depressions of bowl-shaped form, the sink holes to which we have
already adverted. Through the openings in the bottom of these the rain
waters descend into the depths of the earth. Although the most of
these depressions have but small openings in their bottom, now and
then one occurs with a vertical shaft sufficiently large to permit the
explorer to descend into it, though he needs to be lowered down in the
manner of a miner who is entering a shaft. In fact, the journey is
nearly always one of some hazard; it should not be undertaken save
with many precautions to insure safety.

When one is lowered away through an open sink hole, though the descent
may at first be somewhat tortuous, the explorer soon finds himself
swinging freely in the air, it may be at a point some hundred feet
above the base of the bottle-shaped shaft or dome into which he has
entered. Commonly the neck of the bottle is formed where the water has
worked its way through a rather sandy limestone, a rock which was not
readily dissolved by the water. In the pure and therefore easily cut
limestone layers the cavity rapidly expands until the light of the
lantern may not disclose its walls. Farther down there is apt to be a
shelf composed of another impure limestone, which extends off near the
middle of the shaft. If the explorer can land upon this shelf, he is
sure to find that from this imperfect floor the cavern extends off in
one or more horizontal galleries, which he may follow for a great
distance until he comes to the point where there is again a well-like
opening through the hard layer, with another dome-shaped base beneath.
Returning to the main shaft, the explorer may continue his descent
until he attains the base of this vertical section of the cave, where
he is likely to find himself delivered in a pool of water of no great
depth, the bottom of which is occupied by a quantity of small, hard
stones of a flinty nature, which have evidently come from the upper