of the present reefs, new ones might be formed high up round its sides and
ultimately over it. The case resolves itself into: what is the ordinary
height of groups of islands, of the size of existing groups of atolls
(excepting as many of the highest islands as there now ordinarily occur
encircling barrier-reefs in the existing groups of atolls)? and likewise
what is the height of the single scattered islands standing between such
groups of islands? Subsidence sufficient to bury all these islands (with
the exception of as many of the highest as there are encircled islands in
the present groups of atolls) my theory absolutely requires, but no more.
To say what amount of subsidence would be required for this end, one ought
to know the height of all existing islands, both single ones and those in
groups, on the face of the globe--and, indeed, of half a dozen worlds like
ours. The reefs may be of much greater [thickness] than that just
sufficient on an average to bury groups of islands; and the probability of
the thickness being greater seems to resolve itself into the average rate
of subsidence allowing upward growth, and average duration of reefs on the
same spot. Who will say what this rate and what this duration is? but till
both are known, we cannot, I think, tell whether we ought to look for
upraised coral formations (putting on one side denudation) above the
unknown limit, say between 3,000 and 5,000 feet, necessary to submerge
groups of common islands. How wretchedly involved do these speculations
become.


LETTER 534. TO E. VON MOJSISOVICS.
Down, January 29th, 1879.

I thank you cordially for the continuation of your fine work on the
Tyrolese Dolomites (534/1. "Dolomitriffe Sudtirols und Venetiens": Wien,
1878.), with its striking engravings and the maps, which are quite