Dated October 10, 1870, this is a letter from Clarence King, leading the Geological Exploration of the 40th Parallel to Major General A.A. Humphreys, the Army Chief of Engineers. The exploration, also known as the King Survey, began in 1867 with the purpose to examine the geology, topography and natural resources of the territory around the 40th parallel between the Rocky Mountains and the Sierra Nevada. King, a twenty-five-year-old geologist at the start of the survey, would go on to become the founding Director of the U.S. Geological Survey in 1879.