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In stating the reasons why the gaseous contents of a column of sea-water
cannot be discharged at the surface, I endeavoured to show that no analogy
exists between the phenomena and those observable in ascending thermal springs
in which a constant discharge takes place. I f my views are correct, it is obvious
that the arguments apply as forcibly to the saline as to the gaseous contents,
since the same law which prevents the escape of the gaseous matter contained in
each succeeding stratum will enable each to retain in solution its full complement
of saline matter, so long as saline bases are supplied to the ocean.
Analyses of water taken from different latitudes and seas show that the quantities
of saline ingredients vary from local causes. Thus, it has been found that
in the neighbourhood of the equator, where rain-fall and evaporation are both
excessive, they counterbalance each other, and the specific gravity of the surface-
water and the proportion of solid matter undergo but trifling modification ; but
in sub-arctic regions, where the accession of fresh water is excessive during
one season of the year, and evaporation at no time takes place to any great extent,
the effect is much more conspicuous. The admixture of fresh water with
that of the sea takes place gradually, owing to the inferior specific gi'avity of the
former, but its influence extends to considerable depths. I t is of the greatest
importance, therefore, that water obtained from these latitudes for analytical
purposes should not be taken from the surface, and that we should look to the
deep circulation of the ocean for that equable diffusion of its various constituents
on which the equilibrium and permanence of the whole depend.
I t is worthy of remark that the calcareous deposits now forming over a large portion
of the Atlantic and probably of all great sea-beds occur generally at great
distances from coast-lines, and always in deep water ; whereas coral-reefs, which
derive their calcareous matter from the same source, are said to be formed in
shallow water, although, through the subsidence of the sea-bed, these bases often
extend to vast depths, and the reefs themselves may occur in what is now midocean.
Of the rate of growth of coral-reefs very little is known beyond the
fact of its being extremely gradual. Of that of the Foraminiferous deposits
nothing of a definite character has as yet been ascertained. Accordingly we
are not in a position to declare whether the demand for carbonate of lime,
in a given time, is greater in the superincumbent water of the coral-reef or the
Foraminiferous deposit. But, assuming the demand to be equal in equal periods.
it is difficult to account satisfactorily for the presence of carbonate of lime in
the shallow waters near coast-lines, and its entire absence at the surface of the
open ocean, unless we admit that the quantity of the salt increases with the
depth, and that the lowest stratum of water is that in which the conditions most
favourable to the growth of the Foraminiferous shell are present.
Again, since the quantity of carbonic acid present in sea-water increases with
the depth (as proved by the Bonite analyses), and the loose nature of the upper
layer of the Foraminiferous deposit makes it pre-eminently liable to be acted on
by a solvent, it is not improbable that a portion of carbonate of lime may be
redissolved on the spot irrespectively of pressure, and that any deficiency in
the supply may be made good, either by the quantity redissolved, or an increased
quantity brought round by the deep oceanic circulation. I f these views are
correct, it is obvious that the rate at which the increase of the deep-sea calcareous
deposits takes place. depends entirely on the rate at which carbonate of
lime can be poured into the ocean by rivers, since no new deposits could be built
up, did the separation of carbonate of lime from the water holding it in solution
(which is essential to the growth of the shells) and the re-solution of the dead
shells counterbalance each other.
Taking all these circumstances into consideration, it appears probable that
the demand for carbonate of lime at the bottom of the sea is limited only by the
supply; for, inasmuch as the quantity of calcareous matter introduced by rivers
is so great that, in the absence of an unceasing drain upon it by calcareous-
shelled creatures, its waters would very speedily become saturated and give rise
to chemical separation, and since no such separation has been found to take
place (in our own day at least), and the average amount of carbonic add is
not found to decrease, it is evident that, through the increase of the calcareous-
shelled organisms, the quantity of carbonate of lime brought down in solution
to the deeper zones of the ocean must be altogether disposed of.
But, apart from the reasons which render the chemical separation of carbonate
of lime from water holding it in solution improbable, it may be mentioned that
there is no example amongst the North-Atlantic soundings, nor, so far as I am
aware, amongst any of the recorded soundings taken in other portions of the
ocean, of a recent deposit of lime having been met with such as would result
from supersaturation alone. On the other hand, there is abundant proof of a