Others were used as artificial hills on which to build communal houses. … Some of the very large mounds were sites of large communal houses in which entire tribes dwelt. There is still a third class … constructed as places for public assembly. … But to explain the mounds and their uses would expand this article into a book. It is enough to say that the Mound-builders were the Indian tribes discovered by white men. It may well be that some of the mounds were erected by tribes extinct when Columbus first saw these shores, but they were kindred in culture to the peoples that still existed. In the southwestern portion of the United States, conditions of aridity prevail. Forests are few and are found only at great heights. … The tribes lived in the plains and valleys below, while the highlands were their hunting grounds. The arid lands below were often naked of vegetation; and the ledges and cliffs that stand athwart the lands, and the canyon walls that inclose the streams, were everywhere quarries of loose rock, lying in blocks ready to the builder's hand. Hence these people learned to build their dwellings of stone; and they had large communal houses, even larger than the structures of wood made by the tribes of the east and north. Many of these stone pueblos are still occupied, but the ruins are scattered wide over a region of country embracing a little of California and Nevada, much of Utah, most of Colorado, the whole of New Mexico and Arizona, and far southward toward the Isthmus. … No ruin has been discovered where evidences of a higher culture are found than exists in modern times at Zuni, Oraibi, or Laguna. The earliest may have been built thousands of years ago, but they were built by the ancestors of existing tribes and their congeners. A careful study of these ruins, made during the last twenty years, abundantly demonstrates that the pueblo culture began with rude structures of stone and brush, and gradually developed, until at the time of the exploration of the country by the Spaniards, beginning about 1540, it had reached its highest phase. Zuni [in New Mexico] has been built since, and it is among the largest and best villages ever established within the territory of the United States without the aid of ideas derived from civilized men." With regard to the ruins of dwellings found sheltered in the craters of extinct volcanoes, or on the shelves of cliffs, or otherwise contrived, the conclusion to which all recent archæological study tends is the same. "All the stone pueblo ruins, all the clay ruins, all the cliff dwellings, all the crater villages, all the cavate chambers, and all the tufa-block houses are fully accounted for without resort to hypothetical peoples inhabiting the country anterior to the Indian tribes. … Pre-Columbian culture was indigenous; it