-------------------------------------------------------------------------------
Return to article page

To print: Select File and then Print from your browser's menu.
-------------------------------------------------------------------------------
This story was printed from FindArticles.com, located at http://www.findarticles.com.
-------------------------------------------------------------------------------


Natural History
May, 1999

POZZUOLI'S PILLARS REVISITED.

Author/s: Stephen Jay Gould

Even the best of geological theories can be pushed to explain too much.

Between 1830 and 1833, the Scottish barrister-turned-geologist Charles Lyell published his three-volume Principles of Geology--a work that would transform the earth sciences. According to Lyell's uniformitarian approach, the planet's major geological features were formed, not by sudden catastrophic changes, but by the slow, gradual accumulation of small forces acting over immense periods of time. Young Charles Darwin read Lyell's Principles during his voyage of discovery aboard HMS Beagle and began to look at the world with ideas that came "half out of Lyell's brain." In Part One of this essay, the author discussed how Lyell became fascinated by the evidence of sea-level changes on three partially submerged pillars of a Roman ruin at Pozzuoli, a suburb of Naples. Lyell used the pillars as the frontispiece of his classic text, making them an icon of the new geology. Part Two concerns Stephen Jay Gould's own recent journey to Naples, during which barnacles and oyster shells on the famous pillars convinced him that further inundations had occurred since their depiction in Lyell's early volumes. Gould's attempts to understand these more recent incrustations on the pillars lead to musings about how Charles Babbage-impressed by Darwin's theory on the formation of coral reefs--wrongly speculated that the lunar craters were actually coral atolls.--Eds.

Part Two

In exchanging the pillars of Pozzuoli for the fires of Vesuvius as a Neapolitan symbol for the essence of geological change, Charles Lyell made a brilliant choice and a legitimate interpretation. The three tall columns--originally viewed as the remains of a temple to Serapis (an Egyptian deity much favored by the Romans as well) but now recognized as the entranceway to a Roman marketplace--had been buried in later sediment and excavated in 1749. Lyell, who used these pillars as the frontispiece to his Principles of Geology (1830-33), noted that the marble columns, some forty feet tall, are "smooth and uninjured to the height of about twelve feet above their pedestals." He then made his key observation, clearly illustrated in the frontispiece: "Above this is a zone, about nine feet in height, where the marble has been pierced by a species of marine perforating bivalve--Lithodomus."

From this simple configuration, a wealth of consequences follow--all congenial to Lyell's uniformitarian view and all produced by the same geological agents that shaped the previously reigning icon of Vesuvius in flames. The columns, obviously, were built above sea level in the first or second century A.D. But the entire structure then became partially filled by volcanic debris and subsequently covered by seawater to a height of twenty feet above the bases of the columns. The nine feet of holes made by marine clams (the same animals that, under the misnomer shipworms, burrow into piers, moorings, and hulls throughout the world) prove that the columns then stood entirely underwater to this level--for these clams cannot live above the low-tide line, and the Mediterranean Sea experiences little measurable tide in any case. These nine feet of clam borings, underlain by twelve feet of uninjured column, imply that an infill of volcanic sediments had protected the lower parts of the columns--for these clams live only in clear water.

But the bases of the columns now stand at sea level, so this twenty-foot immersion must have been reversed by a raising of land nearly to the level of the original construction. Thus, in a geological moment of fewer than two thousand years, this structure experienced at least two major movements of the surrounding countryside--a subsidence of more than twenty feet, followed by a rise of comparable magnitude. If such geological work can be done in so short a time, how could anyone deny the efficacy of modern causes in rendering the full panoply of geological history in the hundreds of millions of years actually available? And how could anyone argue that the earth has now become quiescent, after a more fiery youth, if so much activity can occupy the mere moment of historical time? Thus, Lyell presented the three pillars of Pozzuoli as a triumphant icon for both key postulates of his uniformitarian system: the efficacy of modern causes and the relative constancy of their strength through time.

The notion of a geologist touring Naples but omitting nearby Pozzuoli makes about as much sense as a tale of a pilgrim to Mecca who visited the Casbah but skipped the Kaaba. Now, I admire Lyell enormously as a great thinker and writer, but I have never been a partisan of his uniformitarian views (my very first scientific paper, published in 1965, identified a logical confusion among Lyell's various definitions of uniformity). But my own recent observations of the pillars of Pozzuoli only seemed to strengthen and extend his conclusions on the extent and gradual character of geological change during historical times.

I had brought only the first edition (1830-33) of Lyell's Principles with me to Naples. In this text, Lyell attributes (tentatively, to be sure) all changes in level to just two discrete and rapid events. He correlates the initial subsidence (to a level where marine clams could bore into the marble pillars) with "earthquakes which proceeded the [1198] eruption of the Solfatara" a volcanic field on the outskirts of Pozzuoli. "The pumice and other matter ejected from that volcano might have fallen in heavy showers into the sea, and would thus immediately have covered up the lower part of the columns." Lyell then ascribes the subsequent rise of the pillars to a general swelling and uplift of land that culminated in the formation of Monte Nuovo (a volcanic mound near Pozzuoli) in 1538.

But at the site, I observed with some surprise that the evidence for changing levels of land seemed more extensive and complex. I noticed the high zone of clam borings on the three columns, but evidence--not mentioned by Lyell--for another discrete episode of marine incursion struck me as far more obvious and prominent, and I wondered why I had never read or heard anything about this event. Not only on the three major columns but in every part of the complex--the minor columns at the corners of the quadrangular market area, the series of still smaller columns surrounding a circular area in the middle of the market, and even the brick walls and sides of structures surrounding the quadrangle--I noted a zone extending two to three feet up from the marble floor and terminated by a sharp line of demarcation. Within this zone, barnacles and oyster shells remain cemented to the bricks and columns--so the distinct line on top must represent a previous high-water mark. Thus, the still higher zone of clam borings does not represent the only episode of marine incursion. This lower, but more prominent, zone of shells must signify a later depression of land. But when?

Lyell's original frontispiece (redrafted from an Italian publication of 1820), which includes the bases of the large columns, depicts no evidence of this zone. Did he just fail to note the barnacles and oysters, or did this period of marine flooding occur after 18307 I scoured some antiquarian bookstores in Naples and found several early-nineteenth-century prints of the columns (from travel literature about landscapes and antiquities, not from scientific publications). None showed the lower zone of barnacles and oysters. But I did learn something interesting from these prints. None depicted the minor columns now standing both in the circular area at the center and around the edge of the quadrangle--although these locations appear in some prints as flat areas strewn with bric-a-brac. But a later print of 1848 shows columns in the central circular area. I must therefore assume that the excavators of Pozzuoli re-erected the smaller columns of the quadrangle and central circle sometime near the middle of the nineteenth century--while we know that Lyell's three major columns had stood upright since their first discovery during excavations in 1749. (A fourth major column still lies in several pieces on the marble floor of the complex.)

All these facts point to a coherent conclusion. The minor columns of the central circle and quadrangle also feature the lower zone of barnacles and oysters. These small columns were not re-erected before the mid-nineteenth century. Lyell's frontispiece, and other prints from earlier in the century, show the three large columns without encrusting barnacles and oysters at the base. Therefore, this later subsidence of land (or rise of sea to a few feet above modern levels) must have culminated sometime after the 1840s--thus adding further evidence for Lyell's claim of substantial and complex activity within the geological eye blink of historical times.

For a few days, I thought I had made at least a minor discovery at Pozzuoli-until I returned home (and to reality) and consulted some later editions of Lyell's Principles, a book that became his growing and changing child (and his lifelong source of income), reaching a twelfth edition by the time of his death. In fact, Lyell documented in two major stages how increasing knowledge about the pillars of Pozzuoli had enriched his uniformitarian view from his initial hypothesis of two quick and discrete changes toward a scenario of more gradual and more frequent alterations of level:

1. In the early 1830s Charles Babbage, Lyell's colleague and one of the most interesting intellectuals of Victorian Britain (more of him later), made an extensive study of the Pozzuoli columns and concluded that both the major fall of land (to the level of the clam borings) and the subsequent rise had occurred in a complex and protracted manner through several substages, not all at once, as Lyell had originally believed. Lyell wrote in his sixth edition of 1840:

   Mr. Babbage, after carefully examining several incrustations as also the
   distinct marks of ancient lines of water-level, visible below the zone of
   lithophagous perforations [holes of boring clams, in plain English], has
   come to the conclusion, and I think, proved, that the subsidence of the
   building was not sudden, or at one period only, but gradual, and by'
   successive movements. As to the re-elevation of the depressed tract, that
   may also have occurred at different periods.

2. When Lyell first visited Pozzuoli in 1828, the high-water level virtually matched the marble pavement. (Most early prints, including Lyell's frontispiece, show minor puddling and flooding of the complex. Later prints, including an 1836 version from Babbage that Lyell adopted as a replacement for his original frontispiece in later editions of the Principles, tend to depict deeper water.) In 1838 Lyell read a precise account of this modern episode of renewed subsidence and then monitored this most recent change in subsequent editions of the Principles. He wrote that Niccolini, "a learned architect [who] visited the ruins frequently for the sake of making drawings," recorded a two-foot sinking of the complex from his first observations in 1807 until 1838, when "fish were caught every day on that part of the pavement where in 1807, there was never a drop of water in calm weather."

Lyell continued to inquire about this active subsidence--from an English colleague named Smith in 1847, from an Italian named Scacchi in 1852, and from his own observations on a last trip in 1858. Lyell acknowledged several feet of recent sinking and decided to blame the old icon Vesuvius. The volcano had been active for nearly a hundred years--including some spectacular eruptions during Sir William Hamilton's long tenure as a British diplomat in Naples during the late eighteenth century--after more than a century of quiescence. Lyell assumed that the general subsidence of surrounding land must represent an adjustment to the loss of so much underground material from the volcano's vent. He wrote: "Vesuvius once more became a most active vent, and has been ever since, and during the same lapse of time the area of the temple, so far as we know anything of its history, has been subsiding."

In any case, I assume that the prominent layer of incrustation by marine barnacles and oysters, unmentioned by Lyell and undepicted in all my early-nineteenth-century sources--but the most obvious sign of former geological activity at Pozzuoli today, and far more striking, in a purely visual sense, than the higher zone of clam borings--occurred during this later period of higher seas, or even more recently. Again, we can only vindicate Lyell's conviction about the continuing efficacy of current geological processes.

Conventional essays in the hagiographic mode would end here, with Lyell triumphant even beyond the grave and his own observations. But strict uniformity, like its older alternative of uncompromising catastrophism, cannot capture all the complexity of a rich and flexible world.

Uniformity provided an important alternative and corrective to strict catastrophism, but not the complete truth. Much of nature does proceed in Lyell's slow and nondirectional manner, but genuine global catastrophes have also shaped our planet's history--an idea again in the ascendant, given virtual proof for the triggering of the Late Cretaceous mass extinction (an event that removed dinosaurs along with some 50 percent of all marine species) by the impact of an extraterrestrial body. A city of intellectual possibilities includes many mansions, and restriction to one great house will keep us walled off from much of nature's truth.

As a closing example, therefore, let us return to Lyell's fascinating colleague Charles Babbage (1791-1871), Lucasian professor of mathematics at Cambridge and inventor of early calculating machines that presaged the modern digital computer. The Encyclopedia Britannica ends its article on this versatile genius with these words: "He assisted in establishing the modern postal system in England and compiled the first reliable actuarial tables. He also invented a type of speedometer and the locomotive cowcatcher." So why not geology as well!

Babbage presented his studies of Pozzuoli to ,the Geological Society of London in 1834 but didn't publish his results until 1847 because, as he stated in a preface written in the third person, "other avocations obliged him to lay it aside" (primarily that cowcatcher, no doubt). Babbage had pursued his studies to affirm Lyell's key uniformitarian postulate, as he clearly indicated in the ample subtitle of his publication, Observations on the Temple of Serapis at Pozzuoli near Naples, with an attempt to explain the causes of the frequent elevation and depression of large portions of the earth's surface in remote periods, and to prove that those causes continue in action at the present time.

In his long-delayed 1847 publication, Babbage added an appendix to describe the recent subsidence also noted by Lyell in later editions of his Principles of Geology. Babbage discussed the observations of Niccolini and Smith as reported to the Geological Society of London: "Mr. Smith found the floor of the temple dry at high water in 1819, and 18 inches on it at high water in 1845." To reach his general uniformitarian conclusions, Babbage then integrated these latest data with his previous observations on earlier changes in historical times:

   The joint action of certain existing and admitted causes must necessarily
   produce on the earth's surface a continual but usually slow change in the
   relative levels of the land and water. Large tracts of its surface must be
   slowly subsiding through the ages, whilst other portions must be rising
   irregularly at various rates.

To generalize this Neapolitan conclusion, Babbage then cited the ongoing work of a young naturalist, based on entirely different phenomena from the other side of the globe: the coral atolls of the tropical Pacific Ocean. This young man had not yet become the Charles Darwin we revere today (publication of the Origin of Species still lay twelve years in the future, and Darwin had revealed his evolutionary suspicions to only a few close confidants, not including Babbage). Therefore, Babbage and the scientific community of Britain knew Charles Darwin only as a promising naturalist who had undertaken a five-year voyage around the world; who had published a charming book on his adventures and three, more technical volumes on the geology of South America and the formation of coral atolls; and who now stood in the midst of a comprehensive treatise, which would eventually run to four volumes, on the taxonomy of barnacles.

Of this interesting work, Darwin's theory on the origin of coral atolls surely struck colleagues as his most important and original contribution. Darwin, labeling his explanation as the subsidence theory of coral reefs, explained the circular form of atolls as a consequence of subsidence on the surrounding sea floor. Reefs begin by growing around the periphery of oceanic islands. If the islands then subside slowly, the corals can continue to grow upward, eventually forming a ring as the central island finally disappears below the waves.

This brilliant--and largely correct--explanation included two implications most favorable to Lyell and his fellow uniformitarians, hence their warm embrace for this younger colleague. First, the subsidence theory provided an excellent illustration for the efficacy and continuity of gradual change--for corals could not maintain their upward growth unless the central islands sank slowly. (Reef corals, filled with symbiotic photosynthetic algae, cannot live below the level of penetration by sunlight into oceanic waters, so any rapid subsidence would extinguish the living reefs.)

Second--and more crucial to the work of Babbage and Lyell at Pozzuoli--the large geographic extent of atolls proves that major regions of the earth's crust must be subsiding, thus also implying that other regions of comparable extent must be rising at the same time. Therefore, the fluctuations recorded on Pozzuoli's pillars do not only represent a local phenomenon but also illustrate one of the most fundamental principles of the gradualist, nondirectionalist, and uniformitarian mechanics of basic planetary behavior.

In fact, and above all other implications, Darwin had emphasized his discovery that coral atolls form only in regions without active volcanoes, while no atolls exist where volcanoes flourish in eruption. This mutual avoidance indicates that large tracts of the earth's crust, not merely local pinpoints, must be subsiding or rising in concert--with atolls as primary expressions of subsidence and volcanoes as signs of uplift.

Babbage wrote to praise the young Darwin but also to assert that he himself had reached the same uniformitarian conclusions independently, during his studies of Pozzuoli:

   Mr. Darwin, whose voyages and travels extended from 1826 to 1836/sic: the
   Beagle voyage lasted from 1831 to 1836], was gradually accumulating and
   arranging an immense collection of facts relating to the formation of coral
   and lagoon islands, as well as to the relative changes of level of land and
   water. In 1838 Mr. Darwin published his views on those subjects, from
   which, amongst several other very important inferences, it resulted that he
   had, from a large induction of facts, arrived at exactly the same
   conclusion as that which it has been the chief object of this paper to
   account for, from the action of known and existing causes.

So far, so good--and so fair and so just. But Babbage then went further into one of the most ludicrously overextended hypotheses ever advanced in the name of uniformitarian geology. He appended a "supplement" to his 1847 publication on the pillars of Pozzuoli, entitled "Conjectures concerning the physical condition of the surface of the moon." In Babbage's day, most scientists interpreted lunar craters as volcanic cones, a catastrophic explanation that he wished to challenge. He noted that a region of lunar craters would look very much like a field of earthly coral atolls standing in the bed of a vanished sea:

   The perusal of Mr. Darwin's explanation of the formation of coral reefs and
   of lagoon islands led me to compare these islands with those conical
   crater-shaped mountains which cover the moon's surface; and it appears to
   me that no more suitable place could be found for throwing out the
   following conjectures, than the close of a paper in which I have
   endeavoured to show, that known and existing causes lead necessarily to
   results analogous to those which Mr. Dam, in has so well observed and
   recorded.... If we imagine a sea containing a multitude of such lagoon
   islands to be laid dry, the appearance it would present to a spectator at
   the moon would strongly resemble that of a country thickly studded with
   volcanic mountains, having craters of various sizes. May not therefore much
   of the apparently volcanic aspect of the moon arise from some cause which
   has laid dry the bottom of a former ocean on its surface?

Babbage became bolder near the end of his commentary, as he explicitly wondered "if those craters are indeed the remains of coral lagoon islands." To be fair, Babbage recognized the highly conjectural nature of his hypothesis:

   The preceding remarks are proposed entirely as speculations, whose chief
   use is to show that we are not entirely without principles from which we
   may reason on the physical structure of the moon, and that the volcanic
   theory is not the only one by which the phenomena could be explained.

But later discoveries only underscore the irony of what may be the greatest overextension of uniformitarian preferences ever proposed by a major scientist. Babbage suggested that lunar craters might be coral atolls because he wished to confute their catastrophic interpretation as volcanic vents and mountains. Indeed, lunar craters are not volcanoes. They are formed by the even more sudden and catastrophic mechanism of meteoritic impact!

Comprehensive worldviews like uniformitarianism or catastrophism provide both joys and sorrows to their scientific supporters: the great benefits of a guide to reasoning and observation, a potential beacon through the tangled complexities and fragmentary character of nature's historical records--ineluctably combined, however, with the inevitable, ever present danger of biases and false assurances that can blind us to contrary phenomena standing right before our unseeing eyes. Lyell himself emphasized this crucial point, with his characteristic literary flair, in the closing paragraph to his discussion of the pillars of Pozzuoli--in this case, to combat the prejudice that landmasses must be rock stable, with all changes of level ascribed to movements of the sea:

   A false theory it is well known may render us blind to facts, which are
   opposed to our prepossessions, or may conceal from us their true import
   when we behold them. But it is time that the geologist should in some
   degree overcome those first and natural impressions which induced the poets
   of old to select the rock as the emblem of firmness--the sea as the image
   of inconstancy.

But we also know that no good deed goes unpunished and that any fine principle can turn around and bite you in the ass. Lyell had used this maxim about the power of false theories to note that conventional preferences for catastrophism had been erroneously nurtured by the differential preservation of such evidence in our imperfect geological records. But Georges Cuvier--Lyell's French colleague, geology's leading catastrophist, and perhaps the only contemporary who could match Lyell's literary and persuasive skills--had issued the ultimate touche in a central passage of the most celebrated defense for geological catastrophism: his Discours preliminaire of 1812.

Here, Cuvier urges an opposite conclusion from the same valid argument about the blinding force of ordinary presuppositions. We are misled, Lyell had remarked, by the differential preservation of catastrophes in the geological record. Cuvier held, au contraire, that we become equally blinded by the humdrum character of daily experience. Most moments, Cuvier argues, feature no local wars or deaths and certainly no global cataclysms. So we do not properly credit these potential forces as agents of history, even though one global paroxysm every few million years (and therefore rarely, if ever, observable in a human lifetime) can shape the pageant of life on earth. Cuvier writes:

   When the traveler voyages over fertile plains and tranquil waters that, in
   their courses, flow by abundant vegetation, and where the land, inhabited
   by many people, is dotted with flourishing villages and rich cities filled
   with proud monuments, he is never troubled by the ravages of war or by the
   oppression of powerful men. He is therefore not tempted to believe that
   nature undergoes her internal wars, and that the surface of the globe has
   been overturned by successive revolutions and various catastrophes.
   (Author's translation)

I must now leave these two great geological gladiators, each using the same excellent tool of reason to battle for his own different theory about the earth's particulars. I return then to the pillars of Pozzuoli, just down the road from the third largest preserved amphitheater of the Roman world (where we may site those warriors for a closing image): When I visited Pozzuoli in early January of the premillennial year of 1999, I noticed, at one entrance to the park that contains the three famous pillars, a small, modern monument--a chipped and neglected slab of marble covered with graffiti scrawled Over a quotation with no identifying author. But I did copy the text as an excellent summary; less literary, to be sure, than the warring flourishes of Lyell or Cuvier but equally eloquent in support of their common principle--a good guide for any scientist and indeed for any person who wishes to use the greatest human gift of independent reason against the presuppositions that bind us to columns of priestly or patriotic certainty, or to mountains of cultural stolidity:

   Cio che piu importa e che il popolo, gli uomini tutti, perdano gli istinti
   e le abitudini pecorili che la millenaria schiavitu ha loro ispirato ed
   apprendano a pensare ed agire liberamente.

   (What is most important is that the populace, all people, lose the
   instincts and habits of the flock, which millennia of slavery have
   inculcated in them, and learn to think and act in freedom.)

Stephen Jay Gould teaches biology, geology, and the history of science at Harvard University. He is also the Frederick P. Rose Honorary Curator in Invertebrates at the American Museum of Natural History.

-------------------------------------------------------------------------------
COPYRIGHT 1999 American Museum of Natural History
in association with The Gale Group and LookSmart. COPYRIGHT 2000 Gale Group
-------------------------------------------------------------------------------