f millions of years.

How much Lucretius has lost, and how much we have gained, is
bound up with the question of the intrinsic value of knowledge
and great ideas. Let us appraise knowledge as we would the
Homeric poems, as some-

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thing which ennobles life and makes it happier. Well, then, we
are, as I think, in possession today of some of those lost Iliads
and Odysseys for which Lucretius looked in vain.[1]

[1] The duration in the past of Solar heat is necessarily bound
up with the geological age. There is no known means (outside
speculative science) of accounting for more than about 30 million
years of the existing solar temperature in the past. In this
direction the age seems certainly limited to 100 million years.
See a review of the question by Dr. Lindemann in Nature, April
5th, 1915.

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DENUDATION

THE subject of denudation is at once one of the most interesting
and one of the most complicated with which the geologist has to
deal. While its great results are apparent even to the most
casual observer, the factors which have led to these results are
in many cases so indeterminate, and in some cases apparently so
variable in influence, that thoughtful writers have even claimed
precisely opposite effects as originating from, the same cause.
Indeed, it is almost impossible to deal with the subject without
entering upon controversial matters. In the following pages I
shall endeavour to keep to broad issues which are, at the present
day, either conceded by the greater number of authorities on the
subject, or are, from their strictly quantitative character, not
open to controversy.

It is evident, in the first place, that denudation--or the wearing
away of the land surfaces of the earth--is mainly a result of the
circulation of water from the ocean to the land, and back again
to the ocean. An action entirely conditioned by solar heat, and
without which it would completely cease and further change upon
the land come to an end.

To what actions, then, is so great a potency of the

30

circulating water to be traced? Broadly speaking, we may classify
them as mechanical and chemical. The first involves the
separation of rock masses into smaller fragments of all sizes,
down to the finest dust. The second involves the actual solution
in the water of the rock constituents, which may be regarded as
the final act of disintegration. The rivers bear the burden both
of the comminuted and the dissolved materi