s, not sculptured like the
figures before described, but designed by the subtile pencil of some
sprite, who, Virginia suggested, must have been the subterranean brother
of the Frost.

"How wonderful!" she said. "And is it not strange how Nature copies
herself, reproducing silently here in the dark the very same forms we
find in the world above! Here is a rose, perfect!"

"With petals of pure white gypsum," said Penn.

Whilst they were talking, Cudjo passed on. They followed a little
distance, then halted. The light of his torch had gone out in the
blackness, and the sound of his footsteps had died away. Carl remained
with the other torch; and there they stood together, without speaking,
in the midst of immense darkness ingulfing their little isle of light,
and silence the most intense.

Suddenly they heard a voice far off, singing; then two, then three
voices; then a chorus filling the heart of the mountain with a strange
spiritual melody. Virginia was enraptured, and Carl amazed.

Penn, who had known what was coming, looked upon them with pride and
delight. At length the music, growing faint and fainter, melted and was
lost in the mysterious vaults through which it had seemed to wander and
soar away.

It was a minute after all was still before either spoke.

"Certainly," Virginia exclaimed, "if I had not heard of a similar effect
produced in the Mammoth Cave, I should never have believed that
marvellous chorus was sung by a single voice!"

"A single woice!" repeated Carl, incredulous. "There vas more as a dozen
woices!"

"Right, Carl!" laughed Penn. "The first was Cudjo's; and all the rest
were those, of the nymph Echo and her companions."

They continued their course through the halls of the echoes, and soon
came to an arched passage, at the entrance of which Penn paused and
placed the torch in a niche. A projection of the rock prevented the
light from shining before them, yet their way was softly illumined from
beyond, as by a dim phosphorescence. They advanced, and in a moment
their eyes, grown accustomed to the obscurity, came upon a scene of
surprising and magical beauty.

"The Grotto of Undine," said Penn.

It was, to all appearances, a nearly spherical concavity, some thirty
yards in length, and perhaps twenty in perpendicular diameter. Carl's
torch was concealed in the niche, and Cudjo's was nowhere visible; yet
the whole interior was luminous with a dim and silvery halo. A narrow
corridor ran r