In the Fall of 1690, following a parley with a delegation of Miamis at the St François-Xavier mission at Green Bay, Nicolas was asked to set up a post on the Mississippi, below the Wisconsin river, for the purpose of fur trading. In addition, the Miami chief gave him a mineral sample, from a lead mine in this region.
Nicolas promised to set up a post "within 20 days below the river Douiskouche" [d'Ouiskouche, Ouiscousing]; «...it's very likely «below» the Wisconsin river, as recorded by Bacquevielle de la Potherie, no doubt quite near the lead mine on the outskirts of today's city of Dubuque, that he had another fort erected.»(1)
Notwithstanding the fact that no trace has survived of this post of Nicolas, the locals believe it to have been situated on the eastern shore, more specifically, within Gramercy park, in the city of East Dubuque, Illinois. The metropolitan region of Dubuque is a tri-state area: Iowa, on the western shore of the Mississippi, and Wisconsin and Illinois, on the eastern shore.
It is doubtful that Nicolas had a permanent post, as he still had to travel continually up and down the river to settle disputes which erupted regularly between Miami, Fox, Sioux, and others.
It is believed that the first mines visited by Nicolas were those of Catfish Creek, a little to the southwest of Dubuque, and of Galena, about 24 km (15 miles) to the southeast, in Illinois, on the opposite shore. This was the start of mining operations by the French, an undertaking which would pick up steam around 1788 with the arrival of Julien Dubuque (1762, St Pierre-les-Becquets - 1810) who would leave his name to this Iowa city.
Louis La Porte, sieur de Louvigny, commander of Michillimackinac at this time (1690-1694), would note the discovery on his personal map of the Mississippi : "il [Perrot] decouvre des mines de plomb tres abondantes dont on en a fait l'espreuve" ["he discovers very abundant lead mines which we have verified"]. [Service historique de la Défense, département Marine, Vincennes, France]
Guillaume Delisle, the king's cartographer, would use this information in his 1703 map. «While passing through in France to settle some family matters, ... Louvigny, ... gave him, in 1697, a map made "by himself, based on his own knowledge and the stories of many Frenchmen".»(2)
«In 1699, [Pierre Lesueur] had been commissioned by the king to explore and work "the mines at the source of the Mississippi," and had thirty miners assigned to him. His reporter and companion, Pénicaut, ...says "We found both on the right and left bank the lead mines, called to this day the mines of Nicolas Perrot, the name of the discoverer."
It was the 13th of August, 1700, when they arrived opposite Fever River [now the Galena river], which Pénicaut calls "Rivière à la Mine". He reports that up this little river, a league and a half [about 4.5 miles, 7.2 km], "there was a lead mine in the prairie". They passed up the Mississippi, Pénicaut mentioning two streams which correspond to the Platte and Grant rivers, in Wisconsin, and says that Lesueur "took notice of a lead mine at which he supplied himself" - supposed to be what came to be known as "Snake Diggings", near Potosi, Wisconsin.»(3)
The Potosi mine
You can visit this "Perrot mine", which is today called St John's Mine, in Potosi, Wisconsin, about 24 km (15 miles) north of Dubuque. In spite of its name, the mine was originally a natural cavern. But there were rich veins of lead ore in this cavern. The story goes that Nicolas, after visiting it, baptised it "Snake Cave" and the valley where it was located was named Snake Hollow.
Between 1822 and 1829, many treaties were signed with the Indians and the "Lead Rush" began. One of the pioneers, Willis St. John, appropriated the mine to himself, whence its name today. In 1839, the inhabitants of the local villages of Van Buren and Lafayette, voted to amalgamate with Snake Hollow to become Potosi [after Potosi in Bolivia ?]. Mine operations ceased definitively in the 1870s. In 1969, the mine was opened as a tourist attraction. You can visit from May to October, every day except Wednesdays.