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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

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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 materials to the sea.  The mud and sand carried by their currents, or gradually pushed along their beds, represent the former; the invisible dissolved matter, only to be demonstrated to the eye by evaporation of the water or by chemical precipitation, represents the latter.

The results of these actions, integrated over geological time, are enormous.  The entire bulk of the sedimentary rocks, such as sandstones, slates, shales, conglomerates, limestones, etc., and the salt content of the ocean, are due to the combined activity of mechanical and solvent denudation.  We shall, later on, make an estimate of the magnitude of the quantities actually involved.