The best conclusions of scientists in regard to the geological periods of our planet consider that the Glacial Epoch began about two hundred and forty thousand, and ended about eighty thousand, years ago.  Traces of the existence of men in North America during that glacial period have been found in abundance, and make it probable that a human population existed, toward the close of that era, all the way from the Atlantic Coast to the Upper Mississippi Valley.  Where these men of the Ice Age originally came from is a matter of conjecture; but it seems probable that they migrated hither from the Old World, since it is certain that during the various elevations and depressions of the two continents, it was possible, several times, for men to go from Europe or from Asia into America without crossing any ocean, either by the northwestern corner of Alaska, which has been repeatedly joined to Siberia through the elevation of the shallow Bering Sea, or by the great Atlantic ridge which more than once has risen above the ocean between Great Britain and Greenland.  Yet, though the first inhabitants of America, in all probability, came thus from the Old World at a very distant period of antiquity, it is believed by the best students of the subject that, until within the last few centuries, there had been no intercourse between America and either Europe or Asia, for at least twenty thousand years.  Hence the Aborigines of this continent developed in the course of ages peculiarities which distinguish them from other races, and justify their being regarded as, practically, native to the soil.

[Illustration:  AN EARLY PLACE OF SHELTER.]

The Indians of New Mexico and Arizona were, probably, fugitives from more fertile lands, whence they had been expelled by the ancestors of the bloodthirsty and cruel Apaches.  The country to which they came, and where they made a final stand against their predatory foes, was well adapted to defense.  For hundreds of square miles the land is cleft with chasms, and dotted with peculiar, isolated table-lands hundreds of feet in height, with almost perfectly level surfaces and precipitous sides.  The origin and formation of these mesas, due to erosion through unnumbered centuries, by water draining from an inland sea, has been already referred to, and it can be readily seen that they originally formed ideal residences for the peace-loving Pueblos, who either made their homes as Cliff Dwellers in the crevices of canon walls, or took advantage of these lofty rocks, already shaped and fortified by Nature, and built on them their dwellings.  These in themselves were no mean strongholds.  Their thick walls, made of rock fragments cemented with adobe, constituted a natural fortress, against which weapons such as savages used before they acquired fire-arms could do little harm; and even these houses the Indians constructed like the cliffs themselves, lofty and perpendicular, tier above tier, and, save for ladders, almost as inaccessible as eagles’