The kiva of Unit type House is architecturally the same as those with vaulted roofs at Spruce-tree House, Cliff Palace, and Far View House on the Mesa Verde. A similar structure, according to Prudden,[48] occurs at Mitchell Spring Ruin in the Montezuma Valley, and near the Picket corral. The same type was found by Morley[49] at the Cannonball Ruin and by Kidder[50] in a kiva on Montezuma Creek in Utah, where clusters of mounds would appear to be composed of single or composite ruins of this type. This small pueblo was probably inhabited by one social unit, and may be regarded as the first stage of a compound pueblo. [48] Circular Kivas in San Juan Watershed. Amer. Anthrop., n. s. vol. 16, no. 1, 1914. [49] Excavation of the Cannonball Ruins in southwestern Colorado. Amer. Anthrop., n. s. vol. x, no. 4, 1908. [50] Explorations in southeastern Utah. Amer. Journ. Archæol., 2d ser., vol. xiv, no. 3, 1910. [Illustration: FIG. 10.—Ground plan of Unit type House.] STRONGHOLD HOUSE (RUIN 11) Ruin 11 is composed of a cluster of several small buildings, one of which is situated on the north edge of the mesa somewhat east of Ruin 10 (pl. 25, _b_); another, called by Morley and Kidder Gibraltar House, formerly of considerable size, was built on the sloping surface of an angular bowlder (pl. 17, 21, _b_). Although many walls have fallen, enough remains to render it a picturesque ruin, attractive to the visitor and instructive to the archeologist, by whom it has been classed as a tower. This building from the east appears to be a square tower, but it is in reality composed of several rooms perched on an inaccessible rock. RUINS IN HOLLY CANYON The towers in Holly Canyon (fig. 11) are in about the same condition of preservation as those in Square Tower Canyon. They cluster about the head of a small canyon (pl. 18, _a_) and may be approached on foot along the mesa above Keeley Camp, about a mile distant. Two of the Holly ruins belong to the tower type and were built on fallen bowlders. One of these has two rooms on the ground floor. (Pls. 19, _a_, _b_; 20, _a_, _c_.) [Illustration: FIG. 11.—Holly Canyon Ruins.] RUIN A, GREAT HOUSE, HACKBERRY CASTLE Ruin A (pl. 21, _a_), the largest building of the group, which stands on the edge of the canyon, is rectangular in form, measuring 31 by 9 feet, and is 20 feet high (fig. 11, _A_). Evidences of two rooms appear on the ground plan, one of which is 14 feet long, the other 12 feet inside measurement. The partition separating the two rooms is not tied into the outer walls, an almost constant feature in ancient masonry. The ends of the rafters are still seen in the wall at a level 12 feet above the base. Fallen stones have accumulated in the rooms to a considerable depth, and the tops of the remaining wall, where the mortar is washed out, will tumble in a short time.