form of maps, are not to be scrutinized as would be
justifiable in the case of surface surveys; for their de-
sign is not to fix the boundary lines of property, but
merely to aid the imagination in following the courses
pursued and the distances traversed in under-ground
explorations. In the nature of the case, much obscurity
must ever rest on regions shrouded in perpetual and
absolute darkness, except as momentarily lighted up by
artificial means; and the hope of the author is only
that he may make certain points clear, and gather into
one volume the winnowed results of long and varied
research by others as well as, himself. My collection of
cave literature contains every thing of the kind that
has been brought to my knowledge; including a con-
siderable amount of material the value of which is
impaired by flights of the fancy, or recklessness of
exaggeration. Obligations to those whose publications
have been of real service, are duly recognized in their
proper place. I desire to make special acknowledg-
ment of the personal attentions paid and the facilities
for exploration furnished by the owners and managers
of the principal American caverns described.
    The illustrations of Mammoth, Sibert, and Wyandot
caves were made under the author's direction, by Mr.
J. Barton Smith, of New Haven, Conn., and were origin-
ally prepared for articles that appeared in Scribner's
Magazine in 1880. The sketches were first done in
black and white, by the light of from twenty to fifty
lamps; after which the places sketched were brilliantly
illuminated by magnesium, that a clearer view might be
had of their outlines. The work thus begun underwent
careful revision in the artist's studio, and was corrected
as to minor details by comparison with photographs
that had been previously taken. Several of the cuts of