One word in regard to the adjectives, prehistoric and historic, applied to southwestern ruins. They are relative ones and obtained from data somewhat diverse in character. Casa Grande on the Gila was called a ruin when first seen by the European. It was inhabited in prehistoric times. From documentary evidence the historian learns that certain other buildings were not inhabited at the advent of the Spaniards, and if their statements are trustworthy these also are prehistoric. Legends of modern Pueblos claim that certain other ruins were inhabited houses of their ancestors before the coming of the white man. The author sees no good reason to throw this evidence out of court without investigation because some of the incidents in it betray late introduction. Many other ruins are classified as prehistoric from the purely negative, but not decisive, evidence that no objects of European make have been found in them. The ruin Sun Temple, on the Mesa Verde, is considered prehistoric from the fact that a tree with over 360 annual rings of growth was found growing on top of its highest wall. We are justified in calling this a prehistoric ruin. The evidences that villages, cliff-dwellings, castles and towers, and other types considered in this article antedate the advent of the white man are as follows: No historian has recorded an inhabited building of this form in this or other regions; no objects of European manufacture have been found in them, and the buildings and pottery which characterize them are different from those of any inhabited when the Spanish entered the Southwest. The complex, which is thought to be the highest form of pueblo architecture, is composed of the following elements united: (1) Several “pure types”[59] representing a religio-sociological complexion of the inhabitants; (2) towers of various forms—round, D-shaped, and rectangular; (3) the great houses; (4) unit type in cave. In Cliff Palace these four types occur united in a pueblo built in a natural cave; in Mud Spring Ruin two and possibly three of these types are found in one open-air village, more spread out as site permits. In Aztec Spring and Mitchell Spring pueblos the arrangement is more defined. In the cluster at the head of South Fork of Square Tower Canyon we have all the elements united in Hovenweep House and Hovenweep Castle. Unit type House shows the single-unit type with tower near by; in Twin Towers we have the great house with cave pueblo and towers separated. Several other towers isolated from other types also occur. [58] These acculturation modifications due to Hispanic influences in modern pueblos are too well marked to need more than a mention. [59] The author uses the words “pure type” instead of “unit type” as a general term to denote “one-unit types,” “two-unit types,” “three-unit types,” etc.