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

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

To the east and to the west are two large depressions, each about 5 feet below the surrounding ground surface, evidently the places whence the material for the construction of the mound was obtained.  Yet the amount of material removed from these excavations must have been considerably in excess of that used in the construction of the mound, and this excess was doubtless utilized in neighboring constructions, since it is hardly to be supposed that it was carried away to any considerable distance.

The purpose of this hollow mound, which is a fair type of many similar structures found in this region, is not clear.  Mr. Frank Hamilton Cushing, while director of the Hemenway southwestern archeological expedition, found a number of these structures and excavated some of them.  From remains thus found he concluded that they were sun-temples, as he termed them, and that they were covered with a roof made of coiled strands of grass, after a manner analogous to that in which pueblo baskets are made.  A somewhat similar class of structures was found by the writer on the upper Rio Verde, but these were probably thrashing floors.  Possibly the structure under discussion was for a similar purpose, yet its depth in proportion to its size was almost too great for such use.  The question must be left for determination if possible by excavation.

In the southern central part of the map is shown another excavation, covering a larger area than any of the others, of very irregular outline and from 3 to 4 feet deep.  It is apparently older than the others and probably furnished the material for the house structures northeast and southwest of it.  Bordering the depression on the south there are some low mounds, almost obliterated, which probably were the sites of other house structures.

Scattered about the area shown on the map there are several small depressions, usually more regular in outline than those described.  The best example is situated near the northeastern corner of the area.  It is situated in the point of a low promontory, is about 3 feet deep, almost regularly oval in outline, and measures about 50 by 100 feet.  A similar depression less than 2 feet deep occurs near the northwest corner of the area, and immediately south of the last there is another, more irregular in outline, and nearly 3 feet deep.  There are also some small depressions in the immediate vicinity of the Casa Grande ruin and of the mounds north of it.