remains, as there shown, extends about 1,800 feet north and south and
1,500 feet east and west, or a total area of about 65 acres.

[Illustration: Pl. LI: Map of Casa Grande Group.]

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