fossil molluscs, and assisted by the English polymath William Whewell, a great inventor of names, Lyell introduced in 1833 the terms “Eocene”, “Miocene” and “Pliocene”. The terms pivoted around the concept of “recent”, expressed by the ancient Greek “kainos”. “Pliocene” came by adding the prefix “pleion”, meaning “major”, given that in the lower formations of this epoch about a half (older Pliocene), or up to 90% (newer Pliocene) of fossil species were still living in modern seas; “Miocene” resulted from “meion”, equal “minor”, since only a minority of species were extant (about 20%); finally “Eocene” came from “eon”, meaning “dawn”, meant for the very beginning of the modern fauna. These beginnings could be verified in marine and freshwater strata directly overlying the Chalk, the last formation of the Secondary. They contained but a very small fraction of extant species, whereas the Chalk had none.
Geological Map of the London Basin, from Charles Lyell’s third volume of the Principles of Geology (1833). Tertiaries are coloured in pink.
Eocene period.—The period next antecedent we shall call Eocene, from ???, aurora, and ??????, recens, because the extremely small proportion of living species contained in these strata, indicates what may be considered the first commencement, or dawn, of the existing state of the animate creation. To this era the formations first called tertiary, of the Paris and London basins, are referrible. (…) The total number of fossil shells of this period already known, is one thousand two hundred and thirty-eight, of which number forty-two only are living species, being nearly in the proportion of three and a half in one hundred. Of fossil species, not known as recent, forty-two are common to the Eocene and Miocene epochs. In the Paris basin alone, 1122 species have been found fossil, of which thirty-eight only are still living. (…) The recent strata form a common point of departure in all countries.—We derive one great advantage from beginning our classification of formations by a comparison of the fossils of the more recent strata with the species now living, namely, the acquisition of a common point of departure in every region of the globe. Thus, of example, if strata should be discovered in India or South America, containing the same small proportion of recent shells as are found in the Paris basin, they also might be termed Eocene, and, on analogous data, an approximation might be made to the relative dates of strata placed in the arctic and tropical regions, or the comparative age ascertained of European deposits, and those which are trodden by our antipodes.Charles Lyell, 1833 (p. 55, 58)
“Synoptical table of the Recent and Tertiary formations”, from Lyell’s third volume of The Principles of Geology.
The species concept adopted by Lyell, to which he dedicated in 1832 a large part of the second volume of the Principles, was an elaboration of the species-individual analogy introduced a few years earlier by the Italian Giambattista Brocchi, refined in the light of the work of Constant Prevost and other early-geology pioneers, who first differentiated the relative age of Tertiary strata (Rudwick 2008). He would then put to work this concept for an all-embracing gradualistic view of Earth’s history, in the prose that was going to make him famous.
In order more distinctly to elucidate our idea of the working of this machinery, let us compare it to a somewhat analogous case that might easily be imagined to occur in the history of human affairs. Let the mortality of the population of a large country represent the successive extinction of species, and the births of new individuals the introduction of new species. While these fluctuations are gradually taking place everywhere, suppose commissioners to be appointed to visit each province of the country in succession, taking an exact account of the number, names, and individual peculiarities of all the inhabitants, and leaving in each district a register containing a record of this information. If, after the completion of one census, another is immediately made after the same plan, and then another, there will, at last, be a series of statistical documents in each province. When these are arranged in chronological order, the contents of those which stand next to each other will differ according to the length of the intervals of time between the taking of each census.Charles Lyell, 1833 (p. 31)
The third of three plates dedicated by Lyell (1833) to fossil shells indicative of the three subdivisions of the Tertiary (species chosen by Deshayes).
The position held by Charles Lyell in the Geological Society of London, his geological lectures at King’s College, and the editorial success of Principles of Geology contributed to bring the names Eocene, Miocene and Pliocene to wide use, although not as straightforward has hoped, and in any case not as implied by the strict application of Lyell’s statistical method (Rudwick 1978, Bergreen 1998). Rather, geologists retained the concept of a formal subdivision of the Tertiary, equating Lyell’s subdivisions to the rock on which they were based. In this sense, the name Eocene was maintained for the rock and time packages represented by marine formations like the Coarse Limestone in Paris (Calcaire Grossier) and the London Clay in London, intercalated in their respective regions with freshwater formations, and gradually changing aspect and thickness moving away from their type areas.
Unrestricted gradualism
Lyell’s gradualistic approach is also evident in his usage of the adjectives “older, middle, and newer”. In the original concept, epochs were in fact not meant to have boundaries: this would give credit to those who claimed the catastrophic nature of the geological record. Instead, Lyell encouraged the usage of “older, middle, and newer” to express departures from the averages suggested by Deshayes’ computations.
Charles Lyell in 1840
In regard to distinct zoological periods, the reader will understand that we consider the wide lines of demarcation that sometimes separate different tertiary epochs, as quite unconnected with extraordinary revolutions of the surface of the globe, and as arising, partly, like chasms in the history of nations, out of the present imperfect state of our information, and partly from the irregular manner in which geological memorials are preserved. We have little doubt that it will be necessary hereafter to intercalate other periods, and that many of the deposits, now referred to a single era, will be found to have been formed at very distinct periods of time, so that we shall continue to use the term contemporaneous with a great deal of latitude. We throw out these hints, because we are apprehensive lest zoological periods in geology, like artificial divisions in other branches of natural history, should acquire too much importance, from being supposed to be founded on some great interruptions in the regular series of events in the organic world, whereas, like the genera and orders in zoology and botany, we ought to regard them as invented for the convenience of systematic arrangement, always expecting to discover intermediate gradations between the boundary lines that we have first drawn. In natural history we select a certain species as a generic type, and then arrange all its congeners in a series, according to the degrees of their deviation from that type, or according as they approach to the characters of the genus which precedes or follows. In like manner, we may select certain geological formations as typical of particular epochs; and having accomplished this step, we may then arrange the groups referred to the same period in chronological order, according as they deviate in their organic contents from the normal groups, or according as they approximate to the type of an antecedent or subsequent epoch. If intermediate formations shall hereafter be found between the Eocene and Miocene, and between those of the last period and the Pliocene, we may still find an appropriate place for all, by forming subdivisions on the same principle as that which has determined us to separate the lower from the upper Pliocene groups. Thus, for example, we might have three divisions of the Eocene epoch—the older, middle, and newer; and three similar subdivisions, both of the Miocene and Pliocene epochs. In that case, the formations of the middle period must be considered as the types from which the assemblage of organic remains in the groups immediately antecedent or subsequent will diverge (p. 56-58).Charles Lyell, 1833 (p. 56-58)
Among others, positive feedbacks were given in France by Ami Boué, who was enthusiastic about Lyell’s subdivisions (Vaccari 1998). Lyell would retain an important role in future debates, until the fifties of his century, when it became clear that a solution had to be found for intermediate strata discovered between the Chalk and the Eocene, or between the Eocene and the Miocene.
Learn more
Bergreen W.A. (1998) The Cenozoic era: Lyellian (chrono)stratigeraphy and nomenclatural reform at the millennium. In: Blundell D.j. & Scott A.C. (eds.). Lyell: the past is the key to the present. Geological Society, London. Special Pubblications 143, 111-132.
Boué A. (1831). Critical observations on the ideas of M. Alexander Brongniart, relating to the classification and probable origin of Tertiary deposits. Edinburgh New Philosophical Journal, 12, 159-172; 340-345.
Lyell C. (1833). Principles of geology, being an attempt to explain the former changes of the Earth’s surface, by reference to causes now in operation. London: John Murray. Volume 3.
Rudwick, M. J. S. (1978). Charles Lyell’s dream of a statistical palaeontology. Palaeontology, 21, 225–244
Rudwick, M. J. S. (2005). Bursting the limits of time. The Reconstruction of geohistory in the age of Revolution. 708 p., University of Chicago Press, Chicago.
Rudwick, M. J. S. (2008). Worlds before Adam. The Reconstruction of geohistory in the age of Reform. 614 p., University of Chicago Press, Chicago.
