The origin of the custom of building mounds has been discussed by Cushing; he conceived the original mound to be a mblden of shells and other refuse accumulated under a shoreland pile dwelling to such height as eventually to form a support for the habitation; and that the asso ciation of mound and dwelling eventually be came so deeply fixed in the minds of the dwellers that when new habitations were erected further inland, the mound was regarded as a necessary aceinnpaniment, and was built of earth in lieu of refuse. During the earlier two-thirds of the Nine teenth Century the opinion prevailed that the "Mound Builders" were a distinct people or race, antedating the Amerind tribes found inhabiting the country by the Caucasian invaders; this was shown, chiefly by Powell and later by Thomas, to be an error. The latter described the earth works of the eastern United States in detail, and identified many of them with the aborigines re siding in their vicinity up to the time of white settlement. The demonstration of the identity of "Mound Builders" and "Indians" may be said to have been completed by Holmes, who in vari ous publications established the unity df msthetie, technie, and symbolic motives in the mounds and among the living tribesmen. The mound proper, with its variants in the form of embank ments. effigies, etc., may be regarded as pertain
ing to humid lands, and the shell-mounds to shorelands; while in arid lands the earth-work ing sometimes differentiated into a style of house-building known in parts of Spanish Amer ica as cajon (so called from the box-like arrange ment of parallel boards \ vhieh piddled earth was laid and allowed to harden in suc cessive ledges, or strata, varying from a few inches to a foot or more in vertical thickness) ; and this type of structure is widely diffused in the more arid regions of both American conti nents, the best example in the United States being the ruin known as Casa Grande (q.v.), near Florence, Ariz. _Modernly the rajon structure grades into adobe—i.e., sun-dried bricks of pid dled silt ; but there is sonic question whether the use of adobe proper in the vernacular) antedated the Caucasian invasion. From cajon to a plaster of earth and stone over wicker walls was an easy step, which was taken by many tribes, as attested by buried ruins of the arid region as well as by vestiges among living tribes, e.g., the Papago; and the step thence to wrought stucco was little harder, and was taken by the ancient Mexicans, Yuratecans, Central Ameri cans, and some South Americans, as well illus trated in several ruined cities (noted under ARCHITECTURE, ANCIENT AMERICAN )