A gypsum mine beneath Sacre Cœur, Montmartre.
This is a painting by Géricault of a Montmartre gypsum mine.
Paris lies within the Paris Basin, a geological bowl-like shape created by millennia of sea immersion and erosion. Much of north-western France spent much of its geological history as a submerged sea water coastline, but towards our era, and the formation of our continents as we know them today, the then relatively flat area that would become the Paris region became increasingly elevated, alternately invaded and sculpted by both sea water, inland sea water “lagoons” and fresh water, in addition to above-water air and river erosion. This cycle resulted a rich and varied geological strata containing many minerals that would become a source of growth and wealth for the Paris region.
Paris has spent most of its geologic history under water, which is why it has such varied and important accumulations of sedimentary minerals.
The Paris area was a relatively flat sea-bottom In the early Cretaceous period: first in a deep-sea environment, then under a more agitated near-shoreline sea towards the end of the same period, Paris’ largely silicium-based sedimentary deposits became, under the action of pressure and the carbonic acid content of seawater, a thick deposit of clay. The invasion of calcium-rich seas then covered this with an even more important layer of chalk. Paris emerged from the sea towards the end of the Cretaceous, and Palaeozoic-era continental shifts, particularly the Variscan orogeny geological upheavals, created a series of hills and valleys throughout the Parisian basin, creating conditions ideal to the mineral deposits that would appear over the next eras.
After a long period above sea level that ended towards the Cenezoic era, Paris began a period of alternation between sea and land environments. Paris was the middle of a shoreline of bays and “lagoons” of still seawater, an environment perfect for the silica-based sea life abundant then. As sea creatures died and settled to the lagoon bottom, their shells would mix with the deposits already present; pressure from additional sea-life sedimentation and the chemical action of the water would transform the result into a sedimentary stone quite particular to the Paris area, “calcaire grossier” (calcaire lutécien in more modern publications). Paris’ most important deposits of this stone occurred during the Eocene epoch’s Lutetian age; in fact, the age itself is named for the sedimentary activity in the Paris region, as Lutèce was the city’s name in Roman times.
Paris’ next important mineral deposit came with the Bartonian age. After a period of land-sea altercation that brought layers of sand and low-quality calcaire grossier, the sea regressed again to return only occasionally to re-fill lagoons with seawater. The result was stagnating pools evaporating seawater: the salts of these, mixed with other organic matter and mineral deposits, crystallised into the calcium sulphate composition that is gypsum. This evaporation cycle occurred several times over this age, creating several layers of gypsum divided by layers of mineral left by the sea’s brief return. In all, Paris’ gypsum deposits are divided into four “masses”, with the last appearing, the “haute masse”, being the most important and most exploited in Parisian history. Gypsum, an “evaporite” mineral, is known for its fragility against freshwater invasion.
The sea returned one last time to the Parisian basin towards the end of the Paleogene period, leaving several layers of varied sediment capped with a thick layer of clay. The latter deposit was important for Paris’ gypsum deposits when the Paris basin rose, this time definitely, from the sea in the early Neogene, as they would protect them from erosion from air and the elements.
This is when Paris began to take the form we know today: huge rivers resulting from the melting of successive ice ages cut through millions of years of sediment, leaving only formations too high or too resistant to river erosion. Paris’ hills of Montmartre and Belleville are the only places where gypsum remained, as the ancestor to today’s Seine once ran as wide, almost along its present path, as half the city, with many arms and tributaries.
In the 18th and 19th centuries, there were a number of gypsum mines in Montmartre. See Mines of Paris. A fossil tooth found in one of these was identified by Georges Cuvier as an extinct equine, which he dubbed Palaeotherium, the “ancient animal”. His sketch of the entire animal in 1825 was matched by a skeleton discovered later.
Principle of the conditions of existence
For Cuvier, the principle of the correlation of parts was theoretically justified by a further principle, that of the conditions d’existence, usually translated as “conditions of existence.” This was his way of understanding function in a non-evolutionary context, without invoking a divine creator. In the same 1798 paper he wrote:
if an animal’s teeth are such as they must be, in order for it to nourish itself with flesh, we can be sure without further examination that the whole system of its digestive organs is appropriate for that kind of food, and that its whole skeleton and locomotive organs, and even its sense organs, are arranged in such a way as to make it skillful at pursuing and catching its prey. For these relations are the necessary conditions of existence of the animal; if things were not so, it would not be able to subsist.