2. That constructed of horizontal courses, constituting by far the larger number of these buildings. Each course of this masonry is made of well-dressed stones, carefully pecked, and of the same size. In this horizontal masonry the thickness of stones used may vary in different courses (pl. 11, _b_). They may be alternately narrow or thick, or layers of thick stones may be separated by one or more layers of tabular or thin stones. This method of alternation may be so regular as to please the eye and thus become decorative, a mode of decoration that reached a high development in the Chaco Ruins. The stones in the horizontal style of masonry are equal in size throughout the whole building in some cases, and show not only care in choice of stones but also in dressing them to the same regulation size. In these cases the joints fit so accurately that chinking has not been found necessary and a minimum use of adobe was required. The inner walls of kivas are much better constructed than the outer walls of the same or of the walls about them. The masonry here is regular horizontal. The sides, lintels, and thresholds of doorways are among the finest examples of construction. With the exception of walls sheltered by overhanging cliffs, the plastering has completely disappeared, but there is no reason to doubt that the interiors of all the great houses and towers were formerly plastered. It is instructive to compare the masonry of the great houses and towers of the Mancos with that of the towers in Hill Canyon (pl. 11, _c_) in Utah, the most northern extension of these two types. In Eight Mile Ruin, one of the largest of these buildings in Hill Canyon, we have a circular tower with annexed great houses, all constructed of well-dressed stones, the masonry in the walls showing on one side of the tower. No excavations, however, have yet been undertaken in Hill Canyon Ruins, and it is not known whether the unit type of kiva is found there, but the combination of great houses and towers is evident from the ground plans elsewhere published.[41] The feature of the towers in Hill Canyon is the clustering into groups, somewhat recalling the condition in Cannonball Ruin, where, however, they are united. In the Eight Mile Ruin one of the towers is separated from the remaining houses. Several towers have accompanying circular depressions with surrounding mounds. This association can well be seen in Holmes Tower on the Mancos Canyon and in Davis Tower and one or two others on the Yellow Jacket. These depressions, sometimes called reservoirs, have never been excavated, but from what is known of rooms accompanying towers in the western section of Hovenweep Castle it may be that they indicate kivas. Some towers have no sunken area in the immediate vicinity, especially those mounted on rocky points or perched on bowlders. At Cannonball Ruin there are several kivas side by side in one section and towering above them is a massive walled tower and other rooms.