much attention within recent years. The age of the
Earth is, indeed, of primary importance in our conception of the
longevity of planetary systems. The essay deals with the
evidence, derived from the investigation of purely terrestrial
phenomena, as to the period which has elapsed since the ocean
condensed upon the Earth's surface. Dr. Decker's recent addition
to the subject appeared too late for inclusion in it. He finds
that the movements (termed isostatic) which geologists recognise
as taking place deep in the Earth's crust, indicate an age of the
same order of magnitude

xi

as that which is inferred from the statistics of denudative
history.[1]

The subject of _Denudation_ naturally arises from the first essay.
In thinking over the method of finding the age of the ocean by
the accumulation of sodium therein, I perceived so long ago as
1899, when my first paper was published, that this method
afforded a means of ascertaining the grand total of denudative
work effected on the Earth's surface since the beginning of
geological time; the resulting knowledge in no way involving any
assumption as to the duration of the period comprising the
denudative actions. This idea has been elaborated in various
publications since then, both by myself and by others.
"Denudation," while including a survey of the subject generally,
is mainly a popular account of this method and its results. It
closes with a reference to the fascinating problems presented by
the inner nature of sedimentation: a branch of science to which I
endeavoured to contribute some years ago.

_Mountain Genesis_ first brings in the subject of the geological
intervention of radioactivity. There can, I believe, be no doubt
as to the influence of transforming elements upon the
developments of the surface features of the Earth; and, if I am
right, this source of thermal energy is mainly responsible for
that local accumulation of wrinkling which we term mountain
chains. The

[1] Bull. Geol. Soc. America, vol. xxvi, March 1915.

xii

paper on _Alpine Structure_ is a reprint from "Radioactivity and
Geology," which for the sake of completeness is here included. It
is directed to the elucidation of a detail of mountain genesis: a
detail which enters into recent theories of Alpine development.
The weakness of the theory of the "horst" is manifest, however,
in many of its other applications; if not, indeed, in all.

The foregoing essays on the physical influence