MINDELEFF 



COMPARATIVE AGE OF RUINS. 



301 



the mass becomes finally rounded. But it will be readily seen that a 

 slight ditference of texture, or thickness, or exposure, or so7ue trifling 

 difference too minute for observation, might easily add many decades 

 to the apparent age of a mound. The walls once fallen, however, the 

 rounding or smoothing of the mounds would probably proceed at an 

 equal rate throughout the group, and study of the profile gives a fairly 

 good estimate as to the comparative age of the mounds. On this basis 

 the most aucient mounds are those specified above, while the most recent 

 are those in the immediate vicinity of the Casa Grande ruin. This esti- 



FlG. 328.— Map of large mound. 



mate accords well with the limited historical data and with the Pima 

 traditions, which recount that the Casa Grande ruin was the last inhab- 

 ited village in this vicinity. 



Probably intermediate in time between the Casa Grande ruin and 

 the rounded mounds described above should be placed the large struc- 

 ture occupying the northern-central part of the map. This mound is 

 deserving of more than a passing notice. It consists of two mounds. 



