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Section of John Senex's map of the Upper Midwest, Upper Mississippi River and the regions drained by several mythical Rivers, based upon and earlier map by John Senex.

The map appeared in Stansbury's Exploration And Survey Of The Valley Of The Great Salt Lake of Utah, Including A Reconnaissance of a New Route Through The Rocky Mountains Senate Special Session, March 1851,

The map focuses on the discoveries of Baron Lahontan, based upon his travel narrative.

Lahontan's maps and most notably the map showing a western River extending to the Pacific, were among the most influential of all mythical cartographic works of North America, effecting the cartographic landscape of the upper Mississippi, Plains and Rocky Mountain Regions, for nearly 50 years.

Lahontan's maps were issued in Memoires de l'Amerique Septentrionale ou la Suite des Voyages ...., one of the most influential and fanciful works of its time. The map depicts the Longue flowing from the mountains in the west (Rocky Mountains), home to the Gnacsitares Indians, and connecting to the Mississippi River. On the western side of the mountains is another river, presumably flowing into the Pacific.

Lahontan's concept was copied by virtually all 18th-century cartographers, including Moll, Senex, Popple, and Delisle, thus perpetuating the myth.

John Senex Biography

John Senex (1678-1740) was one of the foremost mapmakers in England in the early eighteenth century. He was also a surveyor, globemaker, and geographer. As a young man, he was apprenticed to Robert Clavell, a bookseller. He worked with several mapmakers over the course of his career, including Jeremiah Seller and Charles Price. In 1728, Senex was elected as a Fellow of the Royal Society, a rarity for mapmakers. The Fellowship reflects his career-long association as engraver to the Society and publisher of maps by Edmund Halley, among other luminaries. He is best known for his English Atlas (1714), which remained in print until the 1760s. After his death in 1740 his widow, Mary, carried on the business until 1755. Thereafter, his stock was acquired by William Herbert and Robert Sayer (maps) and James Ferguson (globes).