l 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
wonderful from the amount of labour which they exhibit, and its extreme
difficulty. I well remember more than forty years ago examining a
section of Silurian limestone containing many corals, and thinking to
myself that it would be for ever impossible to discover whether the
ancient corals had formed atolls or barrier reefs; so you may well
believe that your work will interest me greatly as soon as I can find
time to read it. I am much obliged for your photograph, and from its
appearance rejoice to see that much more good work may be expected from
you.

I enclose my own photograph, in case you should like to possess a copy.

LETTER 535. TO A. AGASSIZ.

(535/1. Part of this letter is published in "Life and Letters," III.,
pages 183, 184.)

Down, May 5th, 1881.

It was very good of you to write to me from Tortugas, as I always feel
much interested in hearing what you are about, and in reading your
many discoveries. It is a surprising fact that the peninsula of Florida
should have remained at the same level for the immense period requisite
for the accumulation of so vast a pile of debris. (535/2. Alexander
Agassiz published a paper on "The Tortugas and Florida Reefs" in the
"Mem. Amer. Acad. Arts and Sci." XI., page 107, 1885. See also his
"Three Cruises of the 'Blake,'" Volume I., 1888.)

You will have seen Mr. Murray's views on the formation of atolls and
barrier reefs. (535/3