One of the most interesting exhibits of house-building for protection is found in the cliff dwellings, whose ruins are to be seen in Arizona and New Mexico. Tradition and other evidences point to the conclusion that certain tribes had developed a state of civilization as high as a middle period of barbarism, on the plains, where they had made a beginning of systematic agriculture, and that they were afterward driven out by wilder tribes and withdrew, seeking the cliffs for protection. There they built under the projecting cliffs the large communal houses, where they dwelt for a long period of time. Subsequently their descendants went into the valleys and developed the Pueblo villages, with their large communal houses of _adobe_. _Discovery and Use of Metals_.--It is not known just when the human race first discovered and used any one of the metals {101} now known to commerce and industry, but it can be assumed that their discovery occurred at a very early period and their use followed quickly. Reasoning back from the nature and condition of the wild tribes of to-day, who are curiously attracted by bright colors, whether in metals or beads or clothing, and realizing how universally they used the minerals and plants for coloring, it would be safe to assume that the satisfaction of the curiosity of primitive man led to the discovery of bright metals at a very early time. Pieces of copper, gold, and iron would easily have been found in a free state in metal-bearing soil, and treasured as articles of value. Copper undoubtedly was used by the American Indians, and probably by the inhabitants of Europe during the Neolithic Age--it being found in a native state in sufficient quantities to be hammered into implements. Thus copper has been found in large pieces in its native state, not only in Europe but in Mexico and other parts of North America, particularly in the Lake Superior region; but as the soft hematite iron was found in larger quantities in a free state, it would seem that the use of iron in a small degree must have occurred at about the same time, or perhaps a little later. The process of smelting must have been suggested by the action of fire built on or near ore beds, where a crude process of accidental smelting took place. Combined with tin ore, the copper was made into bronze in Peru and Mexico at the time of the discovery. In Europe there are abundant remains to show the early use of metals. Probably copper and tin were in use before iron, although iron may have been discovered first. There are numerous tin mines in Asia and copper mines in Cyprus. At first, metals were probably worked while cold through hammering, the softest metals doubtless being used before others.