n.i\- m
Under these circumstances it may not be out of place to review, as far as
the limited data permit, the conditions under which the most important constituents
of sea-water present themselves at great depths.
According to Bischof*, the total quantity of saline matter present in water
from the Pacific, Atlantic, and German Oceans (as deduced from the analyses of
Bibra) amounts to 3*527 per cent. ; whereas the several ingredients occur in the
following proportions :—
Chloride of sodium ....................................... 75-7S6
Chloride of magnesium.................................. 9-159
Chloride of calcium ....................................... 3-657
Bromide of sodimn ....................................... 1-184
Sulphate of lime ........................................... 4-617
Sulphate of magnesia .................................. 6-597
lOO-OOO
I t will be seen, on reference to this table, that carbonate of lime and silica, the
materials which enter so very largely into the formation of the shells and hard
parts of the marine Invertebrata, ai-e omitted. This is the more remarkable
since the geological phenomena involved in the presence of these substances
in sea-water may justly be regarded as amongst the most important and wonderful
with which we are acquainted. For, although carbonate of lime and
silica occur in proportions which appear small when compared with the mass of
the ocean, they are nevertheless as essential to the existence of marine organisms
of every kind as the carbonic-acid gas which is present in the atmosphere in still
more minute quantity is essential to terrestrial plant-life,—the solid structure of
the former being composed of carbonate of lime, of the latter of carbon. In
both cases, moreover, the demand and supply of the material are balanced with
such unerring nicety, that the failure of either one or the other could not take
place without altogether disturbing some of the existing conditions of the globe.
I t is stated by Bischof f that the vast quantity of carbonate of lime brought
down to the sea by rivers, and the uninterrupted formation of the shells of testaceous
animals, are of ihemsehes primé fa d e evidence of the presence of carbonate
of lime, and that “ the carbonic acid contained in sea-water will constantly dissolve
carbonate of lime when present at the bottom ” ; whilst the fact of “ the
water being so far below its point of saturation as regards carbonate of lime can
* ‘ Chemical and Physical Geology,’ vol. i. p. 379. t Ibid. p. 109.
I
only depend on the constant separation of the carbonate by testaceous animals.”
It is also mentioned, on the authority of Dr. Davy, that carbonate of lime occurs
chiefly along coast-lines, and that it was only detected to the extent of 1 part in
10,000 in water from Carlisle Bay, Barbadoes, whilst in the neighbourhood of
the volcanic island of Fayal the quantity was so small as to be barely discernible,
and in the open sea between the West Indies and Furope it was altogether
absent! Forchhammer, on the other hand, detected it in all sea-water.
The specimens of water analyzed by Bibra were obtained at a depth of only a
few feet below the surface, with a single exception in which the water was
brought up from 7 0 fathoms. Since, in the absence of any distinct statement to
that effect, it is improbable th a t the specimens analyzed by Dr. Davy were
obtained from great depths, or indeed elsewhere than at the surface, his observations
only show that the carbonate of lime met with in the superficial stratum
of water is derived principally from terrestrial sources; and that at a distance
from, coast-lines, in volcanic regions where the limestone-rocks are either wanting
or covered by igneous deposit, or in the open ocean where the depth is too gi*eat
to allow of any supply of the carbonate being furnished from lime held in solution
or calcareous deposits at the bottom, it is entirely absent.
According to M. Bischof, water containing only one-tenth of the quantity
necessary for saturation can dissolve as much carbonate of lime as if it were
saturated; but since the quantity of carbonic acid present in sea-water is sufficient
to dissolve five times as much carbonate of lime as is to be found in it, M. Bischof
infers that its maintenance at a point so far below that of saturation must be
due to the constant abstraction of carbonate of lime by testaceous creatures. He
further states that since water saturated with carbonic acid is capable of dissolving
only 0*1 per cent, of carbonate of lime, and under ordinary atmospheric pressure
water contains its own volume of carbonic acid, a saturated solution “ would
contain 0*2 per cent, by weight, or ten times as much as is necessary for the
solution of carbonate of lime to saturation.” And, lastly, he argues that water
saturated with carbonic-acid gas, even under high pressure, cannot dissolve more
carbonate, because, if it did so, ascending springs would deposit their carbonate
of lime, and choke up their channels, the moment the ascending stream became
subject to a diminished degree of pressure *.
* ‘ Chemical and Physical Geology,’ vol. iii. p. 171.
R 2