1. J. R. V. Prescott, The Geography of Frontiers and Boundaries, London (1965), pp. 34–9.
2. P. E. James, Latin America, 3rd edition, New York (1959).
3. M. W. Mikesell, Comparative studies of frontier history, Annals Assoc. Am. Geogr., 50 (1960), 62–74.
4. R. Hartshorne, Suggestions as to the terminology of political boundaries, Annals Assoc. Am. Geogr., 26(1936), 56–7.
5. See ref. 3.