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Capturing the Center

Natural History, Dec, 1998 by Stephen Jay Gould

This is the second part of a two-part essay. Part 1, "Writing in the Margins," on the great chemist Lavoisier's contributions to the nascent science of geology, appeared in last month's issue.

When Antoine-Laurent Lavoisier began his geological work with Jean-Etienne Guettard in 1766, he accepted a scenario, then conventional, for the history of the earth as revealed by the record of rocks: a simple directional scheme that envisaged a submergence of ancient landmasses (represented today by the crystalline rocks of mountains) under an ocean, with all later sediments formed in a single era of deposition from this stationary sea (on this topic, see Rhoda Rappoport's important article "Lavoisier's theory of the earth," British Journal for the History of Science, 1973). Since geologists then lacked techniques for unraveling the contorted masses of older crystalline rocks, they devoted their research to the later stratified deposits and tried to read history as an uncomplicated tale of linear development. (No fossils had been found yet in the older crystalline rocks, so these early geologists also assumed that the stratified deposits contained the entire history of life.)

Lavoisier's key insight led him to reject this linear view (one period of deposition from a stationary sea) and to advocate the opposite idea, that sea level had oscillated through time and that oceans had therefore advanced and retreated through several cycles in any particular region--a notion now so commonplace that any geologist can intone the mantra of earth history, "The seas go in and the seas go out." Lavoisier reached this radical conclusion by combining the developing ideas of such writers as Georges Buffon and Benoit de Maillet with his own observations on cyclical patterns of sedimentation in vertical sections.

Lavoisier christened his 1789 paper with a generous title fully characteristic of a time that did not separate literature and science: Observations generales sur les couches modernes horizontales qui ont ete deposees par la mer, et sur les consequences qu'on peut firer de leurs dispositions relativement d l'anciennete du globe terrestre (General observations on the recent horizontal beds that have been deposited by the sea, and on the consequences that one can infer from their arrangement about the antiquity of the earth). The title may have been grand, general, and expansive, but the content remained precise, local, and particular--at first. Lavoisier begins his treatise by distinguishing the properties of sediments deposited in open oceans from those formed along shorelines--a device to establish data for his central argument that seas advance and retreat in a cyclical pattern over any given region.

After two short introductory paragraphs, Lavoisier plunges right in by expressing puzzlement that two such contradictory kinds of rock can be found in alternating cycles of a single vertical section. The nature of the fossils and sediments indicate calm and gentle deposition for one kind: "Here one finds masses of shells, mostly thin and fragile, and most showing no sign of wear or abrasion.... All the features [of the rocks] that surround these shells indicate a completely tranquil environment." (I am responsible for these translations from Lavoisier's 1789 paper.) But rocks deposited just above testify to completely different circumstances of formation:

   A few feet above the place where I made these observations, I noted an
   entirely opposite situation. One now sees no trace of living creatures;
   instead, one finds rounded pebbles whose angles have been abraded by rapid
   and long-continued tumbling. This is the picture of an agitated sea,
   breaking against the shore and violently churning a large quantity of
   pebbles.

Lavoisier then poses his key question, already made rhetorical by his observations:

   How can we reconcile such opposite observations? How can such different
   effects arise from the same cause? How can movements that have abraded
   quartz, rock crystal and the hardest stones into rounded pebbles also have
   preserved light and fragile shells?

The simple answer to this specific question may then lead to important generalities for the science of geology and also to criteria for unraveling the particular history of the earth:

   At first glance, this contrast of tranquillity and movement, of
   organization and disorder, of separation and mixture seemed inexplicable to
   me; nevertheless, after seeing the same phenomena again and again, at
   different times and in different places, and by combining these facts and
   observations, it seemed to me that one could explain these striking
   observations in a simple and natural manner that could then reveal the
   principal laws followed by nature in the generation of horizontal strata.

Lavoisier then presents his idealized model of a two-stage cycle as an evident solution to this conundrum: "Two kinds of very distinct beds must exist in the mineral kingdom: one kind formed in the open sea ... which I shall call pelagic beds, and the other formed at the coast, which I shall call littoral beds." Pelagic beds arise by construction as "shells and other marine bodies accumulate slowly and peacefully during an immense span of years and centuries." But littoral beds, by contrast, arise by "destruction and tumult ... as parasitic deposits formed at the expense of coastlines."

Capturing the Center

Natural History, Dec, 1998 by Stephen Jay Gould

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