It is obvious that the population capable of furnishing the requisite labour for works of so extensive a nature must have been numerous, and its resources for the maintenance of such a phalanx of workers proportionally abundant. The garrisons of the great strongholds, and the population that found shelter within such mural defences as “Clark’s Work,” must also have been very large, requiring for their subsistence the contributions of an extensive district. But this only accords with other proofs of the condition of the Mound-Builders as a settled people. When we turn from the consideration of single large fortifications crowning the insulated heights, and estimate the number and extent of mounds, symmetrical enclosures, and works of various kinds connected with the arts of peace and the rites of religious worship, which give so striking a character to the river-valleys and terraces, it is no longer possible to doubt that many sections of this fertile region were once before filled by an industrious, settled population. The Sacred Enclosures have been separated from the military works of the Mound-Builders on very obvious grounds. Their elaborate fortifications occupy isolated heights specially adapted for defence; whereas the broad river-terraces have been selected for their religious works. There, on the great unbroken levels, they form groups of symmetrical enclosures, square, circular, elliptical, and octagonal, with long connecting avenues, suggesting comparisons with the British Avebury, or the Hebridean Callernish; with the Breton Carnac; or even with the temples and Sphinx-avenues of the Egyptian Karnak and Luxor.