Capturing the Center

In a brilliant ploy of rhetoric and argument, Lavoisier then builds his entire treatise as a set of consequences from this simple model of two types of alternating sediments representing the cycle of a rising and falling sea. This single key, Lavoisier claims, unlocks the great conceptual problem of moving from one-dimensional observations of vertical sequences in several localities to a three-dimensional reconstruction of history (I call the solution three-dimensional for a literal reason: the two horizontal dimensions record geographical variation over the earth's surface, while the vertical dimension marks time in a sequence of strata):

   This distinction between two kinds of beds ... suddenly dispersed the chaos
   that I experienced when If first observed terranes made of horizontal beds.
   This same distinction then led me to a series of consequences that I shall
   try to convey, in sequence, to the reader.

The remainder of Lavoisier's treatise presents a brilliant fusion of general methodology and specific conclusions, making the work a wonderful exemplar of scientific procedure at its best. The methodological passages emphasize two themes: the nature of proof in natural history and the proper interaction of theory and observation. Lavoisier roots the first theme in a paradox discussed at the end of last month's installment of this two-part essay: the need to simplify at first in order to generalize later. Science demands repetition for proper testing of observations--for how else could we learn that the same circumstances reliably generate the same results? But the conventional geologies of Lavoisier's time stymied such a goal--for the concept of one directional period of deposition from a single stationary sea offered no opportunity for testing by repetition. By contrast, Lavoisier's model of alternating pelagic and littoral beds provided a natural experiment in replication at each cycle.

But complex nature defies the needs of laboratory science for simple and well-controlled situations, where events can be replicated under identical conditions set by few variables. Lavoisier argues that we must therefore try to impose similar constraints upon the outside world by seeking "natural experiments," where simple models of our own construction might work adequately in natural conditions chosen for their unusual clarity and minimal number of controlling factors.

Consider three different principles, each exploited by Lavoisier in this paper, for finding or imposing a requisite simplicity upon nature's truly mind-boggling complexity.

1. Devise a straightforward and testable model. Lavoisier constructed the simplest possible model of seas moving in and out and depositing only two basic (and strongly contrasting) types of sediment. He knew perfectly well that real strata do not arrange themselves in neat piles of exactly repeating pairs, and he emphasized two major reasons for nature's much greater actual complexity: first, seas don't rise and fall smoothly, but rather wiggle and jiggle in small oscillations superposed upon any general trend; second, the nature of any particular littoral deposit depends crucially upon the type of rock being eroded at any given coastline. But Lavoisier knew that he must first validate the possibility of a general enterprise three-dimensional reconstruction of geological history--by devising a model that could be tested by replication. The pleasure of revealing unique details would have to come later. He wrote: