The most unique feature of this cave, however, is the cupola-shaped
structure which stands in an open space in front of the house group,
near the mouth of the cave, but still under its roof. Its height,
measured inside, is twelve feet, and its widest inside diameter is
eleven feet. Its walls average eight inches in thickness. It has
one aperture three feet wide at the top, another one of the same
dimension near the base, and there are several others nearly opposite
each other. In the two upper ones are seen distinct impressions of
timber in the plaster.

The building was made by twisting long grass into a compact cable
and laying it up, one round upon another. As the coil proceeded,
thick coats of plaster were laid on inside and outside. This plaster,
which is the same material as that of which the houses are constructed,
got thoroughly mixed with the straw during the process of building,
and the entire structure was finished without any opening except
the one at the top. The other apertures were undoubtedly cut out
afterward. There is no trace of withes or other binding material to
hold the straw cables in place. They are kept in position only by
the plaster, which here, as in the houses, is almost as hard as the
conglomerate of the surrounding rocks.

My Mexicans from Sonora called it _olla_, a jar, and insisted that it
was a vessel used for keeping water; but this is entirely improbable,
for several reasons, mainly because the river is in close proximity and
easy of access. It was without the slightest doubt a granary. Similar
structures, used for that purpose to the present day, may be seen in
the States of Vera Cruz and Tlaxcala. In a cave only a short distance
away, the rear portion of which also contained a group of houses, we