Capturing the Center

Lavoisier had spent years watching Guettard fritter away time with an inchoate gathering of disparate hits of information, without any cohesive theory to guide and coordinate his efforts. As a result, Lavoisier pledged to proceed in an opposite manner, while acknowledging that the myth of objectivity had made his procedure both suspect and unpopular. Nonetheless, he would devise a simple and definite model and then gather field observations in a focused effort to test his scheme. (Of course, theory and observation interact in subtle and mutually supporting ways. Lavoisier used his preliminary observations to build his model and then went hack to the field for extensive and systematic testing.) In an incisive contrast between naive empiricism and hypothesis testing as modes of science, Lavoisier epitomized his preference for the second method:

   There are two ways to present the objects and subject matter of science.
   The first consists in making observations and tracing them to the causes
   that have produced them. The second consists in hypothesizing a cause and
   then seeing if the observed phenomena can validate the hypothesis. This
   second method is rarely used in the search for new truths, hut it is often
   useful for teaching,for it spares students from difficulties and boredom.
   It is also the method that I have chosen to adopt for the sequence of
   geological memoirs that I shall present to the Academy of Sciences.

Lavoisier therefore approached the terranes of France with a definite model to test: seas move in and out over geographical regions in cycles of advancing and retreating waters. These oscillations produce two kinds of strata: pelagic deposits in deeper waters and littoral deposits fashioned from eroded coasts near the shoreline. Type of sediment should indicate both environment of deposition and geographical position with respect to the shoreline at that time: Pelagic deposits always imply a distant shore. For littoral deposits, relative distance from shore can be inferred from the nature of any particular stratum; for littoral beds made mainly of flint nodules eroded from the Chalk, the trigger and more angular the nodules, the closer the shoreline.

From these simple patterns, all derived as consequences of an oscillating sea, we should be able to reconstruct the three-dimensional geological history of an entire region from variation in vertical sequences of sediments from place to place. (For example, if a continuous bed representing the same age contains large and angular flint nodules at point A and smaller and more rounded nodules at point B, then A lay closer to the shoreline at the time of deposition.)

Lavoisier devotes most of his paper, including all seven of his beautifully drafted plates, to testing this model, but I can summarize the bulk of his treatise in three pictures and a few pages of text because the model makes such clear and definite predictions--and nature must either affirm or deny. Lavoisier's first six plates--in many ways, the most strikingly innovative feature of his entire work--show the expected geographical distribution of sediments under his model. The first plate, for example, shows the predictable geographical variation in a littoral bed formed by a rising sea. The sea will mount from a beginning position, marked ligne de niveau de la basse mer, "line of low sea level" and indicated by the top of the illustrated waters, to a high stand, marked ligne de niveau de la haute mer, "line of high sea level." The rising sea beats against a cliff, shown at the far left and marked falaise de Craye avec cailloux, "Chalk cliff with pebbles." Note that, as discussed previously, this deposit contains several beds of flint nodules, drawn as thin horizontal hands made up of dark pebbles.