On these crystals the pop-corn is being formed, and specimens can be seen in all stages of development, from the beginning to an approximate degree of finish; and whatever the position it occupies on the receiving surface, either on top, underneath, or on a side exposure, it always maintains the same relative position as growing plants on the mundane sphere.  The water falling on the upper surface in scattering drops forms myriads of minute stalagmites; on side positions the falling drop first strikes the point exposed to its line of descent and then spreads.  The scant moisture slowly makes its way down sloping sides and shelving edges, leaving on each small irregularity a tiny portion of its volume, to deposit an infinitely small charge of solid substance, and the balance finally hangs in moisture less than drops on the growing grains of the under surface.

Pop-corn, therefore, is the globular aragonite of the stalagmitic variety.  A small specimen from Rainy Chamber, placed beside one of the same color from Wind Cave, shows them to be absolutely alike.

Rainy Chamber is the room in which the bones of the three-toed horse, already referred to, were found, but their presence has not yet been explained; therefore the case is open to conjecture and several theories may be advanced and their values considered.  The first question when such a discovery is made, is whether the living animal was possibly a cave-dweller; which, as the horse was not, is quickly disposed of and attention turned to the next, the possibility of a carniverous animal having carried his prey into the dark recesses of the cave in order that the enjoyment of his dinner might be undisturbed.  This theory is equally unavailable by reason of the topographical features presented.  If the present natural entrance to the cave were the only way into this room from the outside, the distance was too great and beset with many difficulties; besides which the final passage is too small to admit an animal of sufficient size to carry any considerable portion of even a very small horse.  But if at that period the room had direct communication with the outside through an opening since closed, the shape of the walls indicate that it must have been a pot-hole in the roof, and through this an animal could have entered by falling, which the horse and others may have done.  But it seems most probable that the remains were carried in by the water through such a hole before it was closed at the beginning of the Quaternary period, when the erosion of the Hills was most active.

Rainy Chamber also contains a large and beautiful assortment of the small polished and coated pebbles called cave pearls.