quite large area, indicated by dotted contour lines. The principal one
occurs a little west of the center of the area, and is worth more than a
passing notice since similar structures are widely distributed
throughout this region. It may be roughly characterized as a mound with
excavated center. The ground for some distance about the structure
(except for two depressions discussed later) is quite flat. From this
flat surface as a base the structure rises to a height of 5 feet. From
the exterior it has the appearance of an ordinary mound, but on reaching
the top the interior is found to be hollowed out to a depth which even
at the present day is below the surrounding surface, although not below
the depressions adjoining. The main structure or mound is shown in
figure 329 (an enlargement from the map). It measures on top of the
crest 150 feet from north to south and about 80 feet from east to west,
but covers a ground area of 200 feet by 120 feet or over half an acre.
The crest is of the same height throughout, except for slight elevations
on the eastern and western sides and a little knoll or swell in the
southwestern corner. There is no indication of any break in the
continuity of the crest such as would be found were there openings or
gateways to the interior. The bottom of the depression in the main
structure is at present about a foot below the surrounding ground
surface, but it must have been originally considerably more than this,
as the profile indicates long exposure to atmospheric erosion and
consequent filling of the interior. No excavation was made and the
character of the construction can not be determined, but the mound is
apparently a simple earth structure--not laid up in blocks, like the
Casa Grande ruin.

[Illustration: Fig. 329.--Map of hollow mound.]

[Illustration: Pl. LIII: General View of Casa Grande.]