Among the scientific instructions for the voyage* drawn up by a
committee of the Royal Society, there is a remarkable letter from Von
Humboldt to Lord Minto, then First Lord of the Admiralty, in which,
among other things, he dwells upon the significance of the researches
into the microscopic composition of rocks, and the discovery of the
great share which microscopic organisms take in the formation of the
crust of the earth at the present day, made by Ehrenberg in the years
1836-39. Ehrenberg, in fact, had shown that the extensive beds of
"rotten-stone" or "Tripoli" which occur in various parts of the world,
and notably at Bilin in Bohemia, consisted of accumulations of the
silicious cases and skeletons of _Diatomaceae_ sponges, and _Radiolaria_;
he had proved that similar deposits were being formed by Diatomaceae, in
the pools of the Thiergarten in Berlin and elsewhere, and had pointed
out that, if it were commercially worth while, rotten-stone might be
manufactured by a process of diatom-culture. Observations conducted at
Cuxhaven, in 1839, had revealed the existence, at the surface of the
waters of the Baltic, of living Diatoms and _Radiolaria_ of the same
species as those which, in a fossil state, constitute extensive rocks
of tertiary age at Caltanisetta, Zante, and Oran, on the shores of the
Mediterranean.
* Of the Challenger.
Moreover, in the fresh-water rotten-stone beds of Bilin, Ehrenberg
had traced out the metamorphosis, effected apparently by the action
of percolating water, of the primitively loose and friable deposit
of organized particles, in which the silex exists in the hydrated
or soluble condition. The silex, in fact undergoes solution and slow
redeposition, until, in ultimate result, the excessively finegrained
sand, each particle of which is a skeleton, becomes converted into
a dense opaline stone, with only here and there an indication of an
organism.
From the consideration of these facts, Ehrenberg, as early as the year
1839, had arrived at the conclusion that rocks, altogether similar to
those which constitute a large part of the crust of the earth, must be
forming, at the present day, at the bottom of the sea; and he threw out
the suggestion that even where no trace of organic structure is to be
found in the older rocks, it may have been lost by metamorphosis.