An 18th century map that changed science

Hello readers! It’s time for some more historical science. This week, we’ll explore how the creation of a map in 18th century France changed the way that scientists viewed the world. The map covered the Auvergne region, which contains the Chaîne des Puys – a line of conical hills that are now known to have formed through volcanic activity. However, before this map was published, the origin of these hills was a mystery. The truth was pieced together by a small team of scientists and engineers, and their findings changed the scientific landscape – although perhaps not to the extent that they deserved.

The aim of this post is to consider historical scientific advances within the broader context of the scientific method. You might be wondering why we’re discussing science facts on a science fiction blog – so let me explain my working. I think that in order to write believable science fiction, you need to understand the scientific process, and the necessary steps in acquiring knowledge. If you’re writing stories set in the far future, you need to consider how your fictional civilisation reached that level of development, and imagine the observations and breakthroughs that would be required. I reckon that the best place to find inspiration for these ideas is by looking to the past. By considering the way we view 18th century scientists from the 21st century, you can imagine how 21st century scientists might be viewed from the 24th century, and so on. With that in mind, let’s put ourselves in the shoes of some 18th century French cartographers!

The map in question…

The map is most commonly titled Desmarest’s Map of the Auvergne. It is attributed to Nicolas Desmarest, a man with many jobs and talents, including cartographer and geologist. However, numerous people were involved with creating the map, including two illustrators, three engineers, and probably an unknown number of local workers – all of whom are uncredited. As such, although I will refer to it as Desmarest’s Map, it should be remembered that he created it with considerable help.

The most important features on the map are the basaltic rocks, which are identified as discrete units of different ages, and can all be traced back to individual conical mountains in the Chaîne des Puys. The distinction and extents of rock units isn’t immediately obvious to someone merely looking across the landscape, admiring the view, so this map was essential in establishing the underlying patterns. Once thousands of observations had been compiled and displayed, the truth was revealed: the basalt rocks of the Auvergne had once been flows of molten lava, emitted from conical mountains that had once been active volcanoes.

Introducing the Chaîne des Puys

The Chaîne des Puys is a 40-km-long chain of mountains, aligned north-south through southern France. Some of its most famous peaks include the Puy de Dôme, which has the greatest prominence at 485 m, and the neighbouring Puy de Pariou, which is depicted on every bottle of Volvic mineral water. The chain contains over 80 volcanic features in total, including 48 cinder cones, 15 explosion craters known as maars, and 8 lava domes. These features, known as puys, form steep-sided hills which are 100-300 m high relative to the ground around them.

Each of the puys formed in separate eruptive events. These eruptions ejected basaltic lava, creating conical hills as millions of airborne chunks landed in a heap around the vent. The eruptions also released lava flows, which trailed into the neighbouring valleys and solidified into layers of basalt. Each eruption created a new cone, and once the eruption ended, the magma pathway sealed itself – which means that none of the puys will ever erupt again. However, the region is still volcanically active, so if another eruption did occur, it would form a new cone in amongst the others. Eruptions started around 95,000 years ago, and the youngest feature is thought to be only 8,000 years old (although the precise age is debated).

But why do we have volcanoes in the Auvergne? The region is nowhere near a tectonic plate boundary – and yet tectonic plate movements are still to blame. The gradual collision of the African and Eurasian continents, which led to the formation of the Alps 35 million years ago, also created a huge region of fracturing and deformation across Europe. This deformation caused thinning of the Earth’s crust, allowing magma to make its way to the surface.

But none of this was known in 1700.

In the 18th century, it was widely believed that basalt (and other igneous rocks) formed from water, rather than magma. This “Neptunist” way of thinking, which we’ve discussed on the blog before, came from Abraham Werner (1749-1817), who was head of the Frieberg mining academy in Germany. Desmarest’s map, which demonstrated a clear link between basalts and volcanoes, would become a crucial piece of evidence in debunking this idea.

The link between basalt and volcanoes might seem obvious to us today. Indeed, it was already obvious to 18th century scientists in Italy (or to anyone who had visited a volcano). However, in northern Europe, Werner’s watery hypothesis was taught as fact, and the works of Italian scientists such as Anton Lazzaro Moro (1687-1764), who advocated for the volcanic formation of igneous rocks, were largely ignored.

As for the Chaîne des Puys, French academics believed the conical mounds to be spoil heaps from giant Roman forges. This idea seems crazy to us today – and ironically, it probably would have seemed crazy to the Romans, who presumably recognised the volcanic cones for what they were (and deemed them to be much more friendly than their own Mount Vesuvius). The first suggestion (or re-suggestion) that the puys were volcanoes came in 1751, from a man named Jean-Étienne Guettard (1715-1786). However, he didn’t make much more of this hypothesis, and didn’t even mention the basalt – so Desmarest took the idea and ran with it.

Who was Desmarest?

Nicolas Desmarest was born in 1725, in Soulaines. Wikipedia states that he had “humble parentage” – but his family were clearly wealthy enough to send him to school. We can only assume that he was academically gifted, as he managed to find employment as a teacher in order to continue his studies. In 1753, he won a prize for an essay detailing the ancient land bridge that once connected England and France. However, he wasn’t wealthy enough to pursue scientific research full-time. Instead, he got a job as an inspector of manufacturing (and I’ll be honest: I’m not quite sure what this job involved). Being a manufactures inspector gave him opportunities to travel, and to make connections with the rich and influential.

Desmarest was keen to join the Royal Academy of Sciences, and he realised that this map was the key to being elected. He knew that the Auvergne region was under-studied, even after Guettard had pointed to its volcanic history, so he seized his chance. He conducted his first reconnaissance mapping work in 1763, alongside his full-time job as an inspector.

The map-making commences

Desmarest secured funds from French government to create his map, and this allowed him to hire additional researchers. He employed three cartographic engineers: Pasumot, Dailley and Pezet. Even with the additional help, it took several years to finish the map, and the work was beset with misfortunes and delays. The main obstacle was that mapping could only be conducted in the summer, when the weather was benign enough to permit observations and note-taking. Another obstacle was the fact that the researchers had to juggle other work commitments: Desmarest was employed as an inspector, and Pasumot was a professor of physics at the college in Auxerre.

One of the first difficulties faced by Desmarest was to determine the size of the mapping area. There were no good maps of France at the time, and to make matters worse, measurement systems varied. A “league” in Auvergne was one third longer than a “league” in Paris. In the end, he decided to focus on a region that, in modern terms, was 40×40 km.

Mapping methods and logistics

Mapping in the 18th century was a difficult and demanding task. Each member of the team worked in isolation in order to cover more ground, only meeting periodically to establish the connections between their separate regions. They were out in the countryside most of the time, and were only expected to return to towns once per week, in order to send letters with updates on their progress.

As for the methods they used, very little information remains. They made use of the best technology available at the time, including a graphometer with telescopic sights, and a portable barometer. However, no notes survive to provide any insight into their calculations of graphical techniques. Somehow, they made their maps at twice the scale of the Carte de l’Observatoire – the most comprehensive map of France at the time.

Misfortunes and delays

Pasumot appears to have done the bulk of the mapping work. He started in June 1764, and was only joined by Desmarest in late August, with the pair finishing work in October. The following year, Pasumot couldn’t start until August, and Desmarest couldn’t join at all, as he was away in Italy. Instead, he enlisted two more cartographers, Dailley and Pezet, who worked with Pasumot until the end of October. Unfortunately, Pezet fell ill with a fever in September and had to drop out – returning home and eventually dying of his illness. He never managed to plot his observations, so his area had to be started from scratch the following summer. 1766 was the final year of mapping, with Pasumot and Dailley starting in August, and Desmarest joining in September, before finishing in October.

Other people who helped with the mapping effort included a young assistant called Dupain, primarily employed as an illustrator. However, his efforts were deemed to be so poor that they were eventually redone by a better artist called Boissieu. Again, it’s worth noting that these people are never usually credited with constructing the map: I had to do quite a bit of digging to discover their names.

Pasumot’s contribution

It is clear that Pasumot did the bulk of the mapping work. Indeed, if we tally up the time spent in the field, Pasumot clocked in a staggering 11 months, while Desmarest managed a respectable 5 (including his reconnaissance work). These days, Pasumot would be listed as a key contributor, probably as a co-author – and yet the map is usually attributed solely to Desmarest. This is because Pasumot is generally regarded as being little more than a cartographic engineer, rather than a geologist, and the volcanic interpretations are regarded as being solely the work of Desmarest.

However, there has been some suggestion in recent times that Pasumot was more than a mere cartographer. Desmarest’s absence from the mapping work has actually proved useful to historians, as it necessitated written correspondence with his researchers – and many of these letters have survived. The letters reveal much about Pasumot’s and Desmarest’s professional relationship, and about Pasumot’s contributions.

Despite being a cartographer by training, Pasumot showed a deep interest in geology. His letters reveal that he was keen to propose and share ideas with Desmarest, and although he never challenged Desmarest’s authority when it came to knowledge of extinct volcanoes, he often provided recommendations for naming features. From a modern perspective, their relationship appears to be one of a student collaborating with his supervisor, rather than a contractor reporting to his boss.

(As an aside, unrelated to the mapping work: Pasumot and Desmarest would eventually go on to have a serious falling-out following a plagiarism dispute in the late 1770s. This tale did not end happily ever after.)

The finished map

Desmarest started to present his findings before the map was finalised. In 1765, while mapping was still in progress, he presented his early interpretations to the Academy of Sciences, and proposed that basalt prisms formed in solidifying lava which had poured from the Chaîne des Puys. Then, in 1768, once data collection had concluded and map consolidation was underway, he contributed to the “Mineralogie” section of the French “Encylopédie” – which was a commendable (and very French) attempt to democratise and secularise scientific knowledge, and educate the masses. Desmarest wrote the captions for images of basalt formations in the Auvergne, which had been done by Boissieu during the mapping project. These captions were essentially abbreviated versions of his mapping report, detailing the volcanic origin of the region’s basalts.

In 1771, Desmarest presented the finished map to the Academy of Sciences. It contained the craters of volcanoes and the extents of basalt lava flows; however, it wasn’t a geological map per se, as it was dominated by landscape features, rather than by the bedrock beneath. Besides the headline discovery that basalt was volcanic, Desmarest also made some other revolutionary interpretations. Firstly, he distinguished older flows from newer ones, giving them different symbols. He also divided the region’s history into three epochs based on the rates of lava production vs. erosion – although he did not attempt to estimate the duration of these epochs, or when they occurred. The academy officially published the map in 1774, over a decade after the mapping project began.

Reception

Desmarest had predicted that this map would provide him a way into the Academy of Sciences, and he was right. He was elected in 1771, having presented his findings. Then, in 1775, the year after the map was officially published, he released a companion paper, detailing the volcanic origins of basalts, and the three epochs of volcanic activity. However, this paper didn’t have much impact at the time. Maybe it could have benefitted from a snappier title, because “Mémoire sur l’origine & la nature du basalte à grandes colonnes polygones, determinées par l’Histoire Naturelle de cette pierre, observée en Auvergne” doesn’t exactly roll off the tongue.

The Royal Academy held its 1775 autumn public assembly in the Palais du Louvre, and Desmarest was one of five invited speakers. However, he was the only speaker not to have his work included in the academy’s memoirs for that year. This exclusion was potentially due to the influence of Georges-Louis Leclerc, the Comte de Buffon, who was publishing a series of volumes on natural history that conflicted with Desmarest’s map. Buffon had written a volume called “Epochs of Nature”, which described entirely hypothetical and unevidenced mechanisms for the formation of the Earth. Due to Buffon’s immense wealth and influence, his work drew much more attention than Desmarest’s small-scale, evidence-based study – and some historians suggest that Buffon used his position as academy treasurer to block the inclusion of Desmarest’s work in the 1775 memoirs.

Outside Paris, the map received a mixed reception – if it was received at all. Werner outright denied Desmarest’s findings, and kept teaching his students that igneous rocks had formed as chemical precipitates in ancient oceans. Thanks to Werner’s denial, the argument between “neptunists” and “plutonists” continued for decades after Desmarest’s map was published. Most northern European geologists refused to believe that the rocks beneath their feet could have a volcanic origin.

Desmarest died in 1815, at a time when many geologists still believed Werner’s water theory. In the UK, understanding had started to shift following James Hutton’s publication of Theory of the Earth (1785), in which he advocated for magmatic origins of igneous rocks. However, it was not until Charles Lyell published Principles of Geology in 1830 (with extensive descriptions of Italian volcanoes), that the volcanic origins of basalt became widely accepted. Depressingly, while Desmarest was alive, his work appears to have been largely ignored.

What does this tell us about scientific enquiry?

The first thing we can learn from Desmarest’s map is that ignorance is eternal and often undetected. To modern day readers, the Chaîne des Puys is obviously volcanic. The puys look like volcanoes, and they are surrounded by rocks that look like solidified lava. However, we only see these facts as obvious because we have seen active volcanoes – either in person, thanks to modern day transport systems, or in photos, thanks to modern-day cameras and communication. If you were an 18th century student at the Frieberg mining academy, looking at a layer of basaltic rock in the peaceful German countryside, would it be easier to believe that it came from a volcano, or from water? You probably wouldn’t even consider a volcanic origin to begin with. Why would you?

Desmarest’s map should have changed minds across northern Europe, alerting geologists to the volcanic origin of basalts. But Desmarest was not even the first to be ignored: Italian scientists such as Anton Lazzaro Moro had described volcanic processes many years earlier. In fact, his works had been translated into German, so it wasn’t as if his message hadn’t been delivered – it just hadn’t been received. It remained incomprehensible to northern Europeans that their volcano-free landscapes could be made of volcano-formed rocks, and their stubbornness was so severe that they denied the informed observations of their southern European counterparts.

It’s easy to look back and call Werner’s supporters blinkered or arrogant (or downright stupid), but we must remember that their framework for understanding the universe was utterly different from ours. They thought the world was only several thousand years old. They had no idea that continents could move. Why would they possibly imagine that their peaceful green countryside had once been the site of an erupting volcano? To them, that would have seemed much less likely than an ancient ocean once flooding the land.

So, what can we learn from this? Firstly, that observations and evidence are the only way to advance our understanding of the world. And secondly, that observations and evidence are entirely useless if we lack the mental capacity or analytical frameworks to interpret them. Collecting data such as maps is of the utmost importance, and storing this data is crucial. We may not be able to interpret collected data with our current level of understanding – but our descendants may still be able to make use of it, when the time comes. And then they will be able to look back at us, and wonder how we could all have been so blind when the solution was so obvious.

In summary…

I hope you enjoyed reading about this lesser-known map, its lesser-known author, and its even lesser-known contributors. Some poor cartographer died in the making of this map, and its findings were still ignored! All because some influential German geologist lacked the means (or the desire) to do some travelling beyond Germany… It’s incredible to think that scientific advancement can be held back by the obstinacy of a single, influential person – or by the voice of a single, informed person going unheard. I’d like to think that the modern scientific community has greatly improved since the 18th century, but unfortunately, much of this mapping tale struck me as familiar.

Happy reading, and have a lovely week!

Some references for those interested:

(During my research into this post, I realised that most papers about Desmarest’s life and work are by a single academic. I hope I have done my bit to make Desmarest’s fanbase a little bit bigger…)

Taylor, K.L., 1994. New light on geological mapping in Auvergne during the eighteenth century: The Pasumot-Desmarest collaboration. Revue d’histoire des sciences, pp.129-136.

Taylor, K.L., 2009. Desmarest’s “Determination of some epochs of nature through volcanic products” (1775/1779). Episodes, 32(2), pp.114-124.