But with all these similarities there are essential differences, due to the place element and the time element. It is evident that the farming frontier of the Mississippi Valley presents different conditions from the mining frontier of the Rocky Mountains. The frontier reached by the Pacific Railroad, surveyed into rectangles, guarded by the United States Army, and recruited by the daily immigrant ship, moves forward at a swifter pace and in a different way than the frontier reached by the birch canoe or the pack horse. The geologist traces patiently the shores of ancient seas, maps their areas, and compares the older and the newer. It would be a work worth the historian’s labors to mark these various frontiers and in detail compare one with another. Not only would there result a more adequate conception of American development and characteristics, but invaluable additions would be made to the history of society.