n thrown by the handful over walls and ceilings, with
the additional ornamentation of calcite crystals. In the crevice beyond
rises the Church Steeple, diminishing regularly, though roughly, in
size, to a height of sixty feet, but not degraded with the little
squirming stairway usually seen in Church spires.

The next room is the Post Office, in which we are for the first time
introduced to the greatest peculiarity and most abundant formation known
to the cave. Being a newly discovered addition to geology it has no
scientific name and therefore is simply called box work, because it
resembles boxes of many shapes and sizes. The formation of the box work
is generally regarded as an unexplained and unexplainable mystery, but a
careful study of various portions of the cave shows it in all stages of
development and suggests a reasonable theory as to the cause of its
origin and variety of development. The volcanic disturbances which have
already been discussed as having been responsible for the various
uplifts and depressions of the Black Hills region, and also for opening
the fissures which gave the cave a beginning, must have supplied the
conditions that were necessary to the formation of box work. And these
preliminary conditions were merely cracks in the rock. By the violence
of earth movement the limestone has been crushed, probably when the land
was undergoing depression, prior to the upheaval which opened the great
parallel fissures. The varying hardness of the rock, as well as
proximity to the surface, would readily account for the difference in
size of the fractures, which is from one-half inch to twelve inches; the
largest being the most distant from the surface. That this crushing was
done before the salt waters retired from the region, which was towards
the close of the Cretaceous Age, is sufficiently evident in the fact
that portions of the Red Beds show similar fractures with the cracks
filled with gypsum, and gypsum, as we have already seen, is a salt
water deposit.

After the crushing was done the cracks in the Carboniferous Limestone
were filled with water heavily charged with calcium carbonate, taken in
solution from the rock, first from pulverized particles, and afterwards
by percolation and contact with exposed surfaces. This calcium carbonate
was slowly deposited in crystalline form, so that in time the cracks
were filled and the crushed rock firmly cemented with calcite seams. But
in the meantime the re