The Birth-Time of the World and Other Scientific Essays

by John Joly

We see, then, that the 27 x 10¹⁶ tonnes of substances leached
from the parent igneous rocks have had a threefold destination.
One part is still in solution; a second part has been
precipitated to the bottom of the ocean; a third part exists on
the land in the form of calcareous rocks.

Observation on the land sediments shows that the calcareous rocks
amount to about 5 per cent. of the whole. From this we find that
3 x 10¹⁶ tonnes, approximately, of such rocks have been taken
from the ocean. This accounts for one of the three classes of
material

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into which the original dissolved matter has been divided.
Another of the three quantities is easily estimated: the amount
of matter still in solution in the ocean. The volume of the ocean
is 1,414 million cubic kilometres and its mass is 145 x 10¹⁶
tonnes. The dissolved salts in it constitute 3.4 per cent. of its
mass; or, rather more than 5 x 10¹⁶ tonnes. The limestones on the
land and the salts in the sea water together make up about 8 x
10¹⁶ tonnes. If we, now, deduct this from the total of 27 x 10¹⁶
tonnes, we find that about 19 x 10¹⁶ tonnes must exist as
precipitated matter on the floor of the ocean.

The area of the ocean is 367 x 10¹² square metres, so that if the
precipitated sediment possesses an average specific gravity of
2.5, it would cover the entire floor to a uniform depth of 218
metres; that is 715 feet. This assumes that there was uniform
deposition of the abstracted matter over the floor of the ocean.
Of course, this assumption is not justifiable. It is certain that
the rate of deposition on the floor of the sea has varied
enormously with various conditions—principally with the depth.
Again, it must be remembered that this estimate takes no account
of solid materials otherwise brought into the oceanic deposits;
_e.g._, by wind-transported dust from the land or volcanic
ejectamenta in the ocean depths. It is not probable, however,
that any considerable addition to the estimated mean depth of
deposit from such sources would be allowable.

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The greatness of the quantities involved in these determinations
is almost awe inspiring. Take the case of the dissolved salts in
the ocean. They are but a fraction, as we have seen, of the total
results of solvent denudation and represent the integration of
the minute traces contributed by the river water. Yet the common
salt (chloride of sodium) alone, contained in the ocean, would,
if abstracted and spread over the dry land as a layer of rock
salt having a specific gravity of 2.2, cover the whole to a depth
of 107 metres or 354 feet. The total salts in solution in the
ocean similarly spread over the land would increase the depth of
the layer to 460 feet. After considering what this means we have
to remember that this amount of matter now in solution in the
seas is, in point of fact, less than a fifth part of the total
dissolved from the rocks during geological time.

The transport by denudation of detrital and dissolved matter from
the land to the ocean has had a most important influence on the
events of geological history. The existing surface features of
the earth must have been largely conditioned by the dynamical
effects arising therefrom. In dealing with the subject of
mountain genesis we will, elsewhere, see that all the great
mountain ranges have originated in the accumulation of the
detrital sediments near the shore in areas which, in consequence
of the load, gradually became depressed and developed into
synclines of many thousands of feet in depth. The most impressive
surface features of the Globe originated

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in this manner. We will see too that these events were of a
rhythmic character; the upraising of the mountains involving
intensified mechanical denudation over the elevated area and in
this way an accelerated transport of detritus to the sea; the
formation of fresh deposits; renewed synclinal sinking of the sea
floor, and, finally, the upheaval of a younger mountain range.
This extraordinary sequence of events has been determined by the
events of detrital denudation acting along with certain general
conditions which have all along involved the growth of
compressive stresses in the surface crust of the Earth.