es of steam must accumulate above the water level in the main
reservoirs before the pressure can become sufficient to expel the water
in the tube, after which steam alone continues to rush out until the
pressure is so relieved that it can no longer force a passage through
the water remaining in the trap, when quiet is restored. By the constant
addition of fresh water from the surface, by percolation or other usual
ways of sinking, the necessary conditions for the generation of steam
are maintained with surprising regularity.

The differences in the shape and general arrangement of the cavities and
tubes of the two caves, indicate that their action as geysers was very
unlike. Wind Cave evidently sent a rather slender column to a great
height, nearly perpendicular, and the water eruption was followed by a
long steam period. Crystal Cave ejected a much larger jet more
frequently, at a low angle of inclination, the eruption was sooner over,
and was not followed by a steam period of any consequence.

Thus it can be seen that the caves of the Black Hills prove the theories
in regard to geyser action in Yellowstone Park, and those theories, in
turn, prove the past history of the caves. The study of geyser action
also shows that the conical or dome shape of some of the cave chambers
is not due to the whirl of incoming floods, as in other regions, but to
jets of water forced up from lower levels.

Perhaps the finest geyser basin, and possible cave, ever in existence
was destroyed when the Grand Canon of the Yellowstone became a canon.
Evidences of the former conditions in control of this gorgeously
brilliant scene are neither wanting nor doubtful. Steam constantly
issues from numerous small vents in the canon walls, and a field glass
reveals miniature geysers in action down in the depth of the canon,
nearly half a mile below the top of the wall; while the entire canon
shows, in both the color and character of its rocks, that chemical
agencies have wrought changes here that have not been effected in other
exposures of similar nature. It seems not improbable that the relation
of Yellowstone River to the Grand Canon was the same as, at the present
time, is that of the Firehole to the Upper, Middle, and Lower Geyser
Basins: and that an explosion of great force was followed by a general
collapse instead of the usual eruption of one of the grandest geysers;
one result being the sudden precipitation of the river into a new,
beautiful