In subsequent pages the author will describe a ruin called the Unit type House, situated in the open on the north rim of Square Tower Canyon. A similar type of unit type house is found in a cave in Sand Canyon. The reader’s attention may first be called to the definition of a unit type, which is a building composed of a circular kiva, with mural banquettes and pedestals supporting a vaulted roof, with ventilator, reflector, and generally a ceremonial opening near a central fire hole in the floor. This kiva (fig. 5) is generally embedded in or surrounded by rectangular rooms. The single-unit type has one kiva with several surrounding rooms; the so-called pure type is composed of these units united. [Illustration: FIG. 5.—Ground plan of Unit type House in cave.] In an almost inaccessible cave (pl. 5, _b_) in Sand Canyon a few miles from the McElmo road near the scaffold already mentioned there is a cliff ruin, so far as known the first described single-unit house in a cave. It covers the whole floor of the cave (fig. 5) and its walls are considerably dilapidated, but the kiva shows this instructive condition: The walls are double, one inside the other, with two sets of pedestals, the outer of which are very much blackened with smoke of constant fires; the inner fresh and untarnished, evidently of late construction. A similar double-walled kiva known as “Kiva A” exists in Spruce-tree House, as described in the author’s account of that ruin.[39] On the perpendicular wall of the precipice at the right hand of the ruin in the cave above mentioned are several pictographs shown in plate 7, _c_. [39] Antiquities of the Mesa Verde National Park: Spruce-tree House. Bull. 41, Bur. Amer. Ethn., 1909. The rectangular rooms about the kiva are in places excavated out of the cliffs, but show standing walls on the front. These were not, however, constructed with the same care as those of the kiva. The cliff-house in Hackberry Canyon (pl. 9, _a_) is one of the most instructive. It lies below Horseshoe House and appears to be a second example of a unit type kiva and surrounding rooms. The cliff-dwelling in Ruin Canyon[40] visible across the canyon from the Old Bluff City Road is well preserved. On the rim of the canyon are piles of stone indicating a very large pueblo, with surface circular depressions indicating unit type houses. [40] The name Ruin Canyon, often applied also to Square Tower Canyon, is retained for this canyon. CLIFF-HOUSES IN LOST CANYON Lost Canyon, a southern tributary of the Dolores River, contains instructive cliff-houses to which my attention was called by Mr. Gordon Parker, superintendent of the Montezuma Forest Reserve, who has kindly allowed me to use the accompanying photographs. This cliff-house (pl. 10, _a_, _b_) belongs to the true Mesa Verde type and shows comparatively good preservation of its walls, some of the beams being in place. It is most easily approached from Mancos.