the Bu- reau of the Census in 1890. then, thanks to population build-up on the Pacific coast, it was no longer possible to draw the traditional na- tional "frontier line" beyond which there were fewer than two people per square mile. Yet, many pockets of land where population density was below that level remained-and most have survived for a century. Today, about one-quarter (949,500 square miles) of the United States is still technically "frontier."
About half of this territory,...