Pozzuoli's Pillars Revisited

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.