HYPSOMETRIC ADDENDA. 281 



gos Lake of my map not having been placed by me suffi- 

 ciently far to the north and west, is to be attributed to the 

 entire want, at that time, of any astronomical determina- 

 tions of the position of Santa Fe, in New Mexico. The 

 error amounts, for the western margin of the lake, to almost 

 50 minutes of arc ; a difference of absolute longitude which 

 will appear less surprising, if it is remembered that my 

 itinerary map of Guanaxuato could only be based for 1 5 

 degrees of latitude on compass surveys, or compass direc- 

 tions, for which I was indebted to Don Pedro de Rivera. 

 (Humboldt, Essai polit. sur la Nouvelle Espagne, T. i. pp. 

 127-136.) These directions being differently combined 

 by my early deceased fellow-labourer, Herr Friesen, and 

 myself, gave him as the result of his combinations 10 7 5 8 

 from Paris as the longitude of Santa Fe, and to me as the 

 result of mine, 107 13'. According to actual astronomical 

 determinations since obtained, the true longitude appears to 

 be 108 22' W. of Paris, or 106 00' W. of Greenwich. 

 The relative position of the beds of fossil salt found in 

 " thick strata of red clay/' on the south east of the island - 

 studded Great Salt Lake (my Laguna de Timpanogos), and 

 not far from the present Fort Mormon and the Utah Lake 

 was given with perfect correctness in my large map of Mexico. 

 I may refer on this point to the latest evidence of the tra- 

 veller who made the first well-assured determinations of 

 geographical position in that district : " 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 



