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Primer: Jean-André de Luc’s Christian Geology April 2, 2009

Posted by Will Thomas in EWP Primer.
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Click for Hasok Chang's page on De Luc

It was nice timing that Michael Robinson happened to bring up at his blog the relationship between Biblical and physical (i.e. “biological” to be somewhat anachronistic) accounts of human history and race.  I’ve just been going over Martin Rudwick’s fantastic Bursting the Limits of Time (2005), and had decided to write a post on Jean-André de Luc (1727-1817), a devoutly Christian natural philosopher of the Earth’s history, and coiner of the term “geology”.

De Luc came from a family of Genevan clock-makers, and had a background in precision engineering.  He was well-known for his design of a portable barometer, and established his reputation as an authority on meteorology.  In 1773 he moved to England, where he resided for the rest of his life, and became a Fellow of the Royal Society and a “reader” (i.e. mentor) for Queen Charlotte, the wife of King George III.

In the mid-to-late eighteenth century, the discipline now understood as geology was comprised of a variety of natural historical and natural philosophical fields of study: mineralogy, physical geography, geognosy (the study of the contents of the earth, related to mining), and the “physics” of the earth (which dealt with qualitative discussions of the earth’s mechanisms).  An important intellectual activity in this period was the construction of vast logical “systems” that connected causal and historical explanations to account for observations, thereby establishing a potential comprehensive “theory of the Earth”.  It was to describe this last activity that de Luc deployed the term “geology”, to distinguish it from more expansive cosmologies, which attempted to account for the workings of the entire universe.

As a devout Christian, de Luc felt embattled in a community of Enlightenment-era savants who embraced deistic, or even atheistic, principles, and who shunned Biblical history as a guide to the history of the Earth.  De Luc was not a Biblical literalist.  But, unlike, say, the powerful savant, the Comte de Buffon—who acknowledged only a weakly allusive relationship between Biblical creation and his own theory of the Earth developing through a series of epochal transformations—de Luc was eager to understand the physical meaning of Biblical revelations.  While he did not imagine physical knowledge had direct moral implications, he did believe that verifying Biblical history would encourage faith in the Bible’s moral teachings.

De Luc developed his geological system over the course of his multi-volume Lettres physiques et morales (Physical and Moral Letters, 1779), addressed to Queen Charlotte.  This system, like Buffon’s, supposed a staged history of the Earth, but, unlike Buffon’s (which supposed the Earth was knocked off the sun by a comet and was cooling from an initial fiery state), imagined no particular origins, nor did it assign causal mechanisms to the changes the Earth experienced.  De Luc supposed that the Earth was of indefinite age, but that most of its existence consisted of a period in which the present continents were at the bottom of the sea—a point most theories had to include to account for fossils of sea animals found well inland.

During this initial period there had been a prior set of continents, which created the present continents’ “secondary” rock formations and their fossils through erosion into the sea.  At some point, however, there was a “revolution” in which those continents collapsed into the sea, and at some point not long thereafter, the present continents emerged, probably slowly.  De Luc identified this emergence with Noah’s flood, and supposed some very few islands remaining above the surface were Noah’s ark.

De Luc argued that the age of the present continents could be measured by the depth of peat soil naturally accumulated over the rock beneath it in places where lands were uncultivated.  Further, since a similar layer of peat could be found on top of ancient burial mounds, it was evidence that the age of the recent continents was roughly the same as the length of time that people had inhabited the Earth, and that both lengths of time were on the scale of thousands of years.  This point was important to De Luc, because some natural philosophers had supposed extremely long or even eternal histories for both, contra Biblical authority.

Rudwick’s take on de Luc is that while his “geology” certainly had its idiosyncratic commitments, it was really no more the case than with other systematic theories, such as Buffon’s or the Scot natural philosopher James Hutton’s.  By the late 19th 18th century, speculative systems were viewed with some dissatisfaction.  Cosmologies and other assorted systems were many in number and all wildly different from each other, and though their proponents were sensitive to the epistemological issues and inevitable controversies involved, they continued to prove to be a popular genre for ambitious savants.  Rudwick identifies important features of de Luc’s geology that proved influential, though de Luc himself did not assert them as progressive methodological innovations.  First, de Luc exercised restraint in attempting to attribute all geological change as consequences of some key phenomenon, such as Buffon’s cooling earth, which meant that he provided a model for geological history that wasn’t deterministic in its broad outlines.  Second, de Luc’s concern for establishing a chronology of the most recent period established a precedent for later attempts to assign specific time frames to past ages.

When de Luc extended his theory in the 1790s, he retained its key elements.  He still firmly connected it to the authority of the Bible, and the most recent period of the Earth’s history remained in the recent past.  However, he also incorporated newer ideas into previous periods in the Earth’s history.  He divided up his account of the formation of the secondary rock formation, taking into account differing fossils to be found in each, and gave an account of extinction and the rise of new species, dividing the prior part of the Earth’s development, like Buffon, into six distinct phases, which he loosely identified with the “days” of the Creation.

Rudwick’s book is superbly detailed and lucidly composed, and I think could be properly studied by all historians of science, not only for its fine description of thinking about the Earth in this period, but also as a fine example of conscientious historical argument.


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