In the 18th century the geological significance of volcanoes was (literally) a hot topic for naturalists - many considered volcanoes only as a local phenomenon, the visible fire feed by underground sulfur veins and the rocks found around them being the ashes of this combustion. Some naturalists considered volcanoes as natural valves of a large underground reservoir of molten rocks, once on the surface the molten mass solidified forming deposits of igneous - "fire-born" - rocks on a large scale, reshaping entire landscapes.
Jean-Ètienne Guettard (1715-1786) was a pharmacist who got interested in geology studying the distribution of medical plants, as he noted that certain plants are restricted to areas where outcrops with certain rocks occur. In 1751 he visited the city of Vichy in the French region of Auvergne, where he noted that a black rock, quarried in the region, was widely used as building material. He recognized the rock of igneous origin, but if this rock was here to be found, where were the volcanoes from which it once poured out?
Fig.1. Basalt columns used as bricks in a stone wall, village of Murat.
Fig.2. Quarry with columnar basalt in situ, located near the village of Le Pont de Alleray.
In the city of Clermont-Ferrand he was accompanied by a local naturalist in a visit to the mountain Puy de Dome (1.465m), here the two men recognized that Le Puys, a chain of mountains stretching from north to south, were in fact a chain of fossil volcanoes, the strange depressions on almost every summit were the eroded remains of the ancient volcanic craters. In 1751 Guettard published his observations, trying to convince his fellow naturalists of the ancient igneous origin of the entire area of today's region of Auvergne - however skepticism prevailed.
In 1763 the naturalist Nicolas Desmarest (1725-1815) decided to test Guettard´s hypothesis by mapping the region. As the rocks where once molten when pouring out from a volcano, he should find "flows" and these should be connected to a volcanic crater. Many contemporary naturalists had rejected the volcanic origin of the Auvergne region as many outcrops of rocks didn´t seem to fit this premise. However Desmarest recognized that isolated areas of igneous rocks were in fact eroded remains of once larger lava covers - the rivers had eroded the now missing parts, but by carefully mapping of elevation and position the isolated cliffs could be fitted in a pattern of lava flows. Desmarest was even able to reconstruct a time sequence (time is an important factor in a geological map) of lava flows, arguing that younger flows were situated higher as older flows, as the younger lava overflow already solidified rock.
Fig.3. "Map of a part of Auvergne, representing the lava flows, where basalt is found in prismatic and round forms, for use in understanding the memoir by M. Desmarest on his basalt." By Nicolas Desmarest (1725-1815) and François Pasumot (1733-1804), published in 1774, image in public domain. Desmarest was the first person to connect the dark rocks - mostly basalt - and volcanoes.
Demarest's map poses the interesting question if it could be classified as a true geological map. It depicts outcrops, connects these outcrops with a rock unit (which at the time was still a novelty) and even features a time sequence. However it only considers igneous rocks, showing them in an oblique landscape view (not as a topographic map).
Most important is the idea behind this map - Demarest's figure was still more a diagram than a map, connecting the dots (or outcrops) to form a line, pointing to the true origin of the rocks, as products from an extinct volcano.
To be continued...
DESMAREST, N. (1771): Mémoire sur l'origine et la nature du basalte à grandes colonnes polygones, determinées par l'histoire naturelle de cette pierre, observée en Avergne In: Mémoires de l'Académie Royale des Sciences à Paris pour 1771.
LEWIS, T.A.(ed) (1985): Volcano (Planet Earth). Time-Life Books: 176
MIALLIER, D. ; MICHON, L. ; EVIN, J. ; PILLEYRE, T. ; SANZELLE, S. & VERNET, G. (2004) : Volcans de la chaine des Puys (Massif central, France): point sur la chronologie Vasset-Kilian-Pariou-Chopine. C.R. Geoscience 336 : 1345-1353