The existence of artificial caves or of ancient dwellings inside natural grottos or recesses, and of villages constructed upon rocky projections or platforms, the so called cave dwellings and Cliff-houses, is dependent upon the geo logical features of the country. Erosion has acted power fully in many parts of the Southwest, — aqueous erosion as well as atmospheric. But the nature of the rock has deter mined the extent of this action. The three kinds of ruins named, therefore, cannot be looked for everywhere. Arti ficial caves require exceptionally soft material in order to be burrowed out with primitive utensils. Light, friable volcanic tufa, or ashes, is the rock Wherein the artificial cave is mostly found; and as this formation is not very common, it follows that the type of dwelling is limited in distribution. Large natural cavities or shelters cannot usually be looked for in granite, syenite, or hard por phyry; and long ledges over precipitous Slopes or overlook ing vertical walls are most abundant in sandstone. The explorer has therefore to pay close attention to the geo logical and lithological features of the country.