high and about half way from the trail below, caves had been quarried or weathered in considerable numbers in the shales which constitute one of the strata in the bluff. A hard platform of rock formed the floor, and afforded sufficient protection for a narrow platform in front of these openings. Immediately above these caves upon the summit of the bluffs, a system of ruined circular towers, enclosed by semicircular walls with the open side of the semicircle facing the precipice, was observed. The caves were accessible from the valley below only by means of ladders, and the towers in turn only by ladders from the caves through the open side of their semicircular enclosures. The walls of these enclosures presented no openings to the plateau above, and it is inferred that the towers which they enclosed served as outlooks from which the sentinel could signal the people who were engaged in tilling the valley below to flee to their cave-shelters at the approach of the enemy, and when too closely pressed by an enemy upon the plateau the sentinel himself could make his retreat by means of his ladder to the caves beneath. The most remarkable cliff-dwellings, discovered by Mr. Holmes, are shown in the cut. [Illustration: Cave-Fortresses of the Rio Mancos.] These extraordinary fortresses, lodged in caves eight hundred feet above the level of the valley, are situated in the cañon of the Mancos, a few miles from its mouth. The first five hundred feet of the ascent from the level of the stream, is over a rough cliff-broken slope, the remainder of massive sandstone, full of niches and caves. The upper house is situated in a deep cavern with overhanging roof about one hundred feet from the cliff’s top. The front wall of the house is built upon the very edge of the giddy precipice. The larger house is lodged in a niche or cave thirty feet below. The lower house was easily accessible. The wall was built flush with the precipice and remained standing to a height of fourteen feet at the highest point, though other portions had crumbled away considerably. The house occupied the entire floor of the niche, which measures sixty feet long by fifteen feet wide. Mr. Holmes described these structures as follows; of the first he says: “The arrangement of the apartments is quite complicated and curious, and will be more readily understood by a reference to the ground-plan (figure 1). The precipice line or front edge of the niche-floor, extends from _a_ to _b_. From this the broken cliffs and slopes reach down to the trail and river, as shown in the accompanying profile (figure 3). The line _b c d_ represents the deepest part of the recess, against which the walls are built. To the right of _b_, the shelf ceases, and the vertical face of rock is unbroken. At the left, beyond _a_, the edge is not so abrupt, and the cliffs below are so broken that one can ascend with ease. Above, the roof comes forward and curves upward, as seen in the profile.