Granting that the great ruins on the Chaco Canyon belong to the same people as those on Mesa Verde, the question arises, Which buildings are the most ancient, those on the Mesa Verde or those on the Chaco? A correct answer to this question should reveal the cradle of the culture indicated by the pure or prehistoric type of pueblo. The author believes that the pure pueblo culture originated in the northern part of the area and migrated southward to the Chaco Valley in prehistoric times, ultimately affecting the people of the Rio Grande, where sedentary people no doubt lived before written history of the area began. The result was a mixture; the mixed population are the modern Pueblos. In the great cliff-houses of the Mesa Verde and the extensive pueblos of the McElmo we find towers combined with pure types of pueblos, either simple or complex. In the Chaco ruins these towers are not found in this combination. To this may be added the great house type of the McElmo, also absent in the Chaco. Here there appears to be an essential difference on which the author ventures a suggestion, but which future research must elucidate. If this pure type originated in the southern tributaries of the San Juan as the Chaco and migrated to the northern we would expect in the latter more distinctly southern objects, as shell ornaments, turquoise mosaics, and a great variety of pottery of a southern type. The pure or unit type is believed to be autochthonous in the San Juan Basin and characteristic of a middle phase of architectural development, the highest north of Mexico. It is self-centered and has preserved its characteristics over an extensive area, influencing regions far beyond. The evolution of this type took place in the region mentioned before the fifteenth century of the Christian era. Traces of its influence have persisted into the country of mixed pueblos down to the present time, but the architectural skill has deteriorated and shows evidence of acculturation[58] from sources outside the San Juan area where it originated.