U.S. Census Office, The Progress of the Nation 1860 (1874)

These maps were featured in the U.S. Census Offices's Statistical Atlas of the United States Based on the Results of the Ninth Census, 1870. This Statistical Atlas, the first of several, was published in 1874 under the supervision of Superintendent Francis Walker, who wanted to mark the upcoming centennial of the United States by illustrating its progress over the last century, including the expansion of settlement. His statisticians did so by plotting the westward encroachment of population density data from each census on a series of maps over eight decades. This map depicts population data from 1860. It can be compared with data maps from 1790-1820, 1830-1840, and 1850, also featured on this site. The grayscale on the maps uses darker colors to indicate higher population densities, with the darkest shade indicating 90 or more inhabitants per square mile and the lightest gray indicating 2 to 5 inhabitants per square mile. The white indicates fewer than 2 inhabitants per square mile. In an accompanying section of the Statistical Atlas titled "The Progress of the Nation: 1790-1870," Walker famously discusses the  population line separating the area of European-American settlement (colored in different shades of gray) and the frontier (shown in white), which would become known as the "frontier line." Of the frontier line in 1860, Walker writes, "At the mouth of the Sioux River, it takes the first step of its great forward movement towards 'The Plains,' and, crossing the Missouri here, it annexes to the body of continuous settlement fully 20,000 square miles in Nebraska and Kansas. Respecting the faith of the Government pledged to the Indian tribes, it still confines itself to the western boundary of Arkansas, and then, running out along the Red River across nearly four degrees of longitude, it stretches with a bold, free course across the vast plains of Texas, almost touching in its furthermost sweep the 100th meridian" (p. 2). Walker calculated that the total settled area of the United States in 1860 had grown to 1,194,754 square miles, with an aggregate population (excluding Native Americans) of 31,443,321, and an average density of settlement of 26.5 inhabitants to the square mile.