98 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



evidence. Some of the mounds of Aztec age appear to be merely accumulations of earth 

 from the adobe roofs and walls of the ancient dwellings, but others appear to have been 

 built of set purpose. This suggests that for some reason the earlier people built their 

 houses directly upon the plain, while the later Aztecs raised theirs upon mounds. To 

 Professor Boas this fact seems to indicate merely that before coming to the Mexican 

 plateau the Aztecs had probably acquired the habit of building elevated structures and that 

 this persisted throughout their history. Possibly, however, the elevation was an advantage 

 for purposes of defense ; or perhaps, at the coming of the Aztecs, the level of the lakes was 

 so high that in times of unusual rain the villages were occasionally in danger of inundation, 

 although during the days of their predecessors, the San Juan people, the plain may have 

 been so dry that no such danger existed. 



In one of the sites marked by San Juan pottery Professor Boas has made an excavation 

 in which he finds the following section from the top downward: 



(A) 1 or 2 feet of fine, dark surface soil full of bits of San Juan pottery. 



(B) 6 inches to 2 feet of "tepetate," or "caliche" as it is called farther north, in layers from 

 1 inch to 1 foot in thickness. It is mixed with bits of San Juan pottery, and is interstratified with 

 layers of well-rounded gravel containing pebbles up to 2 or 3 inches in diameter. The "tepetate" 

 is a white calcareous deposit which is frequently formed in dry regions where a large amount of 

 water evaporates. It is usually considered characteristic of rather arid conditions. Here at 

 Azcapotzaleo it is frequently faulted a few inches, as if the ground had sunken a little. 



(C) 4 or 5 feet of "culture layers" full of San Juan pottery intermingled with ashes, fire- 

 places, and the foundations of ancient houses. 



(D) 5 or 6 feet of fine sand, often in pockets or in slightly cross-bedded bands. This is inter- 

 mixed with finer sandy materials and a certain amount of clay like that which forms the bulk of 

 the overlying culture layers. Fragments of pottery of the same San Juan type, together with 

 l)ones and angular stones as much as a foot in diameter, indicate that men lived here when the 

 layers were being laid down, although there are no foundations. 



(E) 11 or 12 feet of gravel and sand growing coarser downward, and at the base containing 

 cobble-stones several inches in diameter. The pebbles are mostly well rounded, as if they had been 

 carried far in running water, although a few angular pieces are found, especially in the more 

 clayey portions of the sand. San Juan pottery occupies the upper 5 or 6 feet, but only in small 

 quantities. The fragments are often angular, showing that they have not been carried far in running 

 water. The lower 5 or 6 feet contain quite a different kind of pottery, belonging apparently to 

 the type which Professor Boas has called the Mountain culture. It is much more archaic than the 

 San Juan or Aztec types, and it is certainly older, since it lies lower. Whether it persisted until 

 the time of the later cultures we can not tell. Professor Boas says that as yet it has nowhere been 

 found on the surface of the plain, although it is common in small areas scattered among the sur- 

 rounding mountains. Hence its name. The pieces found by Professor Boas in his excavations 

 were all well rounded, showing that they had been carried some distance by running water or, 

 in other words, that they had been l^rought in from the mountains. 



At a short distance from the main excavation Professor Boas found that the gravels of this 

 formation die out. Minor excavations in several places led him to conclude that the main gravel 

 just described indicates the location of a river bed less than 100 meters wide and extending in a 

 north-and-south direction. Outside the river bed, but at the same level, the coarseness of the 

 decomposed tufaceous matter increases a little, and the material is more sandy than above or below, 

 indicating sorting by moving water. In the sandy material the archaic pottery of the Mountain 

 culture is found in large amounts. It is not stream-worn or rounded, and the paints with which 

 it is decorated are still fresh. Clearly it has not been carried far, which indicates that the plain 

 near the old river, or torrent, must have been inhabited. Whether this pottery is of the same age 

 as the worn fragments in the river bed is uncertain. It may be younger, for Professor Boas thinks 

 that there may have been a gradual transition from the Mountain culture to that of Teotihuacan. 



(F) At the base of the gravels a dark, compact clay is found to a depth of about 7 feet. It 

 contains almost no sand, but is full of plant remains, and of hydrated iron which stains it yellow. 

 The formation looks like the deposit of a swamp or of the edge of a lake. It is sharply separated 



