SOUTHERN MEXICO AS ATTEST CASE. 99 



from the overlying gravel in a way to suggest a drying up of the swamp and a sudden bringing 

 in of materials by streams which had formerly had their mouths nearer the mountains. So far 

 as the clays have yet been studied they contain no pottery or other evidences of human occupation. 

 (G) Finally, the lowest formation thus far penetrated is a light-colored sand which Professor 

 Boas thinks to be lacustrine. 



The single section here given is of course inconclusive. The transition from one type 

 of deposits to another may have arisen from a change in the course of streams by reason 

 of an earthquake or volcanic eruption, or it may have been due to a tilting of that particular 

 portion of the earth's crust. The full history of the basin of Mexico can be ascertained 

 only by means of a large number of excavations well scattered over the whole area. Never- 

 theless, the present section is important. Our purpose in Mexico, it will be remembered, 

 is not to build up a new theory, but to test one which is founded upon a great number of 

 facts in widely scattered parts of both Asia and America. We want to discover whether 

 new facts found in other regions disagree with the theory and compel us to modify it, or 

 agree and allow us to carry it into still other fields. Hence it is important to see that in 

 this particular case, the only one of its kind where a rigorous test is yet possible in this 

 particular region, the facts agree closely with what would be expected if the climate of 

 Mexico has varied in harmony with what seems to have been the case in other parts of 

 the world. The apparently lacustrine deposits of (G), and the swampy deposits of (F), to 

 begin with the oldest formation, suggest conditions of decided moisture with such an 

 expansion of the lakes that the floor of the basin was uninhabitable and the people were 

 forced to hve in the surrounding hills where they developed their mountain culture. The 

 succeeding gravels suggest a change to drier conditions whereby the shore of the swamp or 

 lake retreated and streams began to encroach upon the old water-covered bed. At the 

 same time the death of vegetation upon the mountain slopes, because of the aridity, would 

 permit the floods to wash down large amounts of coarse gravel, with which would be 

 mingled rounded, waterworn bits of pottery from the mountain villages, as appears in 

 the lower part of formation (E). During this dry time, if such it really were, the people 

 of the mountain type apparently expanded from their restricted habitat among the arid 

 hills, and spread out over the relatively moist plain as is indicated by the unworn pottery 

 at the base of (E) in the portions of that formation outside the river channel. A little later, 

 the San Juan culture, perhaps that of an invader, made its appearance, the village in 

 question being close to the base of the mountains, or on the very edge of the plam, as is indi- 

 cated by the fact that its pottery is present in the gravels, but is free from marks of wear by 

 running water. By the time that deposit (D) began to be laid down the San Juan people 

 were living not far from the site of the excavations. When (C) was being formed conditions 

 were very much as now. (B), on the contrary, with its layers of "tepetate" and gravel, 

 suggests a return toward aridity, while (A) brings us back to the present conditions. If the 

 elevation of the Aztec mounds, built since the deposition of (A), really has anything to do 

 with the danger of flooding it may indicate a slightly moister time such as that of which 

 the traditions give a suggestion in the fourteenth century, while now in the nineteenth 

 and twentieth centuries we are back once more in dry times. The whole importance of 

 the hne of reasoning here followed is quite independent of the fact that the specific phe- 

 nomena here described are subject to other possible explanations. It Hes rather in the 

 fact that the explanation here offered harmonizes with a vast number of other facts, both 

 in Mexico and elsewhere, while the other explanations take little account of anything 

 outside of the narrow range of the phenomena immediately to be described. 



Turning now from archeology and lake-beds to alluvial terraces, we find that the kind 

 discussed in previous chapters have not been described at any length by the geologists of 

 Mexico. Nevertheless, they are said to be abundant in the states of Chihuahua, Durango, 

 and elsewhere in the northwest, and my own observation proves them to exist in large 



