Terra Incognita. Alain Corbin

freeing the waters of the ‘great deep’.

The theories espoused by so-called ‘Flood geologists’ were criticized by later thinkers. Henri Gautier claimed in 1721 that the globe was completely hollow and full of air. After an eruption near Santorini in 1707, the vulcanist Antonio-Lazzaro Moro theorized that inside the earth was a ball of liquid fire, while Johann Gottlob Krüger suggested in 1746 that the combined action of the Flood and an earthquake had set the entire world ablaze. The historian Vincent Deparis points out that an impressive range of models of the earth³ were put forward over the course of the sixty years preceding the Great Lisbon Earthquake. Fertile as the imaginations of these early geologists were in ascribing a major role to Noah’s Flood, the well of ideas eventually ran dry; theories on the planet’s internal structure took a new direction as research on the issue became less theoretical and imaginative. Kant’s 1756 essay ‘On the causes of earthquakes on the occasion of the calamity that befell the western countries of Europe towards the end of last year’ can be taken as the first step in this new direction.⁴ He began by drawing attention to how little his contemporaries knew about the depths of the earth and reminding readers that no man had ever gone deeper than one-six-thousandth part of the distance to the centre of the earth. He then stated that surface phenomena such as earthquakes will one day shed light on the planet’s internal structure.

      Likewise, Buffon’s two books quoted previously discussed the earth’s interior, drawing on recent geological discoveries to develop his theory. Following Descartes and Leibniz, particularly the latter’s Theodicy, he hypothesized that the globe began as a molten mass, seeing it as akin to a cooler version of the sun. Like Kant, he underlined how much he and his contemporaries did not know, writing in 1749, ‘There are many parts of the surface of the globe with which we are entirely unacquainted, and have but partial ideas of the bottom of the sea, which in many places we have not been able to fathom.’ Men had only been able to explore ‘the mantle of the earth’ since ‘the greatest caverns and the deepest mines do not descend above the eight thousandth part of its diameter’.⁵

Buffon did, however, consider that the earth was a solid, homogeneous globe, not hollow or empty. He theorized that the planet was initially a molten mass that had gradually solidified (see above, chapter 2) and filled with a vitreous material of roughly equal density to sand. He rejected the hypothesis that the core was still on fire. The thinking of later generations was led by geological and physical observations, and the question of the earth’s inner structure was set aside for a while. Late-eighteenth-century scholars seemed tacitly to agree that the earth’s interior was unknowable, focusing their attention instead on visible phenomena such as geological strata, mountains and volcanoes.

      For a period of nearly eighty years, from 1750 to 1830, the main focus of early geologists was developing mineralogy to further their knowledge of the nature of rocks and observing stratigraphy and mineral fossils to identify the successive geological eras that left their mark on the surface of the globe. At this point, it is useful to take a brief look back at a scholar whose work proved far-sighted, though it received little notice in his lifetime. Nicolaus Steno was the founding father of stratigraphy and tectonics and identified the organic origin of fossils, realizing that they were the remains of species now extinct. Decades ahead of his time, he pointed out that strata, or layers of sediment, built up as a result of successive deposits in a liquid environment. He hypothesized that the order of deposits must reflect the order of episodes of global history and that where strata were no longer horizontal, it must mean the earth’s crust had moved vertically at some point. Steno founded modern geology a century before other thinkers caught up, but his writings drew little attention and his ideas had little impact on the history of knowledge – and of ignorance.

As shown in chapter 2, one of the two vital issues in filling in the scientific map was the full acceptance of extremely longue durée geological timescales in the 1760s and 1770s. The observation of strata now meant that geology became a historical source. This new understanding of the geological past challenged the views of Flood geologists; in the nineteenth century, the argument pitted catastrophists against uniformitarians (see below, chapter 13).

      Part I of my book will focus solely on the rise of geology through ever more detailed studies of the earth’s surface and visible morphology, collecting evidence for the ‘constitution and conditions of the depths’.⁶ The debate on representations of the earth in the closing years of the eighteenth century and first half of the nineteenth turned on two main questions. The first was whether tilted strata of sedimentary rocks arose from the way the deposits formed as they were laid down, or whether they bore the traces of fractures from vertical and horizontal tectonic movements, as argued by those who had studied mountain ranges closely. The second, much more heated debate pitted the Neptunists, led by Abraham Gottlob Werner, against the Plutonists, who followed James Hutton. The two scholars studied rocks with a view to ‘identifying, differentiating, and giving precise descriptions of the various minerals and rocks that make up the Earth’s surface’.⁷ Their ambition was to ‘describe the visible surface of the Earth as objectively as possible and to identify its principal components’.⁸

Werner’s and Hutton’s theories – and their followers – were radically opposed. Werner’s Neptunism, which dominated the closing quarter of the eighteenth century, posited that water caused sedimentary deposits. All minerals and rocks, including, most polemically, basalt, were initially ‘in a dissolved state at the bottom of the primeval ocean’. Werner, somewhat influenced by diluvial theory, argued that the various strata were deposited in turn as they filtered out of the water. At the end of the eighteenth century, Werner still believed that water played a fundamental role in creating the earth. Vincent Deparis rightly points out that the theory did at least encourage detailed examination of the various layers of sediment.

      In 1778, however, Déodat de Dolomieu concluded from his study of basaltic lava that it could only come from the interior of the earth as a viscous flow, contradicting Werner’s Neptunism. This led to a lively scholarly argument. In 1785, James Hutton joined in the attack on Neptunism, making the case that the earth’s inner core was still burning. The heat rising from the subterranean regions was what forced rocks upwards as the cycle of sedimentation continued. This Plutonist theory accounted better than Werner’s Neptunism for the very long timescales of the geological past, which many scholars now believed in. Plutonism also had the advantage of accounting for something that was obvious to even the most casual observer: the interior of the earth was still full of fiery matter.

      Neptunism dominated scholarly debate until around 1795. Vincent Deparis records a swift change of opinion between 1802 and 1804, at the very end of the period, when studies of the volcanoes of the Auvergne swayed the debate towards Plutonism.

      How, then, did early-nineteenth-century Europeans building on Enlightenment science picture the earth, understand its surface and its reliefs and imagine its past? It is difficult to give hard and fast answers, but we can put forward some hypotheses.

It seems likely that most of the population, spending their lives within a very small geographical scope, wasted little or no time worrying about such matters. Their lack of knowledge about far-flung regions – and for most people, even ones closer to home – meant that towns, plains, hills and even the mountains that closed off their horizons were simply features of the earth, as mentioned by the priest in the pulpit preaching from the Book of Genesis. That would certainly have been enough to satisfy the curiosity of those few who did wonder about the past and the shape of the earth, which was to be admired as God’s creation. The Flood was generally accepted as historical fact and some might have thought that it was bound to have shaped the planet as they saw it. Almanacs, which were practically the only reading matter available for many people, barely touched on such questions.

      Yet the stratification of ignorance was already beginning to grow more complex: amateurs with a passion for science were starting to gather in academies in the provinces, Paris and London, and reading accounts of