Turner: The Frontier In American History, Frederick Jackson Turner, The Significance of the Frontier in American History, 1920, ISBN 0-486-29167-7, Ch.1: "In a recent bulletin of the Superintendent of the Census for 1890 appear these significant words: "Up to and including 1880 the country had a frontier of settlement, but at present the unsettled area has been so broken into by isolated bodies of settlement that there can hardly be said to be a frontier line. In the discussion of its extent, its westward movement, etc., it can not, therefore, any longer have a place in the census reports." On-line version of the book
