ptional, the mound on the east being less than 3 feet lower,
while the one on the southeast lacks less than 4 feet of its height. The
characteristic feature, however, and one difficult to explain, except on
the hypothesis stated, is the sharp slope of the sides. It will be
noticed that the raised base or terrace on which the mounds are located
is not perfectly flat, but on the contrary has a raised rim. This rim
seems quite inconsistent with the theory which has been advanced that
the terrace was built up solidly as a terrace or base, as in that case
it would seem natural that the slope from the base of the mounds to the
edge of the terrace would be continuous.

There is an abundance of room between the crest of the rim and the base
of the terrace for a row of single rooms, inclosing a court within which
the main structures stood, or such a court may have been covered, wholly
or partly with clusters of rooms, single storied outside, but rising in
the center, in two main clusters, three or more stories high. Such an
agglomeration of rooms might under certain conditions produce the result
seen here, although a circumscribing heavy wall, occupying the position
of the crest of the rim and inclosing two main clusters each rising
three or more stories, might also produce this result. The difficulty
with the latter hypothesis is, however, that under it we should expect
to find a greater depression between the base of the mounds and the edge
of the terrace. The most reasonable hypothesis, therefore, is that the
space between the base of the mounds and the edge of the terrace was
occupied by rooms of one story. This would also help to explain the
steepness of the slopes of the mounds themselves. The walls of the
structures they represent, being protected by the adjacent low walls of
the one-story rooms, would not suffer appreciably by undermining at the
ground level, and if the central room or rooms of each cluster were
higher than the surrounding rooms, as is the case in the Casa Grande
ruin, the exterior walls, being usually heavier than the inner walls,
would be the last to succumb, the clusters would be filled up by the
disintegration of the inner walls, and not until the spaces between the
low one-story walls surrounding the central cluster were nearly filled
up would the pronounced disintegration of the outer walls of the
structures commence. At that period the walls were probably covered and
protected by debris dropping fr