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 removal of the calcium carbonate had started
disintegration of the more exposed portions of the rock, which steadily
continuing, finally reduced the porous body between the crystal seams to
a soft clay which was gradually dissolved and carried out through small
imperfections in the thin crystal sheets, leaving the empty box work as
we find it. But where blasting has exposed fresh surfaces, much of the