buildings, for the whole country is covered with tabular sandstone, often broken up into blocks and flakes ready for immediate use without any preparation whatever. Timber and brush could be procured only with difficulty, and often had to be carried great distances. It has been suggested that the rectangular form of rooms might have been developed from the circular form by the crowding together upon restricted sites of many circular chambers; but such a supposition seems unnecessary. A structure of masonry designed to be roofed would naturally be rectangular; in fact, the placing of a flat roof upon a circular chamber was a problem whose solution was beyond the ability of these people, as has already been shown. Along with this advance, or perhaps preceding it, the social organization of the tribe, or its division into clans and phratries, would manifest itself, and those who "belong together" would build together. This requirement was a very common one and was closely adhered to even a few years ago. Although degeneration in arts is common enough, a peculiar condition prevailed in the pueblo region. So far as the architecture was concerned war and a hostile human environment produced not degeneration but development. This came about partly by reason of the peculiarities of the country, and partly through the methods of war. The term war is rather a misnomer in this connection, as it does not express the idea. The result was not brought about by armed bodies of men animated by hostile intentions or bent on extermination, although forays of this kind are too common in later pueblo history, but rather by predatory bands, bent on robbery and not indisposed to incidental killing. The pueblos, with their fixed habitations and their stores of food, were the natural prey of such bands, and they suffered, just as did, at a later period, the Mexican settlements on the Rio Grande, with their immense, flocks of sheep. It was constant annoyance and danger, rather than war and pitched battles. The pueblo country is exceptionally rich in building material suited to the knowledge and capacity of the pueblo builders. Had suitable material been less abundant, military knowledge would have developed and defensive structures would have been erected; but as such material could be obtained everywhere, and there was no lack of sites, almost if not quite equal to those occupied at any given time, the easiest and most natural thing to do was to move. Owing to the nature of the hostile pressure, such movements were generally gradual, not en masse; although there is no doubt that movements of the latter kind have sometimes taken place.