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 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 solid limestone carries the box-like sheets of crystal.