Near the lower end of the cañon stands one of the most perfect of these towers, rising sixteen feet above the mound on which it is built. It was once attached to an oblong stone building which seems to have been a strongly-fortified house. The rectangular walls, as usual, are prostrate, and have left the tower standing as solitary and picturesque and as full of mystery as the round-towers of Ireland. After the stream breaks from its long confinement out into the open plain of the San Juan Valley the traces of old life are still abundant, but they present no features very different from those above. At the cañon's mouth an Indian trail strikes away toward the north-west. It passes a remarkable group of ruins at a spot called Aztec Springs, and continues to the McElmo, the next _arroya_, or dry stream-bed, west of the Rio Mancos. Aztec Springs no longer deserve the name, for within a short time the last trace of water has disappeared from the spot, showing that the slow drying up of the great South-west country, which has been going forward for ages, and which starved out the old inhabitants, is still progressing. In the dry season there is no water within many miles of this spot, though it is strewn with the remains of stone buildings covering several acres and indicating a large population of industrious people who must have lived by agriculture. Until a long comparative study has been made of all the remains of this race it is mere guesswork to estimate the age of the ruins; but when the prostrate condition of these walls is compared with the state in which the Chaco ruins of New Mexico are found, and when we consider that the latter have no doubt been deserted for at least three hundred and fifty years, it is reasonable to suppose an age of a thousand years for these massive walls at Aztec Springs. Many other great structures of this region, which seem to be coeval with these, are situated many miles away from any perennial water, and the time which has elapsed since those sites were suitable for large farming-towns must be counted by centuries. In this group are two large quadrangular buildings with walls still fifteen feet high, two of the circular estufas, besides a multitude of half-distinguishable walls of dwellings. It is the largest group of ruins in Colorado. [Illustration: CIRCULAR RUIN IN THE CAÑON OF THE MANCOS.]