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[edit] Paris' minerals - geological composition and formation
Paris lies within the Paris Basin, a geological bowl-like formation created by successive millennia of sedimentary deposits and erosion. Upon a thick and relatively level layer of chalk disturbed only to the city's south-west by an arm of the Variscan orogeny (an extension of "the Meudon bump"), Paris' exploited mineral deposits were, in order of deposit: a thick strata of clay, an equally major strata of limestone (15-20m), sand, a band of low-quality limestone, three to four successive bands of plaster (together totalling 40m in places) separated by thin deposits of marl. The whole is capped with another layer of clay, itself topped with sand then organic landfill at the surface.
(what era) erosion sculpted all of the above into a form still visible today: both sea and freshwater eroded most all of Paris' Left Bank water-soluble gypsum deposits away, leaving a surface of sand and limestone; protected from the surface by a waterproof clay cap, it was river erosion that cut the flanks Right Bank gypsum-rich hills of Montmartre and Belleville.
[edit] Mining techniques - the exploitation of the Parisian basin
The first minerals mined in the Paris basin were those visible on the surface: clay and sand in the banks of the strata-eroding river, then limestone where it could be found in the same. Plaster could be found in the flanks of the hills of Montmartre and Belleville, but these would not be exploited to any great extent before the beginning of the first millennium. Pit-mining was the next common step to expose new layers of mineral once it was exhausted on the surface. The technique of piercing vertical "sounding wells" from the surface down to the layer of desired mineral became a general practice only when the mineral accessible through other means was exhausted in the Paris area.
As stone deposits were abundant in the Paris area, most of Paris' buildings throughout its history were built with stone that came from practically under them. Mineral scavenging and rough pit-mining were replaced by more refined techniques with the Roman era. The Romans brought other techniques: Remains of what is thought to have been a Roman clay mine, formerly accessible through deep wells, were discovered twenty metres under Paris Left Bank Montaigne Sainte-Geneviève hill. Pit-mining techniques again became the norm with the decline of the Roman Empire, and would remain the standard until the early 11th century. Another reason for a decline of stone-mining in the Paris area was its population's move to the Right Bank from the early 10th century: rather than undertake the onerous task of finding new deposits and opening new mines, Parisians exploited the vast Left Bank ruins (of buildings levelled by 9th-century Viking raids) as an easily accessible source of building material.
Gypsum, the origin of the famous Plaster of Paris, was most often exploited through the flank of the hill that held it. Tunnelling would begin from where the mineral was visible on the surface, and continue along the strata; the created cavities would be consolidated as the mining progressed. Where the gypsum strata was relatively important, for example the haute masse deposits that reached a thickness of fourteen metres in places, mining would begin at the top of the strata, burrow in, and progress downwards until the bottom of the deposit. The technique used here was piliers et hagues: a first series of tunnels would be bisected by a series of parallel tunnels lateral to these, and the columns of un-mined mineral remaining would serve as of support for the excavation ceiling. The tunnels would become wider as the excavation progressed downward; an exhausted haute masse exploitation, because of its immense height and towering columns culminating in arches at the excavation ceiling, would often have the aspects of a cathedral. Only one such cavity exists in Paris today, renovated into a "grotto" under one of the hills of Paris' Buttes-Chaumont gardens.
[edit] Paris' growth over abandoned mines - a city over an abyss
There are no concrete proof of any mining activity before the late thirteenth century. The earliest text we can find is but a mention in the town commerce register: Paris had 18 "quarriers" in 1292. The first written act concerning any mine dates from almost a century later, 1373, in an authorisation that a certain Dame Perrenelle be permitted to operate the plaster mine already existing in her property to the lower flank of Montmartre.
The majority of Paris' stone deposits were in its Left Bank, and at the time of the city populace's 10th-century move to the Right Bank, well to the suburbs of the former Roman/Merovingian city. As the stone from the abandoned ruins became depleted from the 13th century, new mines began to open further from the city centre. Earlier mines closer to the city centre, when discovered, sometimes served a purpose: when Louis XI donated the former Chateau Vauvert, a property in an area today the northern part of today's Luxembourg Gardens, to the Chartreuse order in 1259, the monks renovated the caverns under their property into wine cellars, and continued the exploitation of stone to the ancient mine's extremities.
By the early 16th century, there were stone excavations operating around today's Jardin des Plantes, Boulevard St-Marcel, Val-de-Grâce hospital, southern Luxembourg (by then the Chartreuse Coventry) and in areas around the rue Vaugirard. Paris' then suburban plaster mines remained for the most part to the Right Bank Montmartre and Belleville hills.
It was only with its expansion past its 13th-century walls that the city began to build on previously-mined land. The left-bank faubourgs were the most at risk: n the 15th century, the largest were the faubourg Saint-Victor (from the eastern extremity of the rue des Écoles and south down the rue Geoffroy St Hilaire); the faubourg St Marcel (rue Descartes, rue Mouffetard) and the faubourg Saint-Jacques (along today’s rue Saint-Jacques below the rue Soufflot); lastly, the faubourg (then bourg) Saint-Germain-des-Prés below today’s church of the same name.
Although seventeenth-century Right Bank Paris had in five centuries expanded past three successive arcs of fortifications, Left Bank Paris was nowhere near as dense in comparison within its unchanged but crumbling 13th-century city walls. Many royal and ecclesiastical institutions came to the area during this period, but by then it seems that the mined state of the Paris faubourg underground had been forgotten by then: The Val de Grâce coventry and the Observatoire observatory, built from 1645 and 1672 respectively, were found to be undermined by immense caverns left by long-abandoned stone mines; reinforcing these took most of the budget consecrated to both projects.
Faubourg growth remained along the main routes from the city, but began to expand at a faster rate with the rise of traffic along the routes to the Fontainebleau and Versailles castles. The route de Fontainebleau (extending to the south of today's Denfert-Rochereau) would be the the site of one of Paris' first major mine collapses in 1774.
[edit] The consolidation period
The above disaster was in part responsible the Conseil du Roi's decision to create a special division of architects responsible for the inspection, reparation and maintenance of the ground under royal buildings within and without Paris. Another division of inspectors created around the same time, but under the direction of the Ministry of Finance, claimed the role of assuring the safety of the national roadways that were their jurisdiction. Created officially on the 24th of April 1777, the entered service on the same eve after a new collapse of the route de Fontainebleau (Avenue Denfert-Rochereau) outside of the barrière d'Enfer city gateway. Although the Ministry of Finance continued to claim jurisdiction over damaged roadways, this rather inept service was eventually succeeded by the Royal-appointed IGC.
As the centuries of mining under Paris' underground went mostly uncharted, thus largely forgotten, the real extent of former mines was unknown then. All important buildings and roadways were inspected then: any signs of shifting were noted, and the ground underneath sounded for cavities. Roadways were particularly problematic: instead of sounding the ground around the route, inspectors instead tunnelled directly under the length of endangered roadway, filling any cavities they found along the way, and reinforcing the walls of their tunnels with solid masonry to eliminate the possibility of any future excavations and disasters. When a length of roadway was consolidated, the date of the work was engraved in the tunnel wall under it, next to the name of the roadway above; today Paris' underground tunnels, dating to as early as 1777, are a living testimony to Paris' old street names and roadways.
