The Casa Grande ruin, as the term is here used, occupies a position near the southwestern corner of the group, and it will be noticed that its size is insignificant as compared with that of the entire group, or even with the large structure in the north-central part of it.  The division of the group into northern and southern portions, which has been made by some writers, is clearly shown on the map; but this division is more apparent than real.  The contour interval on the map is one foot—­a sufficiently small interval to show the surface configuration closely and to bring out some of its peculiarities.  Depressions are shown by dotted contours.  It will be noticed that while most of the mounds which mark the sites of former structures rise but 10 feet or less above the surrounding level, the profiles vary considerably, some being much more smoothed off and rounded than others, the former being shown on the map by even, “flowing” contours, while the latter are more irregular; and it will be further noticed that the irregularity reaches its maximum in the vicinity of the Casa Grande ruin proper, where the ground surface was more recently formed, from the fall of walls that were standing within the historical period.

External appearance is a very unsafe criterion of age, although in some cases, like the present, it affords a fair basis for hypothesis as to comparative age; but even in this case, where the various portions of the group have presumably been affected alike by climatic and other influences, such hypothesis, while perhaps interesting, must be used with the greatest caution.  Within a few miles of this place the writer has seen the remains of a modern adobe house whose maximum age could not exceed a decade or two, yet which presented an appearance of antiquity quite as great as that of the wall remains east and southeast of the Casa Grande ruin.

The application of the hypothesis to the map brings out some interesting results.  In the first place, it may be seen that in the lowest mounds, such as those in the northwestern corner of the sheet, on the southern margin, and southwest of the well-marked mound on the eastern margin, the contours are more flowing and the slopes more gentle than in others.  This suggests that these smoothed mounds are older than the others, and, further, that their present height is not so great as their former height; and again, under this hypothesis, it suggests that the remains do not belong to one period, but that the interval which elapsed between the abandonment of the structures whose sites are marked by the low mounds and the most recent abandonment was long.  In other words, this group, under the hypothesis, affords another illustration of a fact constantly impressed on the student of southwestern village remains, that each village site marks but an epoch in the history of the tribe occupying it—­a period during which there was constant, incessant change, new bands or minor divisions of the tribe appearing on the scene,