population based on the number of ruins within a given region, and even
those based on the size of a given ruin, must be materially revised. As
this subject has been elsewhere[1] discussed, it can be dismissed here
with the statement that the Casa Grande group seems to have formed no
exception to the general rule, but that its population changed from time
to time, and that the extent of the remains is no criterion of the
former population.

[Footnote 1: See pp. 179-261 of this Report, "Aboriginal Remains in
Verde Valley."]

It will be noticed that in some of the mounds, noticeably those in the
immediate vicinity of the Casa Grande ruin, the surface is very
irregular. In this instance the irregularity indicates a recent
formation of surface; for at this point many walls now marked only by
mounds were standing within the historical period. External contour is
of course a product of erosion, yet similarity of contour does not
necessarily indicate either equal erosion or equal antiquity. Surface
erosion does not become a prominent factor until after the walls have
fallen, and one wall may easily last for a century or two centuries
longer than another similarly situated. The surface erosion of a
standing wall of grout, such as these under discussion, is very slight;
photographs of the Casa Grande ruin, extending over a period of sixteen
years, and made from practically the same point of view, show that the
skyline or silhouette remained essentially unchanged during that period,
every little knob and projection remaining the same. It is through
sapping or undermining at the ground surface that walls are destroyed.
An inspection of the illustrations accompanying this paper will show
what is meant by sapping: the external walls are cut away at the ground
surface to a depth varying from a few inches to nearly 2 feet. After a