the igneous
rocks. The following table gives the percentages of the chief
chemical constituents: [1]

[1] F. W. Clarke: _A Preliminary Study of Chemical Denudation_,
p. 13

42

Igneous. Sedimentary.
Silica (SiO2) - 59.99 58.51
Alumina (Al2O3) - 15.04 13.07
Ferric oxide (F2O3) - 2.59 3.40
Ferrous oxide (FeO) - 3.34 2.00
Magnesia (MgO) - 3.89 2.52
Lime (CaO) - 4.81 5.42
Soda (Na2O) - 3.41 1.12
Potash (K2O) - 2.95 2.80
Water (H2O) - 1.92 4.28
Carbon dioxide (CO2) - -- 4.93
Minor constituents - 2.06 1.95
100.00 100.00

In the derivation of the sediments from the igneous rocks there
is a loss by solution of about 33 per cent; _i.e._ 100 tons of
igneous rock yields rather less than 70 tons of sedimentary rock.
This involves a concentration in the sediments of the more
insoluble constituents. To this rule the lime-content appears to
be an exception. It is not so in reality. Its high value in the
sediments is due to its restoration from the ocean to the land.
The magnesia and potash are, also, largely restored from the
ocean; the former in dolomites and magnesian limestones; the
latter in glauconite sands. The iron of the sediments shows
increased oxidation. The most notable difference in the two
analyses appears, however, in the soda percentages. This falls
from 3.41 in the igneous rock to 1.12 in the average sediment.
Indeed, this

43

deficiency of soda in sedimentary rocks is so characteristic of
secondary rocks that it may with some safety be applied to
discriminate between the two classes of substances in cases where
petrological distinctions of other kinds break down.

To what is this so marked deficiency of soda to be ascribed? It
is a result of the extreme solubility of the salts of sodium in
water. This has not only rendered its deposition by evaporation a
relatively rare and unimportant incident of geological history,
but also has protected it from abstraction from the ocean by
organic agencies. The element sodium has, in fact, accumulated in
the ocean during the whole of geological time.

We can use the facts associated with the accumulation of sodium
salts in the ocean as a means of obtaining addi