THE BIRTH-TIME OF THE WORLD AND OTHER SCIENTIFIC ESSAYS

by

J. JOLY, M.A., Sc.D., F.R.S.,
PROFESSOR OF GEOLOGY AND MINERALOGY IN THE UNIVERSITY OF DUBLIN

E. P. DUTTON AND COMPANY
681 FIFTH AVENUE NEW YORK

Produced by Hugh Rance, 2005

Cover

Title page

CONTENTS PAGE

I. THE BIRTH-TIME OF THE WORLD - - - - - - - - - - - 1

II. DENUDATION  - - - - - - - - - - - - - - - - - - 30

III. THE ABUNDANCE OF LIFE  - - - - - - - - - - - - 60

IV. THE BRIGHT COLOURS OF ALPINE FLOWERS - - - - - 102

V. MOUNTAIN GENESIS  - - - - - - - - - - - - - - - 116

VI. ALPINE STRUCTURE - - - - - - - - - - - - - - - 146

VII. OTHER MINDS THAN OURS - - - - - - - - - - - - 162

VIII. THE LATENT IMAGE - - - - - - - - - - - - - - 202

IX. PLEOCHROIC HALOES  - - - - - - - - - - - - - - 214

X. THE USE OF RADIUM IN MEDICINE - - - - - - - - - 244

XI. SKATING  - - - - - - - - - - - - - - - - - - - 260

XII. A SPECULATION AS TO A PRE-MATERIAL UNIVERSE - 288

LIST OF ILLUSTRATIONS

PLATE I. LAKE OF LUCERNE, LOOKING WEST FROM BRUNNEN -
Frontispiece

PLATE II. "UPLIFTED FROM THE SEAS." CLIFFS OF THE TITLIS,
SWITZERLAND - to face p. 4

PLATE III. AN ALPINE TORRENT AT WORK—VAL D'HERENS, SWITZERLAND -
to face p. 31

PLATE IV. EARTH PILLARS—VAL D'HERENS - to face p. 34

PLATE V. "SCENES OF DESOLATION." THE WEISSHORN SEEN FROM BELLA
TOLA, SWITZERLAND - to face p. 40

PLATE VI. ALLUVIAL CONE—NICOLAI THAL, SWITZERLAND. MORAINE ON
ALETSCH GLACIER SWITZERLAND - to face p. 50

PLATE VII. IN THE REGION OF THE CROCI; DOLOMITES. THE ROTHWAND
SEEN FROM MONTE PIANO - to face p. 60

PLATE VIII. FIRS ASSAILING THE HEIGHTS OF THE MADERANER THAL,
SWITZERLAND - to face p. 73

PLATE IX. LIFE NEAR THE SNOW LINE; THE BOG-COTTON IN POSSESSION.
NEAR THE TSCHINGEL PASS, SWITZERLAND - to face p. 80

PLATE X. THE JOY OF LIFE. THE AMPEZZO THAL; DOLOMITES - to face
p. 93

PLATE XI. "PINES SOLEMNLY QUIET." DÜSSISTOCK; MADERANER THAL - to
face p. 100

PLATE XII. ALPINE FLOWERS IN THE VALLEYS - to face p. 105

PLATE XIII. ALPINE FLOWERS ON THE HEIGHTS - to face p. 106

PLATE XIV. MOUNTAIN SOLITUDES; VAL DE ZINAL. FROM LEFT TO RIGHT
ROTHHORN; BESSO; OBERGABELHORN; MATTERHORN; PIC DE ZINAL (THROUGH
CLOUD); DENT BLANCHE - to face p. 116

ix

PLATE XV. SECTOR OF THE EARTH RISE OF ISOGEOTHERMS INTO A DEPOSIT
EVOLVING RADIOACTIVE HEAT - to face p. 118

PLATE XVI. "THE MOUNTAINS COME AND GO." THE DENT BLANCHE SEEN
FROM THE SASSENEIRE - to face p. 133

PLATE XVII. DIAGRAMMATIC SECTIONS OF THE HIMALAYA - to face p.
140

PLATE XVIII. RESIDUES OF DENUDATION. THE MATTERHORN SEEN FROM THE
SUMMIT OF THE ZINAL ROTHHORN - to face p. 148

PLATE XIX. THE FOLDED ROCKS OF THE MATTERHORN, SEEN FROM NEAR
HÖHBALM. SKETCH MADE IN 1906 - to face p. 156

PLATE XX. SCHIAPARELLI'S MAP OF MARS OF 1882, AND ADDITIONS (IN
RED) OF 1892 - to face p. 166

PLATE XXI. GLOBE OF MARS SHOWING PATH OF IN-FALLING SATELLITE -
to face p. 188

PLATE XXII. CANALS MAPPED BY LOWELL COMPARED WITH CANALS FORMED
BY IN-FALLING SATELLITES - to face p. 192

PLATE XXIII. HALOES IN MICA; CO. CARLOW. HALO IN BIOTITE
CONTAINED IN GRANITE - to face p. 224

PLATE XXIV. RADIUM HALO, MUCH ENLARGED. THORIUM HALO AND RADIUM
HALO IN MICA - to face p. 228

PLATE XXV. HALO ROUND CAPILLARY IN GLASS TUBE. HALOES ROUND
TUBULAR PASSAGES IN MICA - to face p. 230

PLATE XXVI. ALETSCH GLACIER, SWITZERLAND - to face p. 282

PLATE XXVII. THE MIDDLE ALETSCH GLACIER JOINING THE GREAT ALETSCH
GLACIER. GLACIERS OF THE LAUTERBRUNNEN THAL - to face p. 285

PLATE XXVIII. PERCHED BLOCK ON THE ALETSCH GLACIER. GRANITE
ERRATIC NEAR ROUNDWOOD, CO. WICKLOW; NOW BROKEN UP AND REMOVED -
to face p. 286

And Fifteen Illustrations in the Text.

x

PREFACE

Tins volume contains twelve essays written at various times
during recent years. Many of them are studies contributed to
Scientific Reviews or delivered as popular lectures. Some are
expositions of views the scientific basis of which may be
regarded as established. Others—the greater number—may be
described as attempting the solution of problems which cannot be
approached by direct observation.

The essay on The Birth-time of the World is based on a lecture
delivered before the Royal Dublin Society. The subject has
attracted 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 influences affecting the
surface features of the Earth are accompanied by one entitled _The
Abundance of Life._ This originated amidst the overwhelming
presentation of life which confronts us in the Swiss Alps. The
subject is sufficiently inspiring. Can no fundamental reason be
given for the urgency and aggressiveness of life? Vitality is an
ever-extending phenomenon. It is plain that the great principles
which have been enunciated in explanation of the origin of
species do not really touch the problem. In the essay—which is an
early one (1890)—the explanation of the whole great matter is
sought—and as I believe found—in the attitude of the organism
towards energy external to it; an attitude which results in its
evasion of the retardative and dissipatory effects which prevail
in lifeless dynamic systems of all kinds.

_Other Minds than Ours_? attempts a solution of the vexed question
of the origin of the Martian "canals." The essay is an abridgment
of two popular lectures on the subject. I had previously written
an account of my views which carried the enquiry as far as it was
in

xiii

my power to go. This paper appeared in the "Transactions of the
Royal Dublin Society, 1897." The theory put forward is a purely
physical one, and, if justified, the view that intelligent beings
exist in Mars derives no support from his visible surface
features; but is, in fact, confronted with fresh difficulties.

_Pleochroic Haloes_ is a popular exposition of an inconspicuous but
very beautiful phenomenon of the rocks. Minute darkened spheres—a
microscopic detail—appear everywhere in certain of the rock
minerals. What are they? The discoveries of recent radioactive
research—chiefly due to Rutherford—give the answer. The
measurements applied to the little objects render the explanation
beyond question. They turn out to be a quite extraordinary record
of radioactive energy; a record accumulated since remote
geological times, and assuring us, indirectly, of the stability
of the chemical elements in general since the beginning of the
world. This assurance is, without proof, often assumed in our
views on the geological history of the Globe.

Skating is a discourse, with a recent addition supporting the
original thesis. It is an illustration of a common experience—the
explanation of an unimportant action involving principles the
most influential considered as a part of Nature's resources.

The address on _The Latent Image_ deals with a subject which had
been approached by various writers before the time of my essay;
but, so far as I know, an explanation

xiv

based on the facts of photo-electricity had not been attempted.
Students of this subject will notice that the views expressed are
similar to those subsequently put forward by Lenard and Saeland
in explanation of phosphorescence. The whole matter is of more
practical importance than appears at first sight, for the
photoelectric nature of the effects involved in the radiative
treatment of many cruel diseases seems to be beyond doubt.

It was in connection with photo-electric science that I was led
to take an interest in the application of radioactivity in
medicine. The lecture on _The Use of Radium in Medicine_ deals with
this subject. Towards the conclusion of this essay reference will
be found to a practical outcome of such studies which, by
improving on the methods, and facilitating the application, of
radioactive treatment, has, in the hands of skilled medical men,
already resulted in the alleviation of suffering.

Leaving out much which might well appear in a prefatory notice, a
word should yet be added respecting the illustrations of scenery.
They are a small selection from a considerable number of
photographs taken during my summer wanderings in the Alps in
company with Henry H. Dixon. An exception is Plate X, which is by
the late Dr. Edward Stapleton. From what has been said above, it
will be gathered that these illustrations are fitly included
among pages which owe so much to Alpine inspiration. They
illustrate the

xv

subjects dealt with, and, it is to be hoped, they will in some
cases recall to the reader scenes which have in past times
influenced his thoughts in the same manner; scenes which in their
endless perspective seem to reduce to their proper insignificance
the lesser things of life.

My thanks are due to Mr. John Murray for kindly consenting to the
reissue of the essay on _The Birth-time of the World_ from the
pages of _Science Progress_; to Messrs. Constable & Co. for leave
to reprint _Pleochroic Haloes_ from _Bedrock_, and also to make some
extracts from _Radioactivity and Geology_; and to the Council of
the Royal Dublin Society for permission to republish certain
papers from the Proceedings of the Society.

_Iveagh Geological Laboratory, Trinity College, Dublin._

July, 1915.

xvi

THE BIRTH-TIME OF THE WORLD [1]

LONG ago Lucretius wrote: "For lack of power to solve the
question troubles the mind with doubts, whether there was ever a
birth-time of the world and whether likewise there is to be any
end." "And if" (he says in answer) "there was no birth-time of
earth and heaven and they have been from everlasting, why before
the Theban war and the destruction of Troy have not other poets
as well sung other themes? Whither have so many deeds of men so
often passed away, why live they nowhere embodied in lasting
records of fame? The truth methinks is that the sum has but a
recent date, and the nature of the world is new and has but
lately had its commencement."[2]

Thus spake Lucretius nearly 2,000 years ago. Since then we have
attained another standpoint and found very different limitations.
To Lucretius the world commenced with man, and the answer he
would give to his questions was in accord with his philosophy: he
would date the birth-time of the world from the time when

[1] A lecture delivered before the Royal Dublin Society, February
6th, 1914. _Science Progress_, vol. ix., p. 37

[2] _De Rerum Natura_, translated by H. A. J. Munro (Cambridge,
1886).

1

poets first sang upon the earth. Modern Science has along with
the theory that the Earth dated its beginning with the advent of
man, swept utterly away this beautiful imagining. We can, indeed,
find no beginning of the world. We trace back events and come to
barriers which close our vista—barriers which, for all we know,
may for ever close it. They stand like the gates of ivory and of
horn; portals from which only dreams proceed; and Science cannot
as yet say of this or that dream if it proceeds from the gate of
horn or from that of ivory.

In short, of the Earth's origin we have no certain knowledge; nor
can we assign any date to it. Possibly its formation was an event
so gradual that the beginning was spread over immense periods. We
can only trace the history back to certain events which may with
considerable certainty be regarded as ushering in our geological
era.

Notwithstanding our limitations, the date of the birth-time of
our geological era is the most important date in Science. For in
taking into our minds the spacious history of the universe, the
world's age must play the part of time-unit upon which all our
conceptions depend. If we date the geological history of the
Earth by thousands of years, as did our forerunners, we must
shape our ideas of planetary time accordingly; and the duration
of our solar system, and of the heavens, becomes comparable with
that of the dynasties of ancient nations. If by millions of
years, the sun and stars are proportionately venerable. If by
hundreds or thousands of millions of

2

years the human mind must consent to correspondingly vast epochs
for the duration of material changes. The geological age plays
the same part in our views of the duration of the universe as the
Earth's orbital radius does in our views of the immensity of
space. Lucretius knew nothing of our time-unit: his unit was the
life of a man. So also he knew nothing of our space-unit, and he
marvels that so small a body as the sun can shed so much, heat
and light upon the Earth.

A study of the rocks shows us that the world was not always what
it now is and long has been. We live in an epoch of denudation.
The rains and frosts disintegrate the hills; and the rivers roll
to the sea the finely divided particles into which they have been
resolved; as well as the salts which have been leached from them.
The sediments collect near the coasts of the continents; the
dissolved matter mingles with the general ocean. The geologist
has measured and mapped these deposits and traced them back into
the past, layer by layer. He finds them ever the same;
sandstones, slates, limestones, etc. But one thing is not the
same. _Life_ grows ever less diversified in character as the
sediments are traced downwards. Mammals and birds, reptiles,
amphibians, fishes, die out successively in the past; and barren
sediments ultimately succeed, leaving the first beginnings of
life undecipherable. Beneath these barren sediments lie rocks
collectively differing in character from those above: mainly
volcanic or poured out from fissures in

3

the early crust of the Earth. Sediments are scarce among these
materials.[1]

There can be little doubt that in this underlying floor of
igneous and metamorphic rocks we have reached those surface
materials of the earth which existed before the long epoch of
sedimentation began, and before the seas came into being. They
formed the floor of a vaporised ocean upon which the waters
condensed here and there from the hot and heavy atmosphere. Such
were the probable conditions which preceded the birth-time of the
ocean and of our era of life and its evolution.

It is from this epoch we date our geological age. Our next
purpose is to consider how long ago, measured in years, that
birth-time was.

That the geological age of the Earth is very great appears from
what we have already reviewed. The sediments of the past are many
miles in collective thickness: yet the feeble silt of the rivers
built them all from base to summit. They have been uplifted from
the seas and piled into mountains by movements so slow that
during all the time man has been upon the Earth but little change
would have been visible. The mountains have again been worn down
into the ocean by denudation and again younger mountains built
out of their redeposited materials. The contemplation of such
vast events

[1] For a description of these early rocks, see especially the
monograph of Van Hise and Leith on the pre-Cambrian Geology of
North America (Bulletin 360, U.S. Geol. Survey).

4

prepares our minds to accept many scores of millions of years or
hundreds of millions of years, if such be yielded by our
calculations.

THE AGE AS INFERRED FROM THE THICKNESS OF THE SEDIMENTS

The earliest recognised method of arriving at an estimate of the
Earth's geological age is based upon the measurement of the
collective sediments of geological periods. The method has
undergone much revision from time to time. Let us briefly review
it on the latest data.

The method consists in measuring the depths of all the successive
sedimentary deposits where these are best developed. We go all
over the explored world, recognising the successive deposits by
their fossils and by their stratigraphical relations, measuring
their thickness and selecting as part of the data required those
beds which we believe to most completely represent each
formation. The total of these measurements would tell us the age
of the Earth if their tale was indeed complete, and if we knew
the average rate at which they have been deposited. We soon,
however, find difficulties in arriving at the quantities we
require. Thus it is not easy to measure the real thickness of a
deposit. It may be folded back upon itself, and so we may measure
it twice over. We may exaggerate its thickness by measuring it
not quite straight across the bedding or by unwittingly including
volcanic materials. On the other hand, there

5

may be deposits which are inaccessible to us; or, again, an
entire absence of deposits; either because not laid down in the
areas we examine, or, if laid down, again washed into the sea.
These sources of error in part neutralise one another. Some make
our resulting age too long, others make it out too short. But we
do not know if a balance of error does not still remain. Here,
however, is a table of deposits which summarises a great deal of
our knowledge of the thickness of the stratigraphical
accumulations. It is due to Sollas.[1]

Feet.

Recent and Pleistocene  - -    4,000
Pliocene                 - -   13,000
Miocene                  - -   14,000
Oligocene                - -     2,000
Eocene                   - -  20,000
                              63,000

Upper Cretaceous        - -   24,000
Lower Cretaceous        - -   20,000
Jurassic                 - -    8,000
Trias                    - -   7,000
                              69,000

Permian                  - -    2,000
Carboniferous           - -    29,000
Devonian                 - -   22,000
                              63,000

Silurian                 - -   15,000
Ordovician              - -   17,000
Cambrian                 - -   6,000
                              58,000

Algonkian—Keeweenawan   - -   50,000
Algonkian—Animikian     - -   14,000
Algonkian—Huronian      - -   18,000
                              82,000

Archæan                   - -   ?
                                   
Total                     - - 335,000 feet.

[1] Address to the Geol. Soc. of London, 1509.

6

In the next place we require to know the average rate at which
these rocks were laid down. This is really the weakest link in
the chain. The most diverse results have been arrived at, which
space does not permit us to consider. The value required is most
difficult to determine, for it is different for the different
classes of material, and varies from river to river according to
the conditions of discharge to the sea. We may probably take it
as between two and six inches in a century.

Now the total depth of the sediments as we see is about 335,000
feet (or 64 miles), and if we take the rate of collecting as
three inches in a hundred years we get the time for all to
collect as 134 millions of years. If the rate be four inches, the
time is soo millions of years, which is the figure Geikie
favoured, although his result was based on somewhat different
data. Sollas most recently finds 80 millions of years.[1]

THE AGE AS INFERRED FROM THE MASS OF THE SEDIMENTS

In the above method we obtain our result by the measurement of
the linear dimensions of the sediments. These measurements, as we
have seen, are difficult to arrive at. We may, however, proceed
by measurements of the mass of the sediments, and then the method
becomes more definite. The new method is pursued as follows:

[1] Geikie, _Text Book of Geology_ (Macmillan, 1903), vol. i., p.
73, _et seq._ Sollas, _loc. cit._ Joly, _Radioactivity and Geology_
(Constable, 1909), and Phil. Mag., Sept. 1911.

7

The total mass of the sediments formed since denudation began may
be ascertained with comparative accuracy by a study of the
chemical composition of the waters of the ocean. The salts in the
ocean are undoubtedly derived from the rocks; increasing age by
age as the latter are degraded from their original character
under the action of the weather, etc., and converted to the
sedimentary form. By comparing the average chemical composition
of these two classes of material—the primary or igneous rocks and
the sedimentary—it is easy to arrive at a knowledge of how much
of this or that constituent was given to the ocean by each ton of
primary rock which was denuded to the sedimentary form. This,
however, will not assist us to our object unless the ocean has
retained the salts shed into it. It has not generally done so. In
the case of every substance but one the ocean continually gives
up again more or less of the salts supplied to it by the rivers.
The one exception is the element sodium. The great solubility of
its salts has protected it from abstraction, and it has gone on
collecting during geological time, practically in its entirety.
This gives us the clue to the denudative history of the
Earth.[1]

The process is now simple. We estimate by chemical examination of
igneous and sedimentary rocks the amount of sodium which has been
supplied to the ocean per ton of sediment produced by denudation.
We also calculate

[1] _Trans. R.D.S._, May, 1899.

8

the amount of sodium contained in the ocean. We divide the one
into the other (stated, of course, in the same units of mass),
and the quotient gives us the number of tons of sediment. The
most recent estimate of the sediments made in this manner affords
56 x 10¹⁶ tonnes.[1]

Now we are assured that all this sediment was transported by the
rivers to the sea during geological time. Thus it follows that,
if we can estimate the average annual rate of the river supply of
sediments to the ocean over the past, we can calculate the
required age. The land surface is at present largely covered with
the sedimentary rocks themselves. Sediment derived from these
rocks must be regarded as, for the most part, purely cyclical;
that is, circulating from the sea to the land and back again. It
does not go to increase the great body of detrital deposits. We
cannot, therefore, take the present river supply of sediment as
representing that obtaining over the long past. If the land was
all covered still with primary rocks we might do so. It has been
estimated that about 25 per cent. of the existing continental
area is covered with archæan and igneous rocks, the remainder
being sediments.[2] On this estimate we may find valuable

[1] Clarke, _A Preliminary Study of Chemical Denudation_
(Washington, 1910). My own estimate in 1899 (_loc. cit._) made as a
test of yet another method of finding the age, showed that the
sediments may be taken as sufficient to form a layer 1.1 mile
deep if spread uniformly over the continents; and would amount to
64 x 10¹⁸ tons.

[2] Van Tillo, _Comptes Rendues_ (Paris), vol. cxiv., 1892.

9

major and minor limits to the geological age. If we take 25 per
cent. only of the present river supply of sediment, we evidently
fix a major limit to the age, for it is certain that over the
past there must have been on the average a faster supply. If we
take the entire river supply, on similar reasoning we have what
is undoubtedly a minor limit to the age.

The river supply of detrital sediment has not been very
extensively investigated, although the quantities involved may be
found with comparative ease and accuracy. The following table
embodies the results obtained for some of the leading rivers.[1]

               Mean annual   Total annual   Ratio of
               discharge in  sediment in    sediment
               cubic feet    thousands      to water
               per second.   of tons.       by weight.

Potomac -        20,160          5,557        1 : 3.575
Mississippi -   610,000       406,250        1 : 1,500
Rio Grande -      1,700          3,830        1 : 291
Uruguay -       150,000         14,782       1 : 10,000
Rhone -          65,850         36,000       1 : 1,775
Po -             62,200         67,000       1 : 900
Danube -        315,200        108,000       1 : 2,880
Nile -          113,000         54,000       1 : 2,050
Irrawaddy -     475,000       291,430        1 : 1,610

Mean -          201,468        109,650       1 : 2,731

We see that the ratio of the weight of water to the

[1] Russell, _River Development_ (John Murray, 1888).

10

weight of transported sediment in six out of the nine rivers does
not vary widely. The mean is 2,730 to 1. But this is not the
required average. The water-discharge of each river has to be
taken into account. If we ascribe to the ratio given for each
river the weight proper to the amount of water it discharges, the
proportion of weight of water to weight of sediment, for the
whole quantity of water involved, comes out as 2,520 to 1.

Now if this proportion holds for all the rivers of the
world—which collectively discharge about 27 x 1012 tonnes of
water per annum—the river-born detritus is 1.07 x 1010 tonnes. To
this an addition of 11 per cent. has to be made for silt pushed
along the river-bed.[1] On these figures the minor limit to the
age comes out as 47 millions of years, and the major limit as 188
millions. We are here going on rather deficient estimates, the
rivers involved representing only some 6 per cent. of the total
river supply of water to the ocean. But the result is probably
not very far out.

We may arrive at a probable age lying between the major and minor
limits. If, first, we take the arithmetic mean of these limits,
we get 117 millions of years. Now this is almost certainly
excessive, for we here assume that the rate of covering of the
primary rocks by sediments was uniform. It would not be so,
however, for the rate of supply of original sediment must have
been continually diminishing

[1] According to observations made on the Mississippi (Russell,
_loc. cit._).

11

during geological time, and hence we may assume that the rate of
advance of the sediments on the primary rocks has also been
diminishing. Now we may probably take, as a fair assumption, that
the sediment-covered area was at any instant increasing at a rate
proportionate to the rate of supply of sediment; that is, to the
area of primary rocks then exposed. On this assumption the age is
found to be 87 millions of years.

THE AGE BY THE SODIUM OF THE OCEAN

I have next to lay before you a quite different method. I have
already touched upon the chemistry of the ocean, and on the
remarkable fact that the sodium contained in it has been
preserved, practically, in its entirety from the beginning of
geological time.

That the sea is one of the most beautiful and magnificent sights
in Nature, all admit. But, I think, to those who know its story
its beauty and magnificence are ten-fold increased. Its saltness
it due to no magic mill. It is the dissolved rocks of the Earth
which give it at once its brine, its strength, and its buoyancy.
The rivers which we say flow with "fresh" water to the sea
nevertheless contain those traces of salt which, collected over
the long ages, occasion the saltness of the ocean. Each gallon of
river water contributes to the final result; and this has been
going on since the beginning of our era. The mighty total of the
rivers is 6,500 cubic miles of water in the year!

12

There is little doubt that the primeval ocean was in the
condition of a fresh-water lake. It can be shown that a primitive
and more rapid solution of the original crust of the Earth by the
slowly cooling ocean would have given rise to relatively small
salinity. The fact is, the quantity of salts in the ocean is
enormous. We are only now concerned with the sodium; but if we
could extract all the rock-salt (the chloride of sodium) from the
ocean we should have enough to cover the entire dry land of the
Earth to a depth of 400 feet. It is this gigantic quantity which
is going to enter into our estimate of the Earth's age. The
calculated mass of sodium contained in this rock-salt is 14,130
million million tonnes.

If now we can determine the rate at which the rivers supply
sodium to the ocean, we can determine the age.[1] As the result
of many thousands of river analyses, the total amount of sodium
annually discharged to the ocean

[1] _Trans. R.D.S._, 1899. A paper by Edmund Halley, the
astronomer, in the _Philosophical Transactions of the Royal
Society_ for 1715, contains a suggestion for finding the age of
the world by the following procedure. He proposes to make
observations on the saltness of the seas and ocean at intervals
of one or more centuries, and from the increment of saltness
arrive at their age. The measurements, as a matter of fact, are
impracticable. The salinity would only gain (if all remained in
solution) one millionth part in Too years; and, of course, the
continuous rejection of salts by the ocean would invalidate the
method. The last objection also invalidates the calculation by T.
Mellard Reade (_Proc. Liverpool Geol. Soc._, 1876) of a minor limit
to the age by the calcium sulphate in the ocean. Both papers were
quite unknown to me when working out my method. Halley's paper
was, I think, only brought to light in 1908.

13

by all the rivers of the world is found to be probably not far
from 175 million tonnes.[1] Dividing this into the mass of
oceanic sodium we get the age as 80.7 millions of years. Certain
corrections have to be applied to this figure which result in
raising it to a little over 90 millions of years. Sollas, as the
result of a careful review of the data, gets the age as between
80 and 150 millions of years. My own result[2] was between 80 and
90 millions of years; but I subsequently found that upon certain
extreme assumptions a maximum age might be arrived at of 105
millions of years.[3] Clarke regards the 80.7 millions of years
as certainly a maximum in the light of certain calculations by
Becker.[4]

The order of magnitude of these results cannot be shaken unless
on the assumption that there is something entirely misleading in
the existing rate of solvent denudation. On the strength of the
results of another and

[1] F. W. Clarke, _A Preliminary Study of Chemical Denudation_
(Smithsonian Miscellaneous Collections, 1910).

[2] _Loc. cit._

[3] "The Circulation of Salt and Geological Time" (Geol. Mag.,
1901, p. 350).

[4] Becker (loc. cit.), assuming that the exposed igneous and
archæan rocks alone are responsible for the supply of sodium to
the ocean, arrives at 74 millions of years as the geological age.
This matter was discussed by me formerly (Trans. R.D.S., 1899,
pp. 54 _et seq._). The assumption made is, I believe, inadmissible.
It is not supported by river analyses, or by the chemical
character of residual soils from sedimentary rocks. There may be
some convergence in the rate of solvent denudation, but—as I
think on the evidence—in our time unimportant.

14

entirely different method of approaching the question of the
Earth's age (which shall be presently referred to), it has been
contended that it is too low. It is even asserted that it is from
nine to fourteen times too low. We have then to consider whether
such an enormous error can enter into the method. The
measurements involved cannot be seriously impugned. Corrections
for possible errors applied to the quantities entering into this
method have been considered by various writers. My own original
corrections have been generally confirmed. I think the only point
left open for discussion is the principle of uniformitarianism
involved in this method and in the methods previously discussed.

In order to appreciate the force of the evidence for uniformity
in the geological history of the Earth, it is, of course,
necessary to possess some acquaintance with geological science.
Some of the most eminent geologists, among whom Lyell and
Geikie[1] may be mentioned, have upheld the doctrine of
uniformity. It must here suffice to dwell upon a few points
having special reference to the matter under discussion.

The mere extent of the land surface does not, within limits,
affect the question of the rate of denudation. This arises from
the fact that the rain supply is quite insufficient to denude the
whole existing land surface. About 30 per cent. of it does not,
in fact, drain to the

[1] See especially Geikie's Address to Sect. C., Brit. Assoc.
Rep., 1399.

15

ocean. If the continents become invaded by a great transgression
of the ocean, this "rainless" area diminishes: and the denuded
area advances inwards without diminution. If the ocean recedes
from the present strand lines, the "rainless" area advances
outwards, but, the rain supply being sensibly constant, no change
in the river supply of salts is to be expected.

Age-long submergence of the entire land, or of any very large
proportion of what now exists, is negatived by the continuous
sequence of vast areas of sediment in every geologic age from the
earliest times. Now sediment-receiving areas always are but a
small fraction of those exposed areas whence the sediments are
supplied.[1] Hence in the continuous records of the sediments we
have assurance of the continuous exposure of the continents above
the ocean surface. The doctrine of the permanency of the
continents has in its main features been accepted by the most
eminent authorities. As to the actual amount of land which was
exposed during past times to denudative effects, no data exist to
show it was very different from what is now exposed. It has been
estimated that the average area of the North American continent
over geologic time was about eight-tenths of its existing
area.[2] Restorations of other continents, so far as they have
been attempted, would not

[1] On the strength of the Mississippi measurements about 1 to 18
(Magee, _Am. Jour. of Sc._, 1892, p. 188).

[2] Schuchert, _Bull. Geol. Soc. Am._, vol. xx., 1910.

16

suggest any more serious divergency one way or the other.

That climate in the oceans and upon the land was throughout much
as it is now, the continuous chain of teeming life and the
sensitive temperature limits of protoplasmic existence are
sufficient evidence.[1] The influence at once of climate and of
elevation of the land may be appraised at their true value by the
ascertained facts of solvent denudation, as the following table
shows.

                 Tonnes removed in      Mean elevation.
                 solution per square    Metres.
                 mile per annum.
North America -         79                  700
South America -         50                  650
Europe -                100                  300
Asia -                   84                  950
Africa -                 44                  650

In this table the estimated number of tonnes of matter in
solution, which for every square mile of area the rivers convey
to the ocean in one year, is given in the first column. These
results are compiled by Clarke from a very large number of
analyses of river waters. The second column of the table gives
the mean heights in metres above sea level of the several
continents, as cited by Arrhenius.[2]

Of all the denudation results given in the table, those relating
to North America and to Europe are far the

[1] See also Poulton, Address to Sect. D., Brit. Assoc. Rep.,
1896.

[2] _Lehybuch dev Kosmischen Physik_, vol. i., p. 347.

17

most reliable. Indeed these may be described as highly reliable,
being founded on some thousands of analyses, many of which have
been systematically pursued through every season of the year.
These show that Europe with a mean altitude of less than half
that of North America sheds to the ocean 25 per cent. more salts.
A result which is to be expected when the more important factors
of solvent denudation are given intelligent consideration and we
discriminate between conditions favouring solvent and detrital
denudation respectively: conditions in many cases
antagonistic.[1] Hence if it is true, as has been stated, that we
now live in a period of exceptionally high continental elevation,
we must infer that the average supply of salts to the ocean by
the rivers of the world is less than over the long past, and
that, therefore, our estimate of the age of the Earth as already
given is excessive.

There is, however, one condition which will operate to unduly
diminish our estimate of geologic time, and it is a condition
which may possibly obtain at the present time. If the land is, on
the whole, now sinking relatively to the ocean level, the
denudation area tends, as we have seen, to move inwards. It will
thus encroach upon regions which have not for long periods
drained to the ocean. On such areas there is an accumulation of
soluble salts which the deficient rivers have not been able to
carry to the ocean. Thus the salt content of certain of

[1] See the essay on Denudation.

18

the rivers draining to the ocean will be influenced not only by
present denudative effects, but also by the stored results of
past effects. Certain rivers appear to reveal this unduly
increased salt supply those which flow through comparatively arid
areas. However, the flowoff of such tributaries is relatively
small and the final effects on the great rivers apparently
unimportant—a result which might have been anticipated when the
extremely slow rate of the land movements is taken into account.

The difficulty of effecting any reconciliation of the methods
already described and that now to be given increases the interest
both of the former and the latter.

THE AGE BY RADIOACTIVE TRANSFORMATIONS

Rutherford suggested in 1905 that as helium was continually being
evolved at a uniform rate by radioactive substances (in the form
of the alpha rays) a determination of the age of minerals
containing the radioactive elements might be made by measurements
of the amount of the stored helium and of the radioactive
elements giving rise to it, The parent radioactive substances
are—according to present knowledge—uranium and thorium. An
estimate of the amounts of these elements present enables the
rate of production of the helium to be calculated. Rutherford
shortly afterwards found by this method an age of 240 millions of
years for a radioactive mineral of presumably remote age. Strutt,
who carried

19

his measurements to a wonderful degree of refinement, found the
following ages for mineral substances originating in different
geological ages:

Oligocene -               8.4 millions of years.
Eocene -                 31    millions of years.
Lower Carboniferous -  150   millions of years.
Archæan -               750    millions of years.

Periods of time much less than, and very inconsistent with, these
were also found. The lower results are, however, easily explained
if we assume that the helium—which is a gas under prevailing
conditions—escapes in many cases slowly from the mineral.

Another product of radioactive origin is lead. The suggestion
that this substance might be made available to determine the age
of the Earth also originated with Rutherford. We are at least
assured that this element cannot escape by gaseous diffusion from
the minerals. Boltwood's results on the amount of lead contained
in minerals of various ages, taken in conjunction with the amount
of uranium or parent substance present, afforded ages rising to
1,640 millions of years for archæan and 1,200 millions for
Algonkian time. Becker, applying the same method, obtained
results rising to quite incredible periods: from 1,671 to 11,470
millions of years. Becker maintained that original lead rendered
the determinations indefinite. The more recent results of Mr. A.
Holmes support the conclusion that "original" lead may be present
and may completely falsify results derived

20

from minerals of low radioactivity in which the derived lead
would be small in amount. By rejecting such results as appeared
to be of this character, he arrives at 370 millions of years as
the age of the Devonian.

I must now describe a very recent method of estimating the age of
the Earth. There are, in certain rock-forming minerals,
colour-changes set up by radioactive causes. The minute and
curious marks so produced are known as haloes; for they surround,
in ringlike forms, minute particles of included substances which
contain radioactive elements. It is now well known how these
haloes are formed. The particle in the centre of the halo
contains uranium or thorium, and, necessarily, along with the
parent substance, the various elements derived from it. In the
process of transformation giving rise to these several derived
substances, atoms of helium—the alpha rays—projected with great
velocity into the surrounding mineral, occasion the colour
changes referred to. These changes are limited to the distance to
which the alpha rays penetrate; hence the halo is a spherical
volume surrounding the central substance.[1]

The time required to form a halo could be found if on the one
hand we could ascertain the number of alpha rays ejected from the
nucleus of the halo in, say, one year, and, on the other, if we
determined by experiment just how many alpha rays were required
to produce the same

[1] _Phil. Mag._, March, 1907 and February, 1910; also _Bedrock_,
January, 1913. See _Pleochroic Haloes_ in this volume.

21

amount of colour alteration as we perceive to extend around the
nucleus.

The latter estimate is fairly easily and surely made. But to know
the number of rays leaving the central particle in unit time we
require to know the quantity of radioactive material in the
nucleus. This cannot be directly determined. We can only, from
known results obtained with larger specimens of just such a
mineral substance as composes the nucleus, guess at the amount of
uranium, or it may be thorium, which may be present.

This method has been applied to the uranium haloes of the mica of
County Carlow.[1] Results for the age of the halo of from 20 to
400 millions of years have been obtained. This mica was probably
formed in the granite of Leinster in late Silurian or in Devonian
times.

The higher results are probably the least in error, upon the data
involved; for the assumption made as to the amount of uranium in
the nuclei of the haloes was such as to render the higher results
the more reliable.

This method is, of course, a radioactive method, and similar to
the method by helium storage, save that it is free of the risk of
error by escape of the helium, the effects of which are, as it
were, registered at the moment of its production, so that its
subsequent escape is of no moment.

[1] Joly and Rutherford, _Phil. Mag._, April, 1913.

22

REVIEW OF THE RESULTS

We shall now briefly review the results on the geological age of
the Earth.

By methods based on the approximate uniformity of denudative
effects in the past, a period of the order of 100 millions of
years has been obtained as the duration of our geological age;
and consistently whether we accept for measurement the sediments
or the dissolved sodium. We can give reasons why these
measurements might afford too great an age, but we can find
absolutely no good reason why they should give one much too low.

By measuring radioactive products ages have been found which,
while they vary widely among themselves, yet claim to possess
accuracy in their superior limits, and exceed those derived from
denudation from nine to fourteen times.

In this difficulty let us consider the claims of the radioactive
method in any of its forms. In order to be trustworthy it must be
true; (1) that the rate of transformation now shown by the parent
substance has obtained throughout the entire past, and (2) that
there were no other radioactive substances, either now or
formerly existing, except uranium, which gave rise to lead. As
regards methods based on the production of helium, what we have
to say will largely apply to it also. If some unknown source of
these elements exists we, of course, on our assumption
overestimate the age.

23

As regards the first point: In ascribing a constant rate of
change to the parent substance—which Becker (loc. cit.) describes
as "a simple though tremendous extrapolation"—we reason upon
analogy with the constant rate of decay observed in the derived
radioactive bodies. If uranium and thorium are really primary
elements, however, the analogy relied on may be misleading; at
least, it is obviously incomplete. It is incomplete in a
particular which may be very important: the mode of origin of
these parent bodies—whatever it may have been—is different to
that of the secondary elements with which we compare them. A
convergence in their rate of transformation is not impossible, or
even improbable, so far as we known.

As regards the second point: It is assumed that uranium alone of
the elements in radioactive minerals is ultimately transformed to
lead by radioactive changes. We must consider this assumption.

Recent advances in the chemistry of the radioactive elements has
brought out evidence that all three lines of radioactive descent
known to us—_i.e._ those beginning with uranium, with thorium,
and with actinium—alike converge to lead.[1] There are
difficulties in the way of believing that all the lead-like atoms
so produced ("isotopes" of lead, as Soddy proposes to call them)
actually remain as stable lead in the minerals. For one

[1] See Soddy's _Chemistry of the Radioactive Elements_ (Longmans,
Green & Co.).

24

thing there is sometimes, along with very large amounts of
thorium, an almost entire absence of lead in thorianites and
thorites. And in some urano—thorites the lead may be noticed to
follow the uranium in approximate proportionality,
notwithstanding the presence of large amounts of thorium.[1] This
is in favour of the assumption that all the lead present is
derived from the uranium. The actinium is present in negligibly
small amounts.

On the other hand, there is evidence arising from the atomic
weight of lead which seems to involve some other parent than
uranium. Soddy, in the work referred to, points this out. The
atomic weight of radium is well known, and uranium in its descent
has to change to this element. The loss of mass between radium
and uranium-derived lead can be accurately estimated by the
number of alpha rays given off. From this we get the atomic
weight of uranium-derived lead as closely 206. Now the best
determinations of the atomic weight of normal lead assign to this
element an atomic weight of closely

[1] It seems very difficult at present to suggest an end product
for thorium, unless we assume that, by loss of electrons, thorium
E, or thorium-lead, reverts to a substance chemically identical
with thorium itself. Such a change—whether considered from the
point of view of the periodic law or of the radioactive theory
would involve many interesting consequences. It is, of course,
quite possible that the nature of the conditions attending the
deposition of the uranium ores, many of which are comparatively
recent, are responsible for the difficulties observed. The
thorium and uranium ores are, again, specially prone to
alteration.

25

207. By a somewhat similar calculation it is deduced that
thorium-derived lead would possess the atomic weight of 208. Thus
normal lead might be an admixture of uranium- and thorium-derived
lead. However, as we have seen, the view that thorium gives rise
to stable lead is beset with some difficulties.

If we are going upon reliable facts and figures, we must, then,
assume: (a) That some other element than uranium, and genetically
connected with it (probably as parent substance), gives rise, or
formerly gave rise, to lead of heavier atomic weight than normal
lead. It may be observed respecting this theory that there is
some support for the view that a parent substance both to uranium
and thorium has existed or possibly exists. The evidence is found
in the proportionality frequently observed between the amounts of
thorium and uranium in the primary rocks.[1] Or: (b) We may meet
the difficulties in a simpler way, which may be stated as
follows: If we assume that all stable lead is derived from
uranium, and at the same time recognise that lead is not
perfectly homogeneous in atomic weight, we must, of necessity,
ascribe to uranium a similar want of homogeneity; heavy atoms of
uranium giving rise to heavy

[1] Compare results for the thorium content of such rocks
(appearing in a paper by the author Cong. Int. _de Radiologie et
d'Electricité_, vol. i., 1910, p. 373), and those for the radium
content, as collected in _Phil. Mag._, October, 1912, p. 697.
Also A. L. Fletcher, _Phil. Mag._, July, 1910; January, 1911, and
June, 1911. J. H. J. Poole, _Phil. Mag._, April, 1915

26

atoms of lead and light atoms of uranium generating light atoms
of lead. This assumption seems to be involved in the figures
upon, which we are going. Still relying on these figures, we
find, however, that existing uranium cannot give rise to lead of
normal atomic weight. We can only conclude that the heavier atoms
of uranium have decayed more rapidly than the lighter ones. In
this connection it is of interest to note the complexity of
uranium as recently established by Geiger, although in this case
it is assumed that the shorter-lived isotope bears the relation
of offspring to the longer-lived and largely preponderating
constituent. However, there does not seem to be any direct proof
of this as yet.

From these considerations it would seem that unless the atomic
weight of lead in uraninites, etc., is 206, the former complexity
and more accelerated decay of uranium are indicated in the data
respecting the atomic weights of radium and lead[1]. As an
alternative view, we may assume, as in our first hypothesis, that
some elementally different but genetically connected substance,
decaying along branching lines of descent at a rate sufficient to
practically remove the whole of it during geological time,
formerly existed. Whichever hypothesis we adopt

[1] Later investigation has shown that the atomic weight of lead
in uranium-bearing ores is about 206.6 (see Richards and Lembert,
_Journ. of Am. Claem. Soc._, July, 1914). This result gives support
to the view expressed above.

27

we are confronted by probabilities which invalidate
time-measurements based on the lead and helium ratio in minerals.
We have, in short, grave reason to question the measure of
uniformitarianism postulated in finding the age by any of the
known radioactive methods.

That we have much to learn respecting our assumptions, whether we
pursue the geological or the radioactive methods of approaching
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
action of the suspended sand upon the river bed.

A view from an Alpine summit reveals a scene of remarkable
desolation (Pl. V, p. 40). Screes lie piled against the steep
slopes. Cliffs stand shattered and ready to fall in ruins. And
here the forces at work readily reveal themselves. An occasional
wreath of white smoke among

32

the far-off peaks, followed by a rumbling reverberation, marks
the fall of an avalanche. Water everywhere trickles through the
shaly _débris_ scattered around. In the full sunshine the rocks are
almost too hot to bear touching. A few hours later the cold is
deadly, and all becomes a frozen silence. In such scenes of
desolation and destruction, detrital sediments are actively being
generated. As we descend into the valley we hear the deep voice
of the torrents which are continually hurrying the disintegrated
rocks to the ocean.

A remarkable demonstration of the activity of mechanical
denudation is shown by the phenomenon of "earth pillars." The
photograph (Pl. IV.) of the earth pillars of the Val d'Hérens
(Switzerland) shows the peculiar appearance these objects
present. They arise under conditions where large stones or
boulders are scattered in a deep deposit of clay, and where much
of the denudation is due to water scour. The large boulders not
only act as shelter against rain, but they bind and consolidate
by their mere weight the clay upon which they rest. Hence the
materials underlying the boulders become more resistant, and as
the surrounding clays are gradually washed away and carried to
the streams, these compacted parts persist, and, finally, stand
like walls or pillars above the general level. After a time the
great boulders fall off and the underlying clay becomes worn by
the rainwash to fantastic spikes and ridges. In the Val d'Hérens
the earth pillars are formed

33

of the deep moraine stuff which thickly overlies the slopes of
the valley. The wall of pillars runs across the axis of the
valley, down the slope of the hill, and crosses the road, so that
it has to be tunnelled to permit the passage of traffic. It is
not improbable that some additional influence—possibly the
presence of lime—has hardened the material forming the pillars,
and tended to their preservation.

Denudation has, however, other methods of work than purely
mechanical; methods more noiseless and gentle, but not less
effective, as the victories of peace ate no less than those of
war.

Over the immense tracts of the continents chemical work proceeds
relentlessly. The rock in general, more especially the primary
igneous rock, is not stable in presence of the atmosphere and of
water. Some of the minerals, such as certain silicates and
carbonates, dissolve relatively fast, others with extreme
slowness. In the process of solution chemical actions are
involved; oxidation in presence of the free oxygen of the
atmosphere; attack by the feeble acid arising from the solution
of carbon dioxide in water; or, again, by the activity of certain
acids—humous acids—which originate in the decomposition of
vegetable remains. These chemical agents may in some instances,
_e.g._ in the case of carbonates such as limestone or
dolomite—bring practically the whole rock into solution. In other
instances—_e.g._ granites, basalts, etc.—they may remove some of
the

34

constituent minerals completely or partially, such as felspar,
olivine, augite, and leave more resistant substances to be
ultimately washed down as fine sand or mud into the river.

It is often difficult or impossible to appraise the relative
efficiency of mechanical and chemical denudation in removing the
materials from a certain area. There can be, indeed, little doubt
that in mountainous regions the mechanical effects are largely
predominant. The silts of glacial rivers are little different
from freshly-powdered rock. The water which carries them but
little different from the pure rain or snow which falls from the
sky. There has not been time for the chemical or solvent actions
to take place. Now while gravitational forces favour sudden shock
and violent motions in the hills, the effect of these on solvent
and chemical denudation is but small. Nor is good drainage
favourable to chemical actions, for water is the primary factor
in every case. Water takes up and removes soluble combinations of
molecules, and penetrates beneath residual insoluble substances.
It carries the oxygen and acids downwards through the soils, and
finally conveys the results of its own work to the rivers and
streams. The lower mean temperature of the mountains as well as
the perfect drainage diminishes chemical activities.

Hence we conclude that the heights are not generally favourable
to the purely solvent and chemical actions. It is on the
lower-lying land that soils tend to accumulate,

35

and in these the chief solvent and the chief chemical denudation
of the Earth are effected.

The solvent and chemical effects which go on in the
finely-divided materials of the soils may be observed in the
laboratory. They proceed faster than would be anticipated. The
observation is made by passing a measured quantity of water
backwards and forwards for some months through a tube containing
a few grammes of powdered rock. Finally the water is analysed,
and in this manner the amount of dissolved matter it has taken up
is estimated. The rock powder is examined under the microscope in
order to determine the size of the grains, and so to calculate
the total surface exposed to the action of the water. We must be
careful in such experiments to permit free oxidation by the
atmosphere. Results obtained in this way of course take no
account of the chemical effects of organic acids such as exist in
the soils. The quantities obtained in the laboratory will,
therefore, be deficient as compared with the natural results.