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 16 © ABORIGINAL: MONUMENTS

Worxs or Drerence.—Those works, which are incon- testably defensive, usually occupy strong natural positions. To understand fully their character and capacity for the purpose assigned to them, it is necessary to notice briefly the predominant features of the country in which they oceur.

The valley of the Mississippi, from the base of the Alle- ghanies to the ranges of the Rocky Mountains, is a vast sedementary basin, and owes its general aspect to the powerful action of water. Its rivers have worn their valleys deep in a vast original plain, leaving in their gradual subsi- dence broad terraces, marking the different eras of their history. The edges of the table lands, bordering on the valleys, are cut by a thousand ravines, presenting bluff headlands and high hills with level summits, sometimes connected by narrow isthmuses with the original table, and sometimes entirely detached. The sides of these elevations are always steep and difficult of ascent in some cases precipitous and absolutely inaccessible. The natural strength of such positions, and their susceptibility of de- fence, would certainly suggest them as the citadels of a rude people, having hostile neighbors or pressed by foreign invaders. Accordingly, we are not surprised at often find- ing these heights occupied by strong and complicated works, the design of which is indicated no less by their position than by their peculiarities of construction. In such cases it is always to be observed that great care has been exercised in their selection, and that they possess peculiar strength and adaptation for the purposes to which they were applied. While rugged and steep on most sides, they have one or more points of comparatively easy approach, in the protection of which the utmost skill of the builders has been expended. They are guarded by double overlapping walls, or aseries of them, haying sometimes an accompanying mound, designed perhaps as a “look-out,” and corresponding to the barbican in the British system of