the age of our era, is, indeed, probable. Whatever the issue it
is certain that the reconciling facts will leave us with much
more light than we at present possess either as respects the
Earth's history or the history of the radioactive elements. With
this necessary admission we leave our study of the Birth-Time of
the World.
It has led us a long way from Lucretius. We do not ask if other
Iliads have perished; or if poets before Homer have vainly sung,
becoming a prey to all-consuming time. We move in a greater
history, the landmarks of which are not the birth and death of
kings and poets, but of species, genera, orders. And we set out
these organic events not according to the passing generations of
man, but over scores or hundreds of 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-
28
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.
29
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 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.
In the Swiss valleys we see torrents of muddy water hurrying
along, and if we follow them up, we trace them to glaciers high
among the mountains. From beneath the foot of the glacier, we
find, the torrent has birth. The first debris given to the river
is derived from the wearing of the rocky bed along which the
glacier moves. The river of ice bequeaths to the river of
water—of which it is the parent—the spoils which it has won from
the rocks
31
The work of mechanical disintegration is, however, not restricted
to the glacier's bed. It proceeds everywhere over the surface of
the rocks. It is aided by the most diverse actions. For instance,
the freezing and expansion of water in the chinks and cracks in
those alpine heights where between sunrise and sunset the heat of
summer reigns, and between sunset and sunrise the cold of winter.
Again, under these conditions the mere change of surface
temperature from night to day severely stresses the surface
layers of the rocks, and, on the same principles as we explain
the fracture of an unequally heated glass vessel, the rocks
cleave off in slabs which slip down the steeps of the mountain
and collect as screes in the valley. At lower levels the
expansive force of vegetable growth is not unimportant, as all
will admit who have seen the strong roots of the pines
penetrating the crannies of the rocks. Nor does the river which
flows in the bed of the valley act as a carrier only. Listening
carefully we may detect beneath the roar of the alpine torrent
the crunching and knocking of descending boulders. And in the
potholes scooped by its whirling waters we recognise the abrasive