:
portion of the mechanically formed calcareous deposits being at times re-dissolved
by water holding carbonic acid in solution, inasmuch as the vast amount of
minute amorphous particles almost invariably present is traceable to no other
source than the disintegrated animal portion of the shells left behind after
the re-separation of the carbonate of lime.
According to the analyses of Forchhammer*, silica (or silicic acid) was found
in all sea-water examined by him, the greatest quantity being 0*3 in 10,000
parts of water. In its pure state it is insoluble in water, but yields to water
containing carbonic acid. In this respect, the conditions favourable to the
presence of carbonate of lime in the ocean are also favourable to that of silica.
With the alkalies silica forms soluble salts, called silicates, wffiich enter very
largely into the composition of rocks; whilst, like carbonate of lime, it is constantly
being conveyed to the sea by rivers, where it furnishes material for the
mineral portion of Sponges, of the Polycystina with .their allies, and the marine
species of the Diatomace®.
Although the whole of these organisms (excepting sponges, which are composite)
far exceed the Foraminifera in minuteness, they nevertheless constitute a
considerable per-centage of some of the oceanic deposits. As is well known,
extensive strata have been found in Europe, in Africa, and America, which are
almost exclusively composed of the siliceous remains of the Diatomace®; whilst
in some parts of America deposits occur of a mixed character, that is to say, in
which the calcareous and the siliceous remains are associated in nearly equal proportions.
I t is a remarkable fact that aU siliceous deposits, such as those of Bilin,
the Berg Mehl, and Tripoli, which consist almost entirely of a few well-marked
species that flourished in the immediate locality, are of fresh-water origin and
never occur in the ocean; whereas in oceanic deposits, although the siliceous
foi-ms may predominate, they are never unmixed with calcareous or other mineral
matter. Again, in those marine deposits in which the species of Diatomace® are
sufficiently abundant and well marked to indicate that they had lived in the
immediate locality, it may be taken for granted either that the water was shallow
or that the deposit was formed along a coast-line, since no Diatomace® live at
greater depths than from 400 to 500 fathoms. In the deep-sea beds, when
Diatomace® occur, the characters of the species, their variety, and their limited
* Bischof, vol. i. p. 109.
*
numbers at once show that they had been drifted from distant shallows, or were
free-floating surface forms which had subsided to the bottom after death.
Whilst, as yet, we have no positive proof that the Polycystina live at extreme
depths, it is a very significant circumstance that the large assemblages of
these organisms hitherto met with in such a recent state as to indicate vitality
occur in deep water, and that the forms taken alive at the immediate surface of
the ocean in some latitudes are sufficiently distinct to prove that the same species
do not occur at the surface and at the bottom without undergoing marked
modification. On the other hand, there is reason to believe that some of the
siliceous organisms met with in a living condition at the surface of the open
ocean cannot live at any great depth, and that, from some peculiarity in molecular
composition, the siliceous portion of their structure yields to the solvent
power of the water. Thus the Acanthemetrina, a small group of organisms with
siliceous frameworks of extreme symmetry and of such characteristic shape as to be
readily distinguishable, occur in tolerable profusion iu tropical and sub-tropical
latitudes; but, strange to say, not a trace of their siliceous remains is to be found
either in recent or fossil oceanic deposits.
The Barbadoes and other Polycystine earths have a calcareous basis derived
from the same source and produced through the same organic agency as the pure
calcareous deposits of the deep-sea bed; and, if we may judge from the composition
of such Polycystine deposits as have heretofore been brought up by the
sounding-machine, it would seem that all partake of the same character.
The character of the pure Diatomaceous earths of the Pliocene and Post-pli-
ocene periods clearly denotes a difference in the chemical constitution of fresh-
and salt-water deposits, which is of the highest importance as affording a means
of determining in which of thesé it may have been formed, supposing the characters
of the species to be uncertain; for, whereas the terrestrial species of
Diatomacese are to be met ivith in water in which carbonate of lime is not
necessarily present, there are no circumstances under which the marine species
of Diatomaceas and the Polycystina (all of which are oceanic) are to be met with
in water devoid of that salt*.
* A systematic chemical analysis of the entire series of soundings hitherto obtained would in all
probability throw a great deal of light on tho geological phenomena of the globe, and, were the means
forthcoming, it might very easily be accomplished.