and west, arose from the entire absence, at that period, of all astronomical determinations of position of Santa Fé in New Mexico. For the western margin of the lake the error amounts to almost fifty minutes, a difference of absolute longitude which will appear less striking when it is remembered that my itinerary map of Guanaxuato could only be based for an extent of 15° of latitude on determinations made by the compass (magnetic surveys), instituted by Don Pedro de Rivera.[IX] These determinations gave my talented and prematurely lost fellow-labourer, Herr Friesen, 105° 36′ as the longitude of Santa Fé, while, by other combinations, I calculated it at 104° 51′. According to actual astronomical determinations the true longitude appears to be 106°. The relative position of the strata of rock salt found in thick strata of red clay, south-east of the Great Salt Lake (Laguna de Timpanogos), with its many islands, and near the present Fort Mormon and the Utah Lake, is accurately given in my large map of Mexico. I may refer to the most recent evidence of the traveller who made the first trustworthy determinations of position in this region. “The mineral or rock salt, of which a specimen is placed in Congress Library, was found in the place marked by Humboldt in his map of New Spain (northern half), as derived from the journal of the Missionary Father Escalante, who attempted (1777) to penetrate the unknown country from Santa Fé of New Mexico to Monterey of the Pacific Ocean. South-east of the Lake Timpanogos is the chain of the Wha-satch Mountains; and in this, at the place where Humboldt has written _Montagnes de sel gemme_, this mineral is found.”[IY]