The structure of the earth

THE STRUCTURE
OFTHE EARTH

T- G-BONNEY,Sc.D.,F-R-S

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THE STRUCTURE OF THE EARTH

THE STRUCTURE OF
THE EARTH

BY T. G. BONNET, Sc.D., F.R.S.

PAST PRESIDENT OF THE GEOLOGICAL SOCIETY AND THE BRITISH
ASSOCIATION, FELLOW OF ST. JOHN'S COLLEGE, CAMBRIDGE

LONDON: T. C. & E. C. JACK
67 LONG ACRE, W.C., AND EDINBURGH
NEW YORK: DODGE PUBLISHING CO.

CONTENTS

CHAP. PAGE

I. THE PROBLEMS AND METHODS OF GEOLOGY. 7

ii. THE EARTH'S CONSTITUTION AND AGE . . 13

III. THE WORK OF HEAT AND COLD ... 34

IV. THE WORK OF- RAIN AND RUNNING WATER . 39
V. THE WORK OF SNOW AND ICE . . 53

VI. THE WORK OF THE SEA .... 62

VII. VOLCANOES AND THEIR LESSONS ... 68

VIII. MOVEMENTS OF LAND AND THEIR RESULTS . 74

IX. THE LIFE HISTORY OF THE EARTH . . 84

BIBLIOGRAPHY 89

INDEX 92

THE STRUCTURE OF THE EARTH

CHAPTER I

THE PROBLEMS AND METHODS OP GEOLOGY

GEOLOGY may be defined as the endeavour to answer
the question, What is the past history of the planet on
which we are living ? As a science it is the outcome
of reasoning inductively from observations. To a cer-
tain, but only a limited, extent, its conclusions can be
verified by experiment, so that, as a rule, its hypotheses
must be tested by ascertaining whether they accord
with facts, and especially those gathered by extending
the field of observation. Many of the mistakes which
have been made in the past, some perhaps which are
even now current, are the result of generalisation from
insufficient knowledge. Hypotheses founded on experi-
ence restricted to one's own back garden are as mis-
chievous in science as they are in politics. As the late
Sir Charles Lyell rightly said, travel is the first and the
second and the third thing necessary in the education
of a geologist, provided he starts with knowledge suffi-
cient to enable him to understand what he sees. Hasty
generalisations, no less than efforts to force the results
of observation into harmony with preconceived hypo-
theses, were for long baneful to the science. It is the
duty of all who desire to be its students to be ever on
the watch, endeavouring to observe accurately and to
reason soundly, to ascertain new facts and apply them
to test accepted conclusions.

One or two simple examples may serve to show the

8 THE STRUCTURE OF THE EARTH

nature of geological problems, and how they present
themselves to anyone who goes about the world with
his eyes open. Some coasts are rock-bound, the waves
even at low tide breaking against rugged clifis, but on
others we can walk then — and sometimes even at high
water — on low banks of shingle or stretches of sand.
Whence have these come ? What has shaped the
pebbles of the one or gathered the grains of the other ?
But we can find pebbles and sand, not only on the sea-
shore but also in the beds of rivers, especially when
their streams are generally strong. Here the same
question is presented in a slightly different form ; and
if we are not content to answer it by saying that the
gravel, sand, or mud were created as they now are,
they must have a history. What that has been it is
our business to ascertain.

Again, the surface of the ground is seldom a dead-
level like a billiard table. Here it is gently undulating ;
there it forms distinct hills. These are often separated
by valleys, which as a rule broaden and diminish in
slope as we pass outward from the higher land. If,
however, as is usual, we approach this in the opposite
direction, we commonly find that the outlines of the
scenery become bolder. The hills are more rugged in
aspect ; their slopes are interrupted by crags ; the
valleys are sometimes bordered by clifis, and we begin
to notice a connection between the nature of the rocks
and the forms which they assume. In another place
we may be travelling towards a mountain range. It
rises against the sky with a sharp and serrate crest,
which becomes yet grander as we approach. We enter
it to find scenery on a larger and more impressive scale
than in any lowland hills : glens and gorges, torrents
and waterfalls, shattered ridges and towering peaks.
What is the reason, the explanation, of these ? Have
the undulating uplands and rounded downs of Kent
and Sussex, the dales and craggy hills of Derbyshire,
the peaks and tarns of North Wales, been as we now
see them since the beginning of the seventh day of
Creation, or have they been shaped in the course of

PROBLEMS AND METHODS 9

countless ages by the slow but unceasing action of
natural forces ? These instances may serve as examples
of one large group of questions — those connected with
the physical history of the earth.

But there is another group which presents itself in
many places. Let us take one of the most striking
instances, and transfer ourselves *n imagination to Alum
Bay in the Isle of Wight. Those singular vertical layers
of diverse-coloured sands call for an explanation, but
let us pass them by for the moment and mount the
rough slopes of Headon Hill on the northern side. Very
soon we find ourselves treading upon shells, in some
respects resembling, in others differing from, those now
to be found in our own or in other countries. Their
condition and aspect suggests that they have been long
dead : they do not correspond in form with those which
we know to be still living. When first we find them
they may be lying loose on the surface, but a little
search, as we mount upwards, shows that they were
once embedded in the successive beds of marl or soft
limestone, over which we pass in mounting upwards.
In one place we find some that so closely resemble those
now living in seas that we feel sure they must have had
their home in salt water ; yet, as their burial-ground is
far above the reach of the waves in even the wildest
storms, they must have lived and died where they are
found. In another place the shells resemble those still
living in streams or lakes, and by degrees we begin to
perceive that the remains found in the different beds
suggest alternations of fresh-water, or estuarine, or
marine conditions. Other places present similar prob-
lems, though in diverse forms, and when we have suc-
ceeded in solving them we find that we have deciphered
a few pages of the earth's history, and begin to wonder
whether it contains many chapters, perchance even
volumes.

Perhaps it was such an occurrence as this of the
relics of a dead past which first set men to consider
what could be their meaning. Certain it is that the
question was asked — probably not for the first tune —

10 THE STRUCTURE OF THE EARTH

more than five-and-twenty centuries ago. In fact
thoughtful men, even if they lived in districts where
fossils are not found, could hardly have avoided asking
themselves the question whether the earth had a be-
ginning, and if so, how it began ? Such questions in
regard to the origin of things generally form a part of
Oriental systems of philosophy and may go back to a
remote antiquity, but, even if they were at first sug-
gested by observation, the treatment of them and its
results have been metaphysical rather than scientific.
But the more practical West took a better course. We
find the results of induction from observation clearly
stated in the writings of Ovid, who professes, probably
with good reason, to represent the opinions of Pytha-
goras,1 who, so far as we know, was one of the first in
Europe to make inductive reasoning a part of his phi-
losophy. He taught his disciples that land had been
converted into sea, and sea had overwhelmed the land,
that valleys had been excavated by running water,
rivers had altered their channels, plains been upheaved
into hills, volcanoes broken out, and other important
changes made in the surface of the earth. In fact,
during the great days of Rome, a fair amount of real
knowledge had been acquired in regard to geology,
though it was mingled with many quaint and erroneous
notions. But culture and learning were submerged at
the fall of the Empire by the invading flood of ignor-
ance and barbarism, and the ultimate triumph of
Christianity unfortunately restricted that liberty of
investigation which paganism had permitted. Though
the advocates of the former had little love for the
Jew, they believed his scriptures to be conclusive in
matters of cosmogony ; thus to dispute the literal accu-
racy of the statements in Genesis was to incur the
censure of the Church, and that, from the seventh to
the fifteenth century, was no trifling matter. On this
subject the Protestant held opinions more definite than
the Roman Catholic, and the former, though less able
to do bodily harm to the supposed heretic, was ready
t1 Pythagoras, cir. 540-510 B.C. ; Ovid, 43 B.C.-A.D. 18.

PROBLEMS AND METHODS 11

to inflict all penalties in his power, or at any rate was
hardly less hostile in spirit to an interference with
traditional opinions. Geologists, even in the earlier
part of the last century, were often vehemently de-
nounced from platform and pulpit, and in its sixth
and seventh decades " the drum ecclesiastic " was
beaten, probably for the last time, vigorously, but in
vain against evolution.

When the path of induction entailed discredit, if not
danger, a desire to escape from its conclusions tempted
geologists, though perhaps unconsciously, to seek refuge
in hypotheses more or less fantastic. One of these cut
the Gordian knot by affirming that fossils were not the
relics of creatures which once had lived, but were " sports
of nature " — mere imitative forms, like the supposed
moss in agate or a portrait in a piece of jasper. Some
persons — but perhaps it would be unjust to number
them among geologists — even went so far as to declare
that fossils were traps, set by the Almighty to ensnare
presumptuous and over-curious inquirers into the earth's
past history. Others considered them to be proof that
nature had been trying her prentice hand in making
models of creatures, which were presently to be animated
with life, before undertaking the more serious work, an
idea which was not without its sturdy advocates, even
when the Royal Society was founded in the days of
Charles II.

Others found a way out of the difficulty by regarding
all fossils as relics of the Noachian deluge. Indeed, at
one time they were considered to be such strong proof
of the accuracy of the book Genesis that Voltaire
attempted to discredit their evidence by suggesting tbs/u
sea-shells which had been discovered, especially in the
Alps, had been accidentally lost by pilgrims to certain
shrines, who had brought them from these places as
sacred souvenirs. That idea was of course too absurd
to win many disciples, but the appeal to the Noachian
deluge found some favour little more than a century
ago, and though a modification of this idea has been
since then occasionally advocated as an explanation of

12 THE STRUCTURE OF THE EARTH

particular phenomena, it would now be unanimously
discarded as accounting for the general distribution of
fossils.

Further study shows that these (we will speak for
the moment of molluscs only) differ not only in their
shapes, but also in their states and modes of preserva-
tion. Some of those from Headon Hill remind us of
species, which, as we can learn from collections in any
good museum, have inhabited rivers or lakes ; others
embedded in the sandy clays which are disclosed at
low tides in Bracklesham Bay are like those which now
live in salt water, and a closer study shows us that
they resemble those now found, not in British but in
tropical seas. The shells also from the latter place
are, as a rule, more friable than those from the former,
having lost a larger proportion of their organic cement.
Both differences, but especially the former, suggest that
the Bracklesham molluscs lived at an earlier date in
the world's history. Again, if we examine the chalk
which forms hill ranges in the Isle of Wight and in
Sussex, we find that it contains shells still more dif-
ferent, both in kind and in mineral condition, from those
now in existence. Not a few of them belong to genera
which do not now live in any part of the globe, and in
most of them the calcareous material of the shell has not
onlyparted with its organic constituent, but also assumed
a crystalline condition. In short, we find, as we pursue
our researches, that the divergence of form and struc-
ture from stiD living organisms becomes, as a rule, yet
more marked as we extend our observation to greater
depths from the surface, and that these dead and gone
organisms in some cases, instead of being converted into
crystalline calcite, exist only as casts in the hardened
rock, or have been replaced by some mineral different
from the original one.

Further examination shows, as we shall presently see,
that the more widely the fossils embedded in a rock
depart from remains of creatures which are still living,
the more ancient that rock will be, and that a study of
the life history of the earth discloses a progress and

THE EARTH'S CONSTITUTION 13

suggests that this is by an evolution, more or less
gradual, rather than by new creations after occasional
destructions.

These instances may suffice to indicate the nature of
the problems presented to the geologist. Both they,
and the methods adopted in solving them, bear some
resemblance to those employed in recovering the history
of a nation whose annals, language, and even its alphabet
have been forgotten. The investigator looks below the
surface of the ground, lays bare the sites of buried
cities, observes the sequence of their ruined foundations
and of other relics, collects every fragment of an inscrip-
tion, and then sets to work by patient research and
repeated comparison of symbol with symbol, of group
with group, by investigations in languages probably
germane to that which has been lost, to recover its
alphabet, its words with their significance and connec-
tion, and at last, as has been done with the hieroglyphs
of Egypt and the cuneiform characters of Assyria, to
reconstruct a history and bring into life a long-forgotten
past. In geology, no less than in archaeology, there are
problems still awaiting solution, but in the one science
no less than in the other we are justified in asserting
that we have obtained a fairly accurate idea of what
has happened in the history both of an ancient people
and of the earth itself, though the latter has extended
over millions of years, and only its final paragraphs
could have been recorded by man.

CHAPTER II

THE earth's shape is very nearly a spheroid, the
polar diameter of which is 7899-1 miles and the equa-
torial 7925-6. As the difference between these is
26*5 miles, the maximum thickness of the equatorial
protuberance, as its gradual departure from a truly

14 THE STRUCTURE OF THE EARTH

spherical form is often called, amounts to rather more
than 13 miles. Strictly speaking, as we shall presently
see, this statement is not quite accurate, but it is suffi-
ciently so for ordinary purposes. The earth, then, may
be defined as a huge ball, partially covered by water
(lakes, seas, and oceans), and wholly enveloped in an
atmosphere that may extend, though in a very
attenuated state, to something like 500 miles above it.
The earth's surface is far from even, though in some
parts which are called plains it is almost level ; other
parts, which are at a greater elevation above the sea,
and are more or less worn into valleys, are called plateaux,
while others are diversified by hills or wrinkled into
mountains. To these reference will be made in later
chapters ; at present it will suffice to say that the
highest summit among the last-named (Mount Everest)
is just over 29,000 feet, several other peaks ranging
between that and 20,000 feet. If all the seas and
oceans were dried up, the part of the crust thus dis-
closed would exhibit irregularities somewhat different
in form and on a rather larger scale, for the submarine
contours are less sharply accentuated than those above
water. It has often been said that, so far as the
gradients go, it would be possible to drive from Valentia
to Newfoundland without putting on the drag, except
perhaps off the Irish coast. In fact, if a cast were
made of the part of the crust now beneath the several
oceans, it would present us, when laid open to view,
with a series of gently shelving plains and vast plateaux,
hardly anywhere assuming a mountainous aspect, though
the maximum elevation would exceed that of any point
on the present land by about 1900 feet.1 The average
depth of the ocean is about 2J miles more than the
average height of the continental land above it, and the

1 The deepest sounding yet obtained is 5155 fathoms (to the east
of the Kermadec Islands) in the South- West Pacific, and several
soundings in that ocean range between 4000 and 5000 fathoms.
The greatest depth obtained in the Atlantic (to the north of the
West Indies) is 4660 fathoms, the largest part of that ocean being
not so much as 3000 fathoms, while the Indian Ocean nowhere
reaches 3300 fathoms (H. R. Mill, International Geography, ch. vi.).

THE EARTH'S CONSTITUTION 15

ratio of the surface of the one to that of the other is
about 72 to 28. It has also been remarked that if
London be taken as the centre of a hemisphere, this
contains far the largest portion of the land surface of
the globe— the whole of Europe and North America,
nearly all Asia and the greater part of South America ;
the remainder of the last, the Antarctic land, and
Australia being the only areas of importance in the
other one. This unequal distribution of land and
water can hardly be fortuitous, and we may refer to
it again. One or two other peculiarities of grouping
have also been noticed, which, if only accidental coin-
cidences, are certainly peculiar ; such as the grouping
of the continental and insular shores of the Arctic
Ocean, which seem to lie along a circular curve inclined,
in the direction of Bearing Strait, at about 5° to
one of latitude. Again, a similar curve, inclined at
about 10° in the direction of Paris, would pass through
the greater part of the inland seas or great lakes of the
Old and New Worlds. Another circle, the normal to
which makes an angle of nearly 20° with the polar axis,
passes through the Isthmus of Panama (the lowest point
in the watershed of the two Americas) and crosses
almost all the great deserts of the Old World.1

The globe revolves once in 23 hours 56 minutes about
its shorter axis. This statement is not quite accurate,
for its axis of rotation varies slightly in position from
time to time ; but the deviation is not cumulative, and
is so slight that we cannot regard it as even sufficiently
important to make any sensible alteration in the climate
of this or that place. The earth also revolves in an
ellipse about the sun, which is situated in one of the
foci, and the plane of this is inclined at an angle of
nearly 23J° to the equatorial plane of the other. This
inclination causes the changes in the length of the
day, in climate, and in other matters, for which
we must refer our readers to some treatise on
astronomy.

1 E. Reclus, The Earth (translated by H. Woodward), Part II.
ch. vii.

16 THE STRUCTURE OF THE EARTH

The mean distance of the earth from the sun is about
92,800,000 miles ; the minimum distance being 91 , 100,000
miles and the maximum 94,600,000 miles. It is so dim-
cult to grasp the significance of such vast figures that
we may venture on a rough illustration, in the hope of
giving some idea of the relative distances and sizes of
the different members of the solar system. Suppose
the sun to be represented by a globe two feet in diameter
and the orbits of the planets by circles, Mercury would
be a grain of mustard seed and the radius of its circle
82 feet ; Venus a pea, with a radius of 142 feet ; the
Earth another pea, its circle having a radius of 215
feet ; Mars a very small pea, with a radius of 327 feet.
The asteroids may be omitted, for none of them would
be bigger than a grain of sand. Jupiter would be a
moderate-sized orange, Saturn a small one, Uranus a
big cherry or a small plum, and Neptune about the
same size ; while the radius of their several circles would
be a quarter of a mile, two-fifths of a mile, three-quarters
of a mile, and a mile and a quarter. In the case of the
sun we can form some idea of the greatness of its distance
from the earth by remembering that light takes 8 minutes
and 16 seconds to come from it to us,1 and that if a heat-
proof baby were born there and its first squall could be
transmitted to us by some multiple megaphone, it would
be fifteen years old before that sound reached our
ears.

We must abstain from discussing a question so diffi-
cult and controversial as the origin of our planetary
system, and take up the history of the earth at the
stage (about which there is less difference of opinion)
when it had become a glowing mass, possibly molten
at the surface, but perhaps solid in the interior. Liquid
rock would then serve for its ocean, for the present one
obviously could only exist in the state of vapour, and
would thus form part of the atmosphere. One conse-
quence of this is important, as we shall presently see ;
namely, that the pressure upon every square inch of the

1 The velocity of light is about 186,000 miles a second ; that of
sound about 1100 feet in the same time.

THE EARTH'S CONSTITUTION 17

earth's surface, instead of being 14 pounds, would be
about 310 times as great. Gradually as this surface
cooled by radiation, a crust would form upon it, at first
neither uniformly nor simultaneously. This, for some
time, would keep breaking up, and would very probably
sink in the underlying "sea of fire," but such disrup-
tions would gradually become rarer until that sea was
permanently frozen over. After this the huge ball
would continue to cool and its crust to thicken. At
last its surface would cease to glow, and water, precipi-
tated in copious showers from the steamy atmosphere,
would begin to rest upon it. Rivers and seas would now
commence the work which will presently be described,
and as time went on life would become possible for
something more than the fabled salamander. Thus the
sun, the earth, and the moon represent three stages
in the history of a celestial system. The first, so far as
we can ascertain, consists of an intensely heated atmos-
phere of a complex character, in which most, if not
all, of the known constituents of this earth are present
in the state of vapour, and which envelops a great
globe, perhaps solid, less luminous, but also at a very
high temperature. The state of the second planet we
may suppose to be generally known ; the third, usually
regarded as the offspring of the earth's hot youth, is
now waterless and rigid, probably without any internal
heat, but alternately scorched by the untempered rays
of the sun and exposed to the cold of space.1

Apart from other considerations, actual experiment
justifies the inference that the interior of the earth is
still at a high temperature. That of the surface and
of the adjacent atmosphere fluctuate simultaneously,
so that they show not only a rise to a maximum during
the summer and a fall to a minimum in the winter, but
also similar oscillations between day and night and
considerable variations from one day to another. If

1 As the moon turns upon its axis in the same time that it
revolves about the earth, the lunar day is almost a fortnight long.
Its volume is about one-fiftieth that of the earth, and its distance
(from the centre) 238,833 miles.

B

18 THE STRUCTURE OF THE EARTH

the observations be plotted down as curves, say, con-
tinuously during each day and for each day in the
year, we shall find that the curves of the former, if
taken just above and just below the ground, present no
sensible difference, but that their diurnal irregularities
disappear as we descend, and the recording curve
assumes the form of one which in the course of a year
rises to a maximum and sinks to a minimum. This will
occur at a depth of about a yard, after which this curve
also will exhibit a similar flattening-out, till at a depth
of about sixty feet the effect of surface changes is no
longer perceived, and the thermometer remains steady.
But after this, if observations be taken at increasing
depths, the temperature is found to rise. The rate of
this is not the same at all places, or strictly proportionate
to the vertical distances between the points of ob-
servation. Evidently it depends to some extent upon
the nature of the rock penetrated and other local cir-
cumstances ; but in 1882 a committee of the British
Association, after studying all the observations then
available, came to the conclusion that a rise of 1° F.
for each 64 feet of descent was, under ordinary circum-
stances, a fairly accurate estimate.1 For a rough
calculation, however, we may take 1° for 60 feet. With
this rate the temperature at a depth of 6000 feet (rather
more than a mile) below London would be about 150° F.,
and we should read 212° (that at which water boils on
the surface) at a little less than 10,000 feet. Lead
would melt (taking no account of the effect of pressure)
at about 35,000 feet, or rather less than seven miles,
while at a depth of from 25 to 30 miles almost all the
materials of which the earth's crust is composed would

1 The local variations are considerable. Taking depths of at least
1000 feet, a rise of 1° F. was observed for 57 feet in a boring at
Grenelle near Paris, and for 55 feet at Kentish Town, London.
The Sperenberg boring to a depth of 4712 feet, almost wholly in
rock-salt, gave 1° in 51£ feet/while the Scarle boring (Lincolnshire)
gave 69 feet, and a coal-pit at Dukinfield 72 feet. A Bohemian
mine gave 1° in 126 feet, and Bootle waterworks (1392 feet) 1° in
130 feet. The slowest increase on record, so far as I know, was in
a mine near Lake Superior, which gave 1° for 223'7 feet.

THE EARTH'S CONSTITUTION 19

be at a temperature which, on its surface, would suffice
to melt them.

But they may be kept solid, at any rate for a still
further distance, by the tremendous pressure which
they suffer from the weight of the overlying material,
so that both the thickness of the solid crust and the
condition of the earth's interior are questions to which
we cannot at present give a definite answer. Three
opinions have been maintained : that our globe con-
sists of a solid shell, not many miles in thickness, en-
closing a liquid interior ; that it is solid to the centre ;
and that a solid shell is separated from a solid core
by a liquid layer. Mathematicians, reasoning from the
phenomena of the tides and the precession of the equi-
noxes, have inferred that the earth must either be
defended by a very thick shell or be solid throughout,
perhaps with the exception of some great reservoirs
of molten matter. For instance, it was maintained by
W. Hopkins that the solid shell could not be less than
800 miles thick (about one-fifth of the radius), and by
Lord Kelvin that the effective rigidity of the globe as
a whole could hardly be inferior to that of a ball of
steel of the same size, in which case the minimum thick-
ness necessary would be at least half its radius, and it
might well be solid throughout. Delaunay and Henessy,
however, questioned the validity of these conclusions,
and argued in favour of a solid crust considerably less
than 100 miles in thickness. This diversity of opinions,
even among skilled mathematicians, is not really sur-
prising, because in order to obtain numerical results
assumptions have to be made in regard to the con-
ductivity of rocks, the effect of pressure on their melting-
point, the increase of temperature with depth,1 the
critical-points of their materials,2 and the like, which

1 It is generally admitted that the temperature does not increase
at a uniform rate in descending. Lord Kelvin supposed that at
about 80 miles it would become 1° F. for 141 feet; at 160 miles,
1° for 2550 feet, or that the temperature at the centre would be
from 6000° to 7000°.

2 The critical temperature is that beyond which no pressure can
keep a substance liquid. For water this is 689° F.

20 THE STRUCTURE OF THE EARTH

often cannot be precisely determined ; so that, how-
ever impregnable the mathematical reasoning may be,
those results may be far from accurate.

During the last few years, Arrhenius, an eminent
Scandinavian chemist, has put forward a view which
is worthy of careful consideration, since it seems to
explain some of the difficulties which have arisen from
the study of volcanoes and earthquakes. In his opinion
water makes its way by capillarity through the sea-floor
towards the increasingly heated interior of the earth.
At a depth approaching eight miles it would reach a
zone where the temperature was higher than 689° F.,
the critical-point of water, which beyond this must be
in a gaseous condition, and rather before reaching this
temperature it begins to surpass silicic acid in its power
of combining with the bases, which are commonly
associated with this acid, in the earth's magma. It
accordingly decomposes them, setting the other free.
But when any kind of pressure squeezes up the softened
magma into pipes or fissures, this becomes cooled and
the silica displaces the water, which produces explosions.

He also argues that, as recent physical investigations
have shown, the rule which holds with water hi regard
to its critical-point probably applies to all known
substances. It therefore follows that, at great depths,
the constituents of the globe must really be in the
gaseous state, since they are at a temperature which
defies the power of pressure to keep them solid or even
liquid. Hence he concludes that (1) the melting-point
of most rocks would be reached at a depth of about
25 miles. At a considerably greater depth the critical-
point is passed and the magma is in a gaseous state.
Its condition, however, under the great pressure is
altogether different from that of a gas as we know it,
for it is intensely rigid.1 Probably this large inner
nucleus consists of some metallic substance, for the
specific gravity of the earth as a whole is about 5*5
tunes that of water, while most of the rocks which form

1 The molecules of the gas are very closely packed by the
pressure, but are nevertheless too hot to stick together.

THE EARTH'S CONSTITUTION 21

its crust (excluding the ordinary metals) are from about
2-5 to 3-5 as heavy as water.1 Be this as it may, recent
investigations in more than one direction suggest the
existence, at a depth of from 20 to 30 miles from the
surface, of a zone the materials of which are in a very
different condition from that of the overlying crust or
of the interior mass.

We pass on to touch briefly on another very vexed
question — the figure of the earth. The singular group-
ing of the larger areas of land and water has been
already mentioned ; the general tendency of the conti-
nental masses either actually to taper to the south or
to throw out promontories in that direction is another
suggestive fact, so that, though we may be content for
general purposes to regard the earth as a spheroid of
revolution, we are prepared to find that the statement
needs some corrections not altogether unimportant.
In 1878 Colonel Clarke, after a very thorough discussion
of all data then available, came to the conclusion that
the earth's form, instead of being a true spheroid, was
an ellipsoid, in which one of the equatorial diameters
was slightly longer than the other. But five years
earlier Mr. Lowthian Green,2 from more general con-
siderations, had maintained the earth to have more
resemblance to a tetrahedron,3 the edges of which
determined the general position of the continents, and
the faces those of the great oceans. At a later date
than both, Mr. Jeans suggested that the figure was
pear-shaped rather than tetrahedral. If it imitated a
stout example of that fruit, the preponderance of
land in a more northern hemisphere and of ocean in
the other one would be explained, and the Antarctic
land-mass would represent the stalk-end of the pear.
We should anticipate a considerable departure from the
strict outline of a geometrical figure if we suppose the
moon, in accordance with the view of Sir G. H. Darwin

1 For a fuller account see R. H. Rastall, Geol. Mag., 1907, p. 173.

2 Vestiges of a Molten Globe, 1873.

3 A regular figure with four faces, each of which is an equilateral
triangle.

22 THE STRUCTURE OF THE EARTH

and other eminent mathematicians, to have been
flung off from the earth while the latter was in process
of consolidation. The pear-shaped form, according to
Mr. Jeans, was due to an effort on the part of the earth
to dismiss a second satellite, which the increasing con-
solidation prevented it from doing. With some modi-
fication, these views are to a considerable extent both
reconcilable and accordant with the facts ; but it is
impossible to pursue further a subject which, like the
condition of the earth's interior, can only be adequately
discussed by masters in mathematical physics.

Yet one more subject, no less difficult, demands a
brief mention — the age of the earth. When geologists
escaped from the shackles of the Mosaic cosmogony and
the Ussherian chronology, Button's dictum — that the
earth indicated to him neither signs of a beginning nor
symptoms of an end — gained more adherents ; and about
three-quarters of a century ago the Uniformitarian
school, of which Sir Charles LyeU may be regarded as
the prophet, began to command a majority among
geologists, and its disciples showed a speculative dis-
position, as if they had an unlimited credit at the bank
of time. Protests, however, began to be raised, more
especially by students of physics, and about 1867
Professor William Thomson (afterwards Lord Kelvin) de-
clared that the earth's history must be compressed into
100,000,000 years, because the laws of cooling and con-
ductivity, the increase of internal heat and the pheno-
mena of the tides, indicated that, assuming the earth
to have been once molten and to have begun to solidify
on reaching a temperature of 7000° F. (a rather liberal
allowance), this could not have happened much more
than 98,000,000 years ago. At a later date, with a
more intimate knowledge of solar physics, he greatly
reduced this period, maintaining that the sun can
hardly have given out light and heat for more than
about 20,000,000 of years. But in the former case,
as many geologists protested, and still more in the
latter, the results, though the general arguments might
be incontestible, involved several elements of uncer-

THE EARTH'S CONSTITUTION 23

tainty, while still more recently the discovery of radio-
active elements has introduced a new factor which cannot
but modify the above-mentioned conclusions. At the
present moment there is perhaps some tendency to
relapse into spendthrift habits in the matter of time ;
but if we bear in mind how much has still to be learnt
about radio-active substances, and those other difficulties
which have been already mentioned, it will be wiser to
suspend judgment, and be content to affirm that though
the age of the earth is to be measured by millions of
years, it must be very far from so boundless as the
earlier Uniformitarians supposed.

Of late years attempts have been made to test these
estimates of the mathematician and approximate to the
earth's age by evidence more directly geological. Much
of its crust is formed, as we shall presently show, of
materials most of which have been deposited by water.
These — the stratified rocks — have been classified, and
attempts have been made to estimate the average
thickness of their several members and the time which
each would require for its deposition. Both these in-
volve, as we can well imagine, great difficulties, and
only the roughest estimates are possible of either, for
as we shall presently see, when materials derived from
the land are deposited in the sea they generally assume
a wedge-like shape, because the coarser are the first to
come to rest. Again, the record is often not con-
tinuous. Nature's work is destructive as well as con-
structive, and hundreds of feet of rock may have been
removed and again incorporated with some deposit
at quite another place and of a much later date. Hence
the estimates vary considerably, and besides this some
of the most ancient members of the stratified group
present difficulties of their own. Putting these aside
for the moment, and beginning with the earliest deposit,
which presents more than mere traces of organic life,
the several " stone books," the volumes in which the
earth's history is written, are supposed to have a total
thickness of thirty-four miles, and those earlier tomes
in which all but the latest pages are wholly blank may

24 THE STRUCTURE OF THE EARTH

be not much less than sixteen miles. We know no
reason why life should not have been possible on the
earth during most of this latter period, and the remains
of it at the base of the other and larger one show, from
their variety and their position in the ascending scale
of organic life, that it must have begun at a much
earlier date. We have at present no means of esti-
mating the pace of the march of evolution, and it is
not surprising that geologists, impressed with the ap-
parent slowness of present change and the number
and variety of the forms which played their part on
this earth's stage, have felt disposed to demand almost
illimitable time in order to bring the drama to the
scene of which we are the spectators. But hints have
of late been given that its action may sometimes be
quickened, so that the sedimentary rocks, though their
rate of deposit involves many uncertainties, may give
us a little more guidance.

It has been suggested that for these rocks one foot
in a century may not be an unfair estimate for their
average accumulation. If that be so, and we take
their total thickness since the beginning of the Cambrian
Period, when the remains of living creatures are neither
very obscure nor extremely rare, to be 183,000 feet
(an estimate which I think does not err on the side
of parsimony), the total time from the beginning of that
(the Cambrian) period would be only 18,300,000 years.
At present it is very difficult, for reasons on which we
must not dwell, to estimate how much more would be
required for the formation of the underlying strata.
According to one estimate, this would bring the total
thickness up to 266,000 feet, and the time to between
26,000,000 and 27,000,000 years. This, however, must
be largely increased by masses of rocks which once were
stratified, but have since undergone great mineral
changes, and by others the origin of which is more
uncertain ; still, so far as can be inferred from the evi-
dence tendered by the crust of the earth, a hundred
million of years would be ample time, though a fifth of
that time would be quite inadequate.

THE EARTH'S CONSTITUTION 25

Sir G. H. Darwin estimates that the time which has
elapsed since the moon parted from the earth may
be about 56,000,000 years, and obviously both the
formation of sediments and the existence of life would
not be possible till long after this event. Again, Pro-
fessor Joly has approached the problem from quite
another point of view. He assumes that the ocean
originally consisted of fresh water ; its saltness being
due to the dissolved matter which has been carried into
it by the water of rivers. From this he concludes
that about 90,000,000 years have elapsed since the
earth became cool enough to allow water to collect upon
it. But for this calculation also he has been obliged
to admit some factors which may easily be far from
correct, so that his estimate is probably a maximum,
and one which may err considerably on the side of
excess. But at any rate it shows that geologists cannot
complain at being restricted to one hundred million of
years for the story of the earth.1

Before quitting this subject we shall find it con-
venient to give a short account of the composition of
that part of the earth which can be examined. The
original crust must have been formed from materials
once molten, and after this had become solid, any changes
in it must have been due either to external agents or
to the invasion from a lower zone of matter still liquid.
The sedimentary rocks have the former origin. The
latter, called igneous rocks, from their past history,
must be the nearest representatives of the primitive
crust, and will therefore be noticed first, though the
brief space at our disposal does not allow of any
approach to a full description.

These igneous rocks vary much in chemical composi-
tion. They consist of silica, sometimes free and crys-
tallised as quartz, but more commonly in combination
with one or more of the following : alumina, potash,
soda, lime, magnesia, and iron-oxides ; the last also
being sometimes free. They may be arranged in a

1 For a discussion of this and other questions, see W. J Sollas,
The Age of the Earth, p. 21 (1905).

26 THE STRUCTURE OF THE EARTH

graduated series, at one end of which are those con-
taining about 75 per cent, of silica, and at the other
those with about 40 per cent. In the former case
alumina and the alkalies are at least 20 per cent, of
the whole ; from the latter they are almost absent,
magnesia and iron being the dominant constituents.
The condition of the material when cooled depends
partly upon its composition, partly on the circumstances
under which it has become solid. Speaking in very
general terms, we may say that a reaotiness to crystal-
Use is in inverse proportion to a richness in silica ; but
much also depends upon circumstances, such as the rate
of cooling and the pressure under which this occurs.
From the same material, as can be demonstrated by
experiment, may be formed either the transparent glass
of our windows or a white opaque mass of small crystals.
Thus it is possible for any rock to be in either a glassy
or a crystalline condition ; in the latter state, however,
the individual crystals may be large enough to be
fairly conspicuous to the eye, or their size may gradually
diminish till they become indistinguishable, and the
whole mass assumes a " stony " aspect like a piece of
very compact porcelain or one of the non-transparent
glasses. In the latter condition the rock may be either
still crystalline, though the individuals are extremely
minute and confusedly crowded, or may consist of a
vast number of minute crystals crowded together in a
residuum of glass. The investigation of these struc-
tures was not really possible until rather more than
half a century ago, when the microscope was applied
by the late Dr. Clifton Sorby to the examination of
very thin sections of rocks.

Volcanic eruptions, as will be described in a later chap-
ter, bring to the surfaces samples, sometimes on a large
scale, of the molten matter beneath the hardened crust.1

1 It is of course possible that, as the solid and the liquid state
depend upon conditions such as pressure, the amount of water
present, and temperature, which may from time to time be varied,
the material of the inner part of the crust may pass more than
once from the one condition to the other.

THE EARTH'S CONSTITUTION 27

These are called lavas, which are sometimes glassy,
sometimes in a more or less minutely crystalline
condition. Rocks of the same chemical composition,
which have cooled at no great distance from the surface
— namely, under conditions sometimes very similar to
those upon it — will differ little in structure from lavas,
though less frequently glassy, but as the depth at
which they solidify increases they will become, if other
conditions remain the same, more coarsely crystalline.1

Petrologists have divided the igneous rocks into a
number of species and varieties, but we must be content
to mention only two or three of the commoner. A
magma with a high percentage of silica and some
20 per cent, of alumina and alkalies, more commonly
potash, when coarsely crystalline forms granite.2 When
the rock is very minutely crystalline, presenting a
" stony " instead of a speckled aspect, we may call it
a felstone ; and when it is glassy an obsidian or a
pitchstone (the latter having a more resinous appear-
ance) . Those lavas, however, which consist of an intimate
mixture of minute crystals and glass are generally called
trachytes (because they frequently have a rough feeling
to the hand).

A magma containing from a little more than 40 to
about 50 per cent, of silica, with a low proportion of
alkalies, but a fair amount of alumina and a high one
of lime, magnesia, and iron, is called a dolerite or a
basalt, according to its crystalline condition. And the
latter term is popularly applied to all the varieties
which are sufficiently compact to look black at a short
distance. This material but rarely and locally forms

1 Those which we can examine have been subsequently exposed
to view by the removal of the overlying rocks.

2 When soda is the dominant alkali, it bears another name; but
as there is no hard and fast division between the two this name
may suffice for general purposes. When there is little free quartz
and more of the lime magnesia and iron bases, the rocks are
named syenite and diorite. Some "practical men" apparently
think that almost anything which can be used for paving-stones
or road metal can be called granite. That, however, is wholly
unjustifiable.

28 THE STRUCTURE OF THE EARTH

a glass. The older kinds often assume a green colour,
and are inclusively called greenstones. The rocks con-
sisting of a still lower percentage of silica with a high
one of magnesia and iron-oxides are comparatively rare ;
seldom, if ever, forming glasses, and, so far as is at
present known, they never quite reached the surface.
These are called the olivine-rocks or peridotites, and are
rather liable to alteration.

The sedimentary rocks must have been derived from
the igneous. When the agents of denudation, as
will presently be described, act upon such a rock as a
granite, the felspar is " rotted " by the removal of its
alkalies and other changes, so that it gradually becomes
a clay ; the quartz, which is a very insoluble mineral,
is liberated to form sand, and the other silicates either
form some variety of clay or enter into other chemical
compounds such as carbonates. Thus the igneous
rocks are directly or indirectly the source of the sedi-
mentaries, and the material derived from them is
transported by moving water to other places. This
process will be described in later chapters ; at present
it will suffice to say that only those materials which are
deposited on the bed of the sea can occupy a large
area, and that they will become more finely grained
as the distance from the source of supply increases.
Thus each sedimentary deposit will be more or less
wedge-shaped as the materials change from coarse to
fine, so that we may go on from gravel and sand to
clay, which ultimately disappears after passing through
the state of the very finest mud.

But the deposit of material may still continue, though
in quite a different way. In the destruction of rock-
masses water carries off in solution some of their con-
stituents, especially silica and lime (the latter as a
carbonate), together also with a little sulphur and
phosphorus. Living organisms now begin their work,
removing from the water all the constituents of which
they have need, and making use especially of the first
and second to build up the hard frame of their bodies.
Silica is removed by the little plants called diatoms,

THE EARTH'S CONSTITUTION 29

by radiolarians, and by certain sponges— all very low
in the scale of animal organisation, but able to construct
beautiful though minute " skeletons." The carbonate
of lime is built up into another minute group of plants —
certain algae — the tests of foraminifera, generally minute
but often wonders of construction, into corals, the
shells of molluscs, and other marine and fresh-water
organisms. These after death are buried in the sand
and mud, thus augmenting its volume, but as the
process of life and death is continued in the clear
waters, the making of limestone goes on there; since,
where the depth is too great for the larger organisms to
flourish or even to exist, there is a constant rain of
those minuter creatures which have been floating like
a cloud in the upper waters of the ocean. Limestones,
then, are mainly organic in origin. When formed in
the shallower waters they may develop with moderate
rapidity, but are likely to be limited in extent, since
they require rather exceptional conditions ; while in
the deep water, since they are formed of very minute
organisms, their growth will be very slow. Thus
whenever we can follow a deposit far enough from a
shore, we may expect to find gravel graduate into sand,
and this into clay, which gradually dies out and is
replaced by limestone. It is therefore obvious that the
deposits, which are strictly contemporaneous records
of any one epoch in the earth's history, will differ con-
siderably in their thickness, mineral character, and
organic remains. Also, that as the conditions of deposit
must change from time to time, the results will show
corresponding changes, so that we may find in any one
place a variable succession of strata, and may sometimes
discover that Nature not seldom destroys what she has
constructed, and has torn whole pages out of the life-
history of any particular district. In such a case the
new deposits will not, as a rule, lie quite evenly on the
old ; the crust will very probably have been moved,
the older strata tilted into a different position, so that
the newer sometimes rest upon the truncated ends of
the others. Geologists call this uneven fitting an un-

30 THE STRUCTURE OF THE EARTH

conformity, and it must always indicate a considerable
interval of time. Changes of this kind are accordingly
associated with changes in the life-history of the
place, so that we must be prepared for palaeontological
as well as stratigraphical breaks in the succession of
strata.

Rocks are grouped together, as the numbers of a
periodical are bound into volumes, by making use of
convenient changes in their physical and palseontological
characters, and we have to do the best we can with
a series from which pages and even whole sheets are
missing. Thus it follows that a successive grouping
of strata adopted for any one district or country may
not be strictly accurate for another, or the characters
of the several members may be very different. The
chalk of England is represented by a fairly strong
yellowish limestone in the south-west of France and
by a hard sandstone in Saxony ; and even where this
change has not occurred we cannot prove that the
deposits began and ended quite simultaneously in
countries some distance apart. Still less can this be
done when we are dealing with the larger groups, each
of which may be regarded as including several parts
or even volumes ; and the difficulty of correlation is
likely to increase with the distance, because we may
expect that in past times the creatures living on the
earth would show differences corresponding with the
climatal and other conditions much as they now do,
though possibly not to quite such a marked extent.
It must not therefore be supposed that the geological
epochs, periods or eras, have nearly so precise a meaning
as they have in human history. As Huxley once
pointed out, deposits though homotaxial — that is,
occupying the same position in the progressive record
of life — may not be contemporaneous in any strict
sense of the word; still, as a classification is necessary,
we may arrive at one which is sufficiently accurate for'
practical purposes, though we cannot date the beginning
of a geological formation with the same precision as
the reign of a king. The divisions in the succession of

THE EARTH'S CONSTITUTION 31

stratified rocks are drawn at any convenient horizon
where there is either an actual gap in the record, or
some marked change in the character of the deposit
or the fossils suggests that, especially in the latter case,
more time has really elapsed than would at first sight
be supposed.

The following grouping has been adopted for the
strata in our own Islands, and it holds good for the
adjacent parts of the Continent, and may be extended,
if we allow of a gradually increasing elasticity in the
terms, to other parts of the world. Large associated
successions of the stratified rocks are called Systems,
and the times occupied in their deposit Periods. The
Systems are subdivided into Groups and Stages, and
their durations are expressed respectively as Epochs
and Ages.1 The fossififerous systems are associated
into three great sets named Series, which are some-
times called Primary, Secondary, and Tertiary, but
now perhaps more often Palaeozoic, Mesozoic, and
Kainozoic — the Ancient-life, the Middle-life, and the
New-life eras — to which we must again refer in the
chapter dealing with the life-history of the earth. This,
then, is a list of the systems in descending order — that
is, as they occur hi the earth's crust — each above its
predecessor in age — together with the general characters
of the deposits representing them in the United King-
dom. No sharp line separates the latest from the time
when history begins.

Recent, Prehistoric and Sands, Gravels and Clays
Pleistocene

Pliocene .... Gravels, Sands and soft Lime-
stones

Miocene .... Wanting
'5 Eocene .... Clays, Sands and a little Lime-
stone

1 There is unfortunately some diversity in the use of their
names, and the proposals of the International Geological Congress
in 1881 did not really help to secure uniformity.

32 THE STRUCTURE OF THE EARTH

' Cretaceous 1 Soft white Limestone, with

sandy and clayey base

Neocomian . . . Sands and Clays
Jurassic .... Limestones and Clays
Triassic .... Clays, Sands and Gravels

' Permian 2 Sandy rocks with some mag-

nesjan Limestone
Carboniferous . . . Clays, Sandstones and Coals,

with Limestones below
Devonian 3 Sandstones, Shales and local

Limestones
Silurian 4 ... Sandstones, Shales and local

Limestones
Ordovician . . . Sandstones, Shales (often

Slates 5) ; little Limestone
Cambrian . . . Sandstones and Shales, often

Slates

Beneath the Cambrian is a considerable thickness of
rocks, in which the uppermost differ little from it in
mineral character but retain very few, and these
commonly obscure, traces of living creatures ; and the
lower, though often certainly sedimentary in origin,
have undergone so much mineral change that even if
living creatures had existed when they were deposited,
all traces of them must have been obliterated. These
rocks are now commonly called Archaean, and in them
we meet with a third great group of rocks, the Meta-
morphic, or those which have undergone such great
changes that it is difficult to determine their original
condition. The term should be used only in this sense,
for of course hardly any rock is now quite in the same
state as when it was deposited. The organic fragments
in one of the Jurassic limestones have been cemented

1 Some geologists call Cretaceous and Neocomian respectively
Upper and Lower Cretaceous.

2 The Permian and Trias, which in some districts are not easily
separated, were formerly grouped together as New Red Sandstone.

3 As this system is represented over a large area chiefly by Sand-
stone of a reddish colour, it is often called the Old Red Sandstone.

4 Some geologists call Silurian and Ordovician respectively
Upper and Lower Silurian.

5 Slates split (from pressure) independently of bedding.

THE EARTH'S CONSTITUTION 33

together by the deposit of calcite (crystallised carbonate
of lime) ; even in the apparently unchanged chalk the
silica, once disseminated through it in the form of minute
organisms, is now aggregated as flints ; there is mineral
deposit among the grains in a sandstone and slight
change among the constituents of an ancient shale.
Rocks truly metamorphic often exhibit a parallel
arrangement of their component minerals, and are
called schists from a tendency to split parallel with
this structure. Igneous rocks also may undergo meta-
morphism, but in consequence of the greater chemical
stability of their constituents this is often less con-
spicuous than among those of sedimentary origin. The
agents of change are water, pressure, and heat, of which
sometimes the one, sometimes the other, may be pre-
dominant. Water produces chemical changes in the
mineral constituents of a rock by subtraction, addition,
and rearrangements ; pressure causes crushing, and
thus facilitates in more than one way the attack of
water ; heat, besides intensifying the effects of the
other two, brings about alterations which would be im-
possible without it. One example only must suffice —
the effect of an igneous rock when intrusive in a clay
or shale. If the former cools quickly, the latter is
simply hardened — nature plays the brickmaker — but
with slower cooling, as generally happens when the
intruder is a great mass of granite, the mineral character
of the other rock may be so completely changed that an
aggregate of clayey particles has become a crystalline
rock consisting mainly of some kind of mica and quartz.
The igneous rocks are also metamorphosed, but as a
rule not so conspicuously as the sedimentary ; for
instance, when water enters into chemical combination
with the magnesian silicate in a peridotite, it forms
serpentine, well known as an ornamental rock ; and
when a granite has been exposed to a severe pressure
rude cleavage planes are produced, along which mineral
changes take place, so that the granite is converted into
a gneiss or mica-schist. But some of these two rocks
in the Archaean series are probably igneous in origin,

c

34 THE STRUCTURE OF THE EARTH

and may have acquired the foliated structure at the
outset under conditions of cooling very different from
the present. If the temperature at the surface were
high enough to prevent water from accumulating upon
it, the pressure there would be augmented by the weight
of the ocean. " In that case the very lava-stream
would consolidate under a pressure of about 310 atmo-
spheres, equivalent to about 4000 feet of average rock ; " 1
and besides this, the rise of temperature beneath the
earth's surface would be much more rapid — for instance,
after about one twenty-fifth of the whole time which has
elapsed since the first consolidation, the rate would be
one degree for every 10 feet of descent. The earlier
geologists supposed that sedimentary rocks of a com-
paratively late geological age might have been con-
verted into crystalline schists and gneisses, but the
evidence advanced in favour of this view has always
broken down when it has been closely scrutinised, and
few would now deny that such crystalline rocks are not
only Archaean, but also do not belong to the latest
part of that era.

CHAPTER III

THE WORK OF HEAT AND COLD

WE may define a rock as an aggregate of mineral
particles, generally more or less diverse. In ordinary
use the term generally connotes a certain amount of
consistency and hardness, but that, strictly speaking,
is not the case in geology. In that science, clay or
even the sand of a dune are rocks no less than a fime-
stone of the Portland quarries or the granite of Dart-
moor. But the materials of which the Earth's crust is
formed are, as a rule, fairly hard, so that the geologist
often for convenience adopts by implication the ordi-
nary significance, as we shall hereafter do, unless the

1 The author, Foundation Stones of the Earth's Crust, 1888, p. 13.

THE WORK OF HEAT AND COLD 35

contrary is stated. We shall also assume that they are
practically free from water.

With these limitations, all rocks expand with a rise
and contract with a fall in temperature, and the effects
of the strains thus set up are often far more consider-
able than would be expected by those who live, as we
do in Great Britain, in a temperate climate. In regions
nearer to the Equator, and especially in lands almost
without rani, like the deserts of Africa and Central
Asia, where the sky may be clear for weeks or even
months together, the difference between the day and
the night temperature is often great. For instance, in
Western America a difference of 90° F. between the
extremes of day and night temperature is not un-
common. At 12° S. latitude in Central Africa, Living-
stone noted a maximum of 137° F. and a minimum of
42°, while these on the thirtieth parallel in South Aus-
tralia are said to be 131° and 24°, which give a range
of 107°. Such changes as these, daily repeated, though
not always so great, cannot occur without setting up
severe strains in the exposed portions of a rock, especi-
ally in fragments, where the shape is irregular and little
more than one surface is exposed to the sky. Travellers
have noted the results of these continued expansions
and contractions. Fragments are constantly splitting
off from the faces of crags or other exposed masses of
rock, and these are again broken up, so that the ground
is strewn with sharp-edged angular pieces, which vary
in weight from a few ounces to as much as two hundred
pounds. Extreme cold would be quite as effective as
extreme heat, but the consequences of this cannot -be
so readily distinguished, because water in freezing
expands with great force, and in regions where the rain-
fall is more normal and the winters are severe, the ice-
wedge, as we may call it, becomes a much more effective
agent in rupturing rocks than any molecular strains
from expansion and contraction in a dry condition.
But in dealing with past episodes in the history of the
globe we are often unable to prove whether in a par-
ticular region the range of the thermometer was great

36 THE STRUCTURE OF THE EARTH

and the rainfall slight, and thus we take it to be prob-
able that important changes of temperature have more
commonly produced effects in the past, as they still do
in the present, by the intervention of water.

Heat and cold set the air in motion, and are the causes
of winds.1 But winds catch up the lighter materials
on the earth's surface and transfer them from place to
place. The dust, like " the windy ways of men," is
" stirred only to be laid again," but not exactly on the
spot which it previously occupied. It is carried through
the air, it strikes against obstacles in its course, and
sooner or later comes again to rest. Those who live in
temperate regions have little notion of the effects,
though to some extent indirect, brought about by the
winds. Now and then a gale of exceptional strength
may devastate our pleasure-grounds and forests ; we
may see dust careering along our roads or clouds of
sand sweeping along a flat shore ; we may watch the
gradual building up of dunes on our coast or even their
slow march inland as they retreat before the invading
sea ; 2 but these are hardly more than feeble imitations
of what can be witnessed in arid regions like the Sahara
or the Central Asian deserts. Dust, like a fog, blots
out the light of the sun ; it fills the air, making respira-
tion difficult ; it penetrates almost everywhere ; it
piles up itself in all sheltered places and against every
obstacle. Dunes or sandhills on our own shores are
monuments of the transporting power of wind, and
their development can often be studied. In the path
of drifting sand a tuft of grass may be enough to form
a tiny mound ; a groyne gathers a bank of sand in its

1 For a discussion of this subject, and an account of the air
currents, regular or irregular, on the globe, we must refer the
reader to any treatise on meteorology.

8 The tower of the ruined church of Eccles, near Happisburgh in
Norfolk, projected from the dunes in 1839 on the landward side of
their crest ; in 1862 it rose from the bottom of their seaward slope
(Lyell, Principles of Qeology, llth edition, i. pp. 518, 519). In
April 1892 it cleared the dunes by nearly three yards ; during a
storm, Jan. 23, 1895, it was overthrown by the waves (E. Hill,
Geological Magazine, 1895, p. 229).

THE WORK OF HEAT AND COLD 37

lee, and some accidental check may be the beginning
of a dune. Even one of these is seldom long at rest.
When the wind is high it drives the sand up the slope
on which it impinges, carries the grains over the crest,
and lets them come to rest on the other side. Thus
a dune is commonly crescent-shaped; its sides, which
are the lower, advancing more rapidly than the central
part, so that wave follows wave on the surface of the
desert as they do on the sea, except that their forward
movement is extremely slow.

But the wind-driven dust and sand takes some share
in sculpturing the face of the earth. In the National
Museum at Washington, according to Sir A. Geikie,1
is a sheet of plate-glass, once a window in the lighthouse
at Cape Cod, which was so worn by the impact of sand
grains driven against it by a gale of not more than
forty-eight hours' duration as to be no longer trans-
parent. Drifting sand, as I once observed at Barmouth,
had in the course of a few years distinctly smoothed
the masonry of a stone wall, and on the Fifeshire coast
had actually polished the surface of a projecting hum-
mock of basalt. Its effects are greater in a region like
Egypt. The limestone rocks are furrowed and hollowed
out by the desert sand. The face of the Sphinx is com-
paratively smooth on one side, on the other it is deeply
grooved; for the stratified mass, from which it was
hewn many centuries ago, is unequal in its power of
resistance, and in the latter case exposed to the pre-
valent winds. The abrasive power of wind-driven dust
and sand is amply illustrated in the Egyptian and
other deserts. Its effects, perhaps, may occasionally
have been a little over - estimated, but it is un-
doubtedly an agent of some importance hi producing
changes more or less superficial, developing structures
latent in rocks or corroding them into strange forms.
If a bed here and there be harder than the rest, it may
ultimately stand out from the face of a cliff in a sharply
defined ridge or, if little more than a lenticle, may bring
about the formation of a pinnacle capped by a protective

1 Text-Book of Geology, p. 436 (1903).

38 THE STRUCTURE OF THE EARTH

turban. Drifting sand also sometimes wears away the
surfaces of rounded pebbles which are most exposed to
its action, and gives them a definitely angular form.
If the pebble was originally egg-shaped, and the winds
are very persistent in direction, its cross-section may
become a triangle, so that these smoothed and wind-
worn stones are inclusively called dreikanter, though
the number of their faces may exceed three. Another
effect is produced, but it is on a much smaller scale
and on the sand grains themselves. Quartz, of which
they often mainly consist, is a very hard mineral, and
its surface, when it is first removed from such a rock as
granite (commonly its original home), is slightly irregular.
When such grains are transported by water, as we shall
presently describe, they are very slowly rounded, be-
cause the fluid acts like a lubricant in preventing
friction, but when they are driven along by the wind
they are constantly impinging one on the other and on
any projecting rock-surface till they become models in
miniature of a pebble on a beach. Thus a geologist,
when he finds a sandstone in which many of the grains
are well rounded, has little doubt that, even if it is not
directly of desert origin, these in some past period of
their history have been driven about by the wind.
Such grains may be recognised in some of the oldest
stratified rocks,1 showing that even in those remote
ages the winds swept over barren sands as they con-
tinue to do at the present time.

In some regions the advancing dunes or the accu-
mulating dust completely buries fields and forests and
even, as Sven Hedin and Stein have recently described,
the works and homes of man. Many geologists believe
that the peculiar sandy earth, which in some of the
more central parts of Europe lies like a cloak over the
rougher features of the country, often to a height of
some 1200 feet above the sea-level, and is called loess
by Continental geologists, is really a wind-borne dust,
Hike that of Turkestan and some districts in Northern

1 Quart. Jour. Ged. Soc., xlvii. (1891), p. 90; Brit. Assoc. Rep.,
1886, p. 612.

THE WORK OF RAIN 39

China; where, as Richthofen informs us, it sometimes
exceeds 1500 feet in thickness, and has been carved into
deep valleys and precipitous ravines with cliffs 500 feet
in height, in which dwellings have been excavated by
the inhabitants of the region.

CHAPTER IV

THE WORK OF BAIN AND RUNNING WATER

RUNNING water is the most important of Nature's
graving tools. It destroys, transports, and deposits;
its action in each of these processes being partly chemi-
cal, partly mechanical. The three come in the order
enumerated, but their operation is sometimes all but
simultaneous. Rain, when it falls from the sky, is
almost pure water, for in its descent it can absorb only
a small quantity of air with a little carbonic and other
acids, with some sodium chloride — especially near sea-
coasts. It also brings down floating dust, whether
inorganic or organic, the former especially in the neigh-
bourhood of large towns where the air is polluted by
the smoke of countless chimneys. But however pure
rain water may be when it descends upon the earth,
it will be found before long to have taken up mineral
substances varying with the nature of the ground over
which it has passed, and to be sweeping onwards mud,
sand, or gravel, according to the velocity of the stream.
We are prevented by the limitations of our space
from describing in detail the distribution and amount
of rainfall and the laws by which these are governed.
It must suffice to say that they depend upon the cur-
rents of the atmosphere, the shape of the land surface,
and the relative position of the seas. Thus, in England,
winds from western to southern quarters often bring
rain because they have taken up moisture in passing
over the Atlantic, while winds from the east are
commonly dry because they have made a long journey

40 THE STRUCTURE OF THE EARTH

overland, where their expenditure has much exceeded
their receipts. Thus the annual rainfall hi Norfolk
and Cambridgeshire is about 23 inches, while on the
lower ground in Southern Lancashire it is at least
10 inches more. The rainfall is increased by hills
rising in the path of moist air-currents; for instance,
its annual average is nearly 38 niches at Manchester,
and over 51 inches about the Woodhead reservoirs
(800 feet above sea-level) on the western side of the
Pennine range. Some regions of the earth, such as the
Sahara and similar deserts, are almost rainless, while in
others the fall is much greater than in any part of the
British Isles ; though in some of the mountainous dis-
tricts it may vary from 60 to 80 niches, and at Seath-
waite in Borrowdale (the wettest place in Britain) is
slightly more than 129 inches. But the wettest place
in the world, so far as our information goes, is Cherra-
punji in the Khasia Hills, where the annual rainfall
amounts to at least 472 inches — or nearly 40 feet —
the larger part of which descends during the monsoon —
that is, in about four months of the year. Here as much
as 40-8 inches has been measured in a single day.

When a building is fresh from the mason's hands
the surfaces of its stones, where so desired by the archi-
tect, are smooth, but in old buildings these have become
rough to the touch. On a limestone such as one from
Portland, Bath, or Ketton, tiny fragments of shells and
little rounded grains, from which the rock gets the
name of oolite, become conspicuous to sight and touch ;
projecting grains of quartz make a sandstone like a
rasp; the polish disappears from marbles, porphyries,
and granites, and the last of these after many cen-
turies may even begin to crumble. The falling rain
smites the surface with its hammers, tiny but per-
sistent, and as the old proverb says, " constant dropping
wears away stones " ; it sinks into the rock wiierever
that is permeable, and sets up chemical changes which
destroy its coherence. The rain no sooner collects into
streamlets than its action though now localised is inten-
sified. In some of the streets of Cambridge a runlet of

THE WORK OF RAIN 41

water flows along the gutter. In fine weather this is
clear, and before sanitation was regarded people might
be seen filling their kettles in front of their own door-
steps. But after a heavy shower the water is muddy,
for the rain has carried with it the dust from the street.
Thus every brook and every river runs more swiftly
after wet weather, and the volume of the water increases
more rapidly when the fall of rain has been heavy.
Rivers, ordinarily sluggish, quicken their pace and
become turbid with mud ; the swifter sweep along sand
and gravel coarser than that usually moved; and in
mountain regions we may stand by the swollen torrents
and listen to the " grumbling " of boulders as they are
hurried onwards. During this process fragments broken
from neighbouring crags gradually lose edges and angles
by friction and mutual impact ; for the making of mud,
sand, and pebbles is mainly a result of mechanical
forces, though, as we shall see, chemical action plays
some, though a variable, part in the work, and these
forces are at work, not only on the surface but also
underground.

At this place it will be convenient to mention two
cases, in one of which the action of water, speaking in
general terms, is wholly mechanical, in the other wholly
chemical. Earth-pillars are the best examples of the
former. These are pinnacles of a stiff, stony clay,
capped by a cushion-like boulder. Occasionally they
are isolated ; more often they form linear groups. Two
very noted examples occur in upland valleys a few miles
from Botzen in the Italian Tyrol. A little examina-
tion shows that they have been carved out of a much
larger mass of clay by runlets of rain as they hurried
down either side of the valley towards the central
stream. In fine weather the path of these is dry and
the clay is hard ; after heavy rain it is softened and a
little stream runs down every furrow. The bigger
boulders act like an umbrella and protect the clay
beneath from being washed away, but when one falls
off the unprotected pinnacle is gradually destroyed.
In the Alps these earth-pillars often vary from about

42 THE STRUCTURE OF THE EARTH

4 to 8 yards in height, but in some places, as in the
Sierra Nevada, they are much more lofty. But they
may also be, and are so frequently, on quite a small
scale. Such miniature pillars, often only one or two
inches high, may sometimes be found in our own Islands ;
in fact they may be looked for whenever a rather stifi
clay contains fairly flat bits of stone.

Sand-pipes, as they are called, are the best instances
of the direct chemical action of water. These occur
most frequently in chalk, but are occasionally found in
other limestones, where they also have been covered
with a sandy gravel. Into the latter ram water has
sunk, has made its way down to the chalk, and has
begun to dissolve this, at some " vulnerable " point,
forming a cup-like hollow. As this is gradually
deepened it is kept filled by sand or gravel slipping
from above, and may thus be prolonged downwards
to a depth of several feet, while it is enlarged side-
ways, though much more slowly.

The corrosive action of rain and of the atmosphere
generally is conspicuous in limestone districts such as
the hill regions of the Mendips, Derbyshire, or Western
Yorkshire, where the bare rock is pitted, furrowed, and
sometimes traversed by channels (a feature especially
noteworthy in the Eastern Alps) which sooner or later,
like the gutters along house roofs, end in a pipe plunging
downward into the rocks. The surface of the limestone
is bare; the furrows afford shelter to ferns and other
Alpine plants. Sometimes, however, where these are
shallow and the rock contains but little insoluble
material, hardly a tuft of grass or any herbage mitigates
the austerity of the landscape. But where the struc-
ture of the rock permits the rain to remain on the sur-
face long enough to be gathered into rills, these may
form brooks, which however are at last swallowed up.
Here a " pot" or natural shaft is formed, down which
the water plunges, thus adding a mechanical to its
chemical action. These shafts are common in districts
where the limestone is pure, compact, and thick, and
nowhere more so than in the district around Ingle-

THE WORK OF RAIN 43

borough ; l Gaping Gill, one of the most noted, en-
gulfing a stream perhaps half a dozen yards wide and
usually a few inches deep. The shaft here is rather
more than 300 feet deep, and expands at the bottom
into a bulbous shape.

The water swallowed up in these natural shafts con-
tinues its underground course, carving out a channel
for itself, so that many districts have a subterranean
as well as a subaerial drainage system, streams com-
bining down below just as they do upon the surface,
and it may often be that the former system is on the
larger scale. Here also the work is obviously to some
extent mechanical, but that it is mainly chemical is
proved by the fact that caves are either unimportant
or altogether wanting in any but limestone regions.
Sometimes a river which has cut its way down to a
bed of rock more than usually permeable disappears
from sight, leaving a channel which is only used in times
of flood, and perhaps afterwards it emerges to resume
a subaerial path. That happens to the Manifold, near
Ham in Derbyshire ; and in any part of our Islands,
where that thick mass of pure grey limestone which
belongs to the lower part of the Carboniferous system
comes to the surface, swallow-holes, caves, and sub-
terranean streams are likely to be found. This is also
true of any similar kind of rock in other parts of the
globe. The fine caves of Le Han in the Belgian
Ardennes, and the more gigantic Mammoth Cave of
Kentucky, are striking instances of those occurring in
the Carboniferous Limestone ; while in the Jura and in
most of the valleys in the Limestone Alps the traveller
sees streams leaping out from the face of a cliff or
emerging full-born on the bed of a glen. The water
swallowed up on the higher ground has made its way
by subterranean channels, which are no doubt often
enlarged into caves — those at Adelsberg in Carinthia
are on an exceptionally grand scale — until at last its
course is intercepted by a valley and it again returns
to the light of day. Instances are common enough in

1 See Boyd Dawkins, Cave Hunting, 1874, chap. ii.

44 THE STRUCTURE OF THE EARTH

our own Islands. In the ponds among the gardens near
Wells Cathedral copious springs spout up, which are fed
by the rain that has been swallowed up on the Mendip
Hills ; the Axe, when it emerges from the cave by the
hyaena-den at Wookey Hole, has been supplied, at any
rate in part, by water swallowed up on the hills in the
neighbourhood of Priddy ; and the stream which plunges
down Gaping Gill returns to the light of day on the
bed of a valley near the village of Clapham, not many
yards away from a line of caves which it must once
have excavated. As a rule, the subterranean channel
cannot be followed for the whole distance from intake
to outlet ; it may be too narrow or low, or blocked by
fallen rocks ; but, notwithstanding this, the connection
between the two ends can in many cases be placed
beyond reasonable doubt, if not actually demonstrated.
The fact that various mineral substances are present
in a greater or less amount in the water of springs and
rivers is another proof of its action on the rocks over or
through which it passes. As already said, when rain
reaches the earth away from the polluting atmosphere
of towns, it is practically pure water, but that of streams,
lakes, and springs contains in solution an appreciable
quantity of mineral salts, which prove by their amounts
and differences that they have been derived from the
rocks over and through which the water has been
running. For instance, the Scotch Dee, above Aber-
deen, contains 312 parts of mineral matter in 10,000,000
of water; the Rhine, near Bale, 1712 parts; and the
Thames, at Ditton, 2720 parts in the same amount.
Of the solid matter in the first river, 205 parts are
salts of lime or magnesia, 122 of them being carbonate
of lime. Of that in the second, those salts form 1607
parts, 1279 being carbonate of lime; and in the third
river the proportions are 2302 and 1684. It is easy to
account for these differences. The Dee flows over
crystalline rocks, the constituents of which contain but
little lime (for though marble is among them, its amount
is relatively small), and are but slightly soluble in ordi-
nary water. The Rhine is fed by streams from the

THE WORK OF RAIN 45

Alps, where limestones are abundant as well as crystal-
line rocks, and the river system is on a much grander
scale than any in Britain. Thus, not only is the pro-
portion of dissolved mineral salts much higher than in
the Dee, but also that of the carbonate of lime fully
ten times as great, while the rise in the corresponding
percentage of the Thames water, though it is a shorter
river, is mainly due to its having traversed sedimentary
rocks, the constituents in which are rather more readily
attacked by reason of their fine state of division and
the loose texture of the limestones, especially the chalk.
An idea of the quantity of material thus removed from
the basin of the Thames may be obtained from the
fact that on an average its waters carry in solution
1000 tons of chalk daily under Kingston Bridge. As
the volume of a ton of chalk is about 15 cubic feet, this
would be enough in the course of a year to form a
solid mass 365 feet long, 150 feet broad, and 100 feet
high.

Springs afford confirmatory evidence. Everyone
knows that the water is " hard " in limestone districts,
that is, contains much dissolved carbonate of lime.
This is often deposited as " tufa " when a spring comes
to the surface and the soluble bicarbonate is converted
by evaporation into the insoluble carbonate. That is
the origin of the "petrifying" springs so common in
Derbyshire and other limestone districts. In this way,
as may be seen for instance near Matlock, masses of
tufa are formed large enough to be quarried. This is
done on a still greater scale by some of the rivers issuing
from the western slopes of the Apennines (mainly lime-
stone). The chief buildings of Imperial Home were
constructed of travertine, which has been deposited
around and below Tivoli by the Anio and other streams ;
and the grand ruins of the three Greek temples at
Psestum consist of a coarse tufa formed by water from
the neighbouring uplands. Other, and less common,
kinds of mineral springs admit of a similar explanation.
The brine wells of Cheshire, Staffordshire, and Worcester-
shire, are supplied from the rock-salt dissolved by their

46 THE STRUCTURE OF THE EARTH

waters in percolating through the Red Marls, where it
was in all probability deposited from an inland sea.
By processes of this nature a vast quantity of soluble
material must be conveyed from the body of the earth
to its surface, over which it is transported to such seas
and to oceans. The mineral constituents in these
must have been derived, with slight exceptions, from
the solid matter in the earth. They may alter their
relations, may enter into new combinations, but they
cannot be spontaneously generated in the water.

Vast quantities also of material, as already implied,
are transported by the mechanical action of water, and
are swept along as mud, sand, or gravel ; the amount
depending partly on the strength of the stream, partly
on the nature of the rock over which it flows. Tables
often quoted show that a current, going at the rate of
15 feet a minute, can move soft clay ; fine sand will be
carried along by double that velocity, and stones as
large as big peas by the treble of it. Currents flowing
from 135 feet to 200 feet a second can transport pebbles
from 1 inch to 1J inches in diameter. We must not
forget that the moving force of running water varies
as the sixth power of its velocity, so that if the latter
be doubled the former becomes 64 times as great. If
it can roll along a stone in the one case an inch in
diameter, this will be 4 inches in the other. Thus the
material moved at flood times is much coarser than
that ordinarily transported, so that gravelly seams in a
mass of sand may be regarded as the records of excep-
tionally heavy rains in the remote past. The amount
of material thus transferred also depends on the nature
of the rocks over which a river passes. One flowing
over clays, shales, and soft slates is generally more or
less muddy, but where the rock is hard, like some sand-
stones or limestones and most crystalline rocks, the
water is clear except after heavy storms. The limpidity
of the streams is one of the most attractive features
in the valleys of the Italian Alps, near Monte Viso,
where glaciers have given place to permanent snow-
beds.

THE WORK OF RAIN 47

Thus torrents, as we can see in many Alpine valleys,
sweep along boulders and coarse gravel, as well as the
sand and mud with which their waters are turbid, drop-
ping the heavier material whenever the current slackens,
but transporting the lighter till that also becomes bur-
densome. Experiments have been made to determine
the amount of material which is actually travelling down
the channel of a river, and a study of the flat beds of
valleys and of the deltas formed in lakes and seas leads
to a general estimate. But the quantity, it must be
remembered, in any river varies at different seasons.
The Rhone, for instance, is believed to transport when
low one part (by weight) in 7000 ; when at its mean
height, one in 2000; and when in flood, the same in
230 parts. The Ganges, before joining the Brahma-
pootra, is said to be transporting annually enough sedi-
ment to cover 172 square miles with a layer one foot
deep. But the Mississippi brings down to the sea
enough to cover to the same depth 268 square miles,
and the Hoango could do this with no less than 730
square miles ; the material in all cases being partly in
suspension, partly pushed along the bottom by the
moving water. The flat valley-beds on the course of
many rivers are formed by material which they have
dropped on their journey, and it is still augmented
when they overflow their banks. The cultivated land
of Egypt, as Herodotus observed between twenty-three
and twenty-four centuries ago, is " the gift of the
Nile " ; the level plain separating the Lakes of Brienz
and Thun has been built up from a depth of fully
700 feet, with debris brought by the Lutschine from the
Oberland, and to a less extent by the Lombach torrent
from the valley of Habkern. The delta of the Rhone
is gradually trespassing on the upper end of the Lake
of Geneva, and its margin is now half a league in advance
of its position in Roman times. The Adige and the Po
have been extending Italian territory at the expense of
the Adriatic, and the delta of the latter in one place
has done this so rapidly that Adria, which was a seaport
nineteen centuries ago, is now 14 miles inland.

48 THE STRUCTURE OF THE EARTH

Thus rivers demolish, transport, and build, but their
effects on the whole are more destructive than con-
structive, because they remove so much material which
they do not obviously restore. If time enough were
given, rain and rivers would ultimately plane off all
inequalities from the land — would bring it down to a
dead-level, and spread it out over the floor of the sea.
The ocean, in fact, is the grave of the land. Some of
the transported material is dropped on the way, but
this halt after all is only temporary — gravitation, aided
by water, will again be at work upon it. Some will
be added to the land as an irregular fringe, and still
more will be carried many miles away from this and
spread over the floor of the sea, gradually diminishing
its depth ; but most if not all the matter dissolved in
the river water is carried away into the ocean, where
it maintains, or rather increases, the saltness of the
sea, and supplies to living organisms the minerals which
their solid parts require, such as silica for the diatoms,
the radiolarians, and many sponges, and carbonate of
lime for most foraminifera and mollusca. Some idea
of the quantity of the latter mineral thus invisibly
transferred may be obtained from an estimate made
many years ago by Bischof, that the Rhone annually
discharges into the sea enough carbonate of lime to
make more than 332,500 million full-grown oyster-shells.
Thus the chalk which the Thames and the Colne have
secretly " conveyed " from the hills of Eastern England
is once more restored to a solid form in the oyster-beds
of Whitstable and Colchester.

From what has already been said, it follows that
rivers make the valleys rather than, as many formerly
supposed, the valleys make the rivers. Movements in
the earth's crust are of course necessary to set the
water to work, and to counteract its levelling tendency.
Folds, and even certain kinds of fissures, in the rocks
may be helpful hi determining its course, but this is
regulated by gravitation rather than by gaping cracks.
The process of erosion and its changes can usually be
most readily understood by following the course of a

THE WORK OF RAIN 49

valley from its beginning in a mountain region to its
emergence on the lowlands. For instance, in many
parts of the Alps the upper pastures are smooth slopes
of turf. On these the herbage has a protective effect
— enough to prevent the rain from washing away the
soil or gathering into rills. Sooner or later, however,
the latter is accomplished, and the continuity of the
slope is quickly interrupted by a little furrow. As the
area drained is enlarged and these rills are combined,
the furrow is deepened and widened, so as to become
a more conspicuous feature on the mountain side,
Presently the stream in cutting downwards may en-
counter some harder stratum, which causes it to set
up a plunging movement. That, where circumstances
are favourable, may initiate a waterfall, but it will in
any case modify the shape of the glen. This, when the
rock is rather friable, has a V-like section, for as the
bed is deepened the sides slip down, but where the
rock is strong this action almost ceases, cliffs replace
the slopes, and the valley becomes a gorge. As the
angle of descent diminishes, and the brook, augmented
by tributary streamlets, grows into a river, the valley
is widened, for the stream begins to oscillate and to
press more on the sides than on the bottom of its
bed. Under these circumstances a section of the valley
(apart from any change of form due to deposit on its
bed) is gradually altered from a V to a kneading-
trough. The slope of the sides is generally different,
because the velocity of the water is not identical in all
parts of a river channel ; it is greater at the surface
than at the bottom, and, if the stream be straight,
greater in the middle than at the sides. Hence with
a curving channel the line of quickest motion trans-
gresses toward the concave bank, on which the water
presses rather more strongly than on the convex one.
Thus the slopes of a valley are generally steeper on
the concave side than on the convex. But in studying
these features we must bear in mind that the volume
and the velocity of the river have not always been
constant, and thus be prepared for what may at first

50 THE STRUCTURE OF THE EARTH

sight, be regarded as anomalies. The valleys, for
instance, in our English lowlands must have been ex-
cavated, and their dominant contours have been im-
pressed upon them by much larger rivers than those
at present flowing along their beds.

In several such valleys, especially in South-Eastern
England, beds of coarse gravel, often of considerable
thickness, may be found to at least 100 feet above the
present level of the water. These were evidently
deposited by the river when it was flowing at a corres-
ponding height above its modern channel, along which
also it can only transport alluvial mud, except possibly
during a very high flood. Besides this, the true floor
of the valley sometimes lies many feet below that now
visible ; so much so that it may even be below the
present sea-level. Thus the river in its days of youth-
ful vigour must have carved out a channel for itself,
which afterwards in an enfeebled phase it could no
longer keep clear and is now filling up. A channel
choked with drift is hidden beneath the present bed of
the Thames at London, of the Humber at Hull, of the
Mersey at Liverpool, and sometimes away from any
course visible on the surface.1 The alluvial flats on
either side, as these rivers approach their estuaries, are
significant of a loss of power and a consequent dropping
of their burdens. One cause of this has obviously been
a lowering of the land, which has diminished the velocity
of the current, but another has been a change in the
volume of the stream. When these coarse gravels were
deposited, both the snowfall in winter and the total
annual precipitation were greater than they now are.
All through the spring, and in some regions well into
the summer, the rivers were kept flowing strong and
full ; here wearing down their beds, there overflowing

1 Hidden valleys, filled with drift to depths from 60 to over 100
feet below ordnance datum, have been occasionally found. The
most remarkable case was at Glemsford in the valley of the Stour,
above Sudbury, where 477 feet of drift was pierced before reaching
the chalk. See F. W. Harmer, Quart. Jour. Oeol. Soc., Ixiii. (1907),
p. 494.

THE WORK OF RAIN 51

their banks to drop sand and gravel on either side, so
that such an one as the Thames must have swept along
for some months in every year, with a strength which
it now manifests only for a day or two, perhaps half a
dozen times hi a century.

Changes such as we have mentioned, in the level of
the land, in rainfall, and in climate generally, make
the story of the sculpture of a country complicated and
sometimes difficult to interpret. It is like a palimpsest,
on which the process of erasing and rewriting has been
more than once repeated, so that the earlier records
can hardly be deciphered. In illustration of this we
may mention a district the features of which long per-
plexed geologists. This is the Weald of Kent and
Sussex. From the South Foreland to Beachy Head
a line of chalk downs sweeps round through Hampshire ;
their inner slopes descending steeply to a fairly wide
valley which separates them from a more sandy and
generally lower range, and their outer shelving down
more gently in the direction of their planes of bedding
shown by the included layers of flints. This valley is
excavated in the Gault, a soft dark clay, and its inner
boundary is formed by a range of brownish sands and
sandstones with a little clay — the deposit often called
Lower Greensand. This second range encloses another
and much wider valley, something like the imprint of a
horse-shoe, which is carved out of the Weald Clay, and
within it, to represent the " frog," rises a group of hills
chiefly sandstone. Each of these ranges occasionally
overtops the 800-foot contour line, while the floors of
the enclosed valleys usually lie between those of 150
and 250 feet. This district, since the rocks are older
towards the middle part, must have formed part of an
elliptical dome which once extended across the Channel
into France, but has now been severed and to no small
extent effaced by the sea. This was formerly supposed
not only to have isolated the smaller French from the
larger English portion, but also to have in some way or
other scooped out the valleys, so that the steep inward-
facing chalk cliffs were regarded as memorials of an

52 THE STRUCTURE OF THE EARTH

ancient coast-line. This explanation more than half a
century ago was shown to be untenable by C. Le Neve
Foster and W. Topley.1 On the northern side of the
area (for this will suffice to illustrate their method of
reasoning) the drainage from its interior zones is carried
to the Thames by the Wey, the Mole, the Darent,
and the Medway, all of which cut completely through
the North Downs. In the valley of the last-named
river old gravels, evidently once deposited by it, may
be traced to a height of sometimes 300 feet above its
present level. These gravels indicate that, speaking in
general terms, the bed of the Medway, together with
its tributaries, must once have been higher than it now
is by about that amount, and the same is true of the
other river systems. There is ample evidence to show
that all this work must have been done while the land
was above sea-level, so that rain and running water must
have deepened the Wealden area by some 300 feet.
But the physical features of the upper part are similar
to those of this lower one, so that we are justified in
inferring that even if the top of the dome was planed
off by the waves when first it rose from the sea, the
existing system of hills and valleys must be attributed
to subaerial agencies.

In a region undergoing denudation of this kind streams
struggle one against the other. The more active of two
flowing in opposite directions cuts back more quickly
into the intervening watershed, thus tending to lower
its crest, to push back the dividing line into its neigh-
bour's territory, and ultimately capture some of its
tributaries. The Alps afford many notable instances
of this kind of trespass, but it also occurs, though less
obviously, in Britain. Another form of trespass is
exhibited when one of the branches of a river system
cuts back into the ridge separating it from the main
channel of another one, " taps " the latter, and by
diverting the whole of its water leaves the part below
dry till streamlets from either flank combine to supply
another but feebler occupant.

1 Quart. Jvwr. Qed. Soc., vol. xxi. (1865), p. 443.

THE WORK OF SNOW AND ICE 53

When a dome-like area of stratified rock is being
gradually upheaved, the water which runs off it natur-
ally follows the line of quickest descent, thus taking a
radial or " transverse " course. But when this inter-
sects beds alternating in hardness, as in the region of
the Weald, the rain which falls on the exposed surfaces
of the softer outcrops gradually lowers these, as it more
slowly makes its way — for it will find one somehow —
to one of these transverse furrows, and thus excavates
another set of valleys more vague and irregular in
outline, gradually removing these softer materials, and
leaving the harder rising on either hand as lines of
hills. As this second set of valleys follows the general
direction (or strike) of the strata, they are called longi-
tudinal valleys. In the Wealden district they follow
the outcrops of the Gault and of the Weald Clay, and
in some parts of the Alps a geological map shows that
a river makes more than one change from a transverse
to a longitudinal course.

CHAPTER V

THE WORK OF SNOW AND ICE

ICE no less than running water is an agent of denuda-
tion, transport, and deposition. When the air tempera-
ture falls below the freezing-point, rain is replaced by
snow, which lies upon the ground till warmer weather
causes it to melt. When the mean annual temperature
of a district is below 32° F., the snow will not all
be liquefied, and there will be more or less accumula-
tion. In equatorial regions, where the temperature at
the sea-coast never descends to the freezing-point, snow
and ice are unknown, but in high latitudes running
water is a rarity, often only to be found in summer.
But snow may be seen, even in the Tropics, because
the air becomes gradually cooler in ascending from the
sea-level. The rate at which the mercury of the thermo-

54 THE STRUCTURE OF THE EARTH

meter drops is not quite uniform, but about 3° F.
for each 1000 feet of ascent is a rough approximation.1
The elevation at which the snow, instead of being com-
pletely melted away, just manages to linger through the
summer, is called the snow-line, and it more nearly
corresponds with a mean annual temperature of 30° F.
than of 32° F., because the frozen material loses slightly
by evaporation even during the coldest weather. If
then the mountains in a tropical region rise high enough,
their upper parts will be snow clad. Suppose, for
instance, the mean annual temperature at the sea-
coast to be 75°, the snow-line would be at or slightly
under 15,000 feet, and all mountains that exceeded this
elevation — like, for instance, the principal summits on
the Ecuadorian Andes, which vary from about 15,500 feet
to 20,500 feet — would be snow clad to much the same
extent as peaks in the Alps which range from 8500 feet
to 13,500 feet, for in the latter chain the snow-line
ranges, according to latitude, from slightly below
8000 feet to nearly 9000 feet. So that in the Oberland
there will be permanent snow on any summit which
overtops the former limit, and on the higher peaks
rain rarely or never falls. But on these the snow does
not accumulate indefinitely. When the slopes are steep
the loose new-fallen material slips away from the frozen
surface of the older, which has anchored itself to the
irregularities of the underlying rock, and slides down
to the upland glens beneath. Such a discharge is called
a dust-avalanche, and these are common after a spell
of bad weather in summer or at the beginning of winter ;
but when the approach of the former enables the moun-
tains to throw off the burden of snow which has been
laid upon them by the latter, this slips away in huge
more or less hard-frozen slab-like masses which plough
their downward way through forests, obstruct roads,
and bury villages, so as to be much the more destruc-
tive to property and life. These are called from their

1 As a rule the fall is slightly more rapid, and sometimes 1° for
300 feet would be rather more exact. At Ben Nevis it is 1° for 277
feet.

THE WORK OF SNOW AND ICE 55

closer texture " ground-avalanches." Both, however,
sweep along with them fragments of rock, earth, and
other material, thus transporting and depositing as
well as destroying.

But there are many parts of a mountain where
avalanches would not be very effective hi relieving the
accumulation of snow. They could carry it from the
crags to the upper parts of valleys, but the beds of these
would not slope rapidly enough to rid themselves of the
burden by a second set of avalanches. When snow falls
on a fairly level surface, like a flat-topped mountain
or the head of an upland glen, it accumulates layer
upon layer. The surface during fine weather melts a
little, the water trickles down into the underlying mass,
and is there again frozen. The pressure also of the
upper layers upon the lower causes these to consolidate,
and thus the texture of the snow-bed is by slow degrees
changed into fairly solid ice. This is the beginning of a
glacier. The head of an elevated mountain valley forms
a reservoir occupied by snow which is in process of
conversion into ice, and which is prevented from inde-
finite accumulation by a slow downward movement of
the frozen material, which may be said to creep along the
bed of the valley by the action of gravitation. On the
history of that change, the physical cause of the motion,
and the precise nature of glacier ice, we have not space
to dwell ; it must suffice to say that the movements of
this ice resemble those of a plastic solid (for instance,
some kinds of wax or even clay), which is rather easily
ruptured by strain, but is readily re-cemented when
fragments are pressed together. Thus any inequality
of movement (and this is more rapid in the central
part of the ice-stream than at the sides) or irregularity
in the slope of the valley — anything which sets up a
distinct strain — causes fissures, or crevasses, as they
are called, to open. If, for instance, there is a sudden
descent — a rocky step — in its bed, the glacier may be
almost broken up into a wilderness of white crags,
parted by blue chasms — affording often scenes of weird
beauty. Such a part is called an ice-fall.

56 THE STRUCTURE OF THE EARTH

So long as the glacier is above the snow-line its volume
is more or less augmented, but below this limit it
dwindles as it descends into the warmer air, until at
last it is altogether melted away.1 The great Aletsch
glacier is much the longest in the Alps, for it is rather
more than 16 miles ; others, like the Unteraar,
the Gorner, and the Mer de Glace vary from 8 to
10 miles, but the majority are considerably shorter.
In fact every stage may be found, from a glacier which
is little more than a weve-basin, and that a small one,
to the long ice-stream supplied by great reservoirs of
snow like one of those just enumerated. These grander
flows of ice descend between pine-woods and grassy
alps to about 4000 feet (and formerly some 600 feet
lower) above sea-level, but the smaller often do not
extend for more than about a thousand feet below the
snow-line, and as a rule a glacier does not begin to form
till about the same amount above this limit. Its place
of birth and of death are, of course, at a less distance
from the sea-level as either pole is approached. In
Scotland the snow-line would be at about 5000 feet, so
there are neither glaciers nor, strictly speaking, any
permanent snow.2 But in the north of Norway moun-
tains lower than Ben Nevis are draped with snow and
give birth to glaciers, which descend almost, and hi
one case quite, down to sea-level. In this district the
mean temperature is about 36° P., so the snow-line
must be near 2000 feet, and the glaciers take their origin
about 1000 feet higher — that is to say, the conditions
here are similar to those in an Alpine district where the
higher peaks rise rather above 12,000 feet. In Green-
land, where the mean annual temperature is only just
above 32° F. in the extreme south, the glaciers increase
rapidly in volume, and descend in latitude 64° 50' from

1 The rate of motion is dependent on more than one condition.
In the Alps it averages about a foot a day, but the corresponding
advance of great Greenland glaciers is 20 feet or even more.

2 Ben Nevis, the highest mountain, is 4406 feet. The mean
annual temperature is barely 31° F. , and though a little snow may
remain near its summit, it is only in places sheltered from the
sun.

THE WORK OF SNOW AND ICE 57

a divide of unbroken snow between eight and nine
thousand feet above sea-level down into the fjords,
where they terminate in great cliffs of ice, and huge
blocks are detached which sail away to increase the
dangers of an Atlantic voyage. When valley glaciers
expand and become confluent at the foot of a mountain
range, as on the Alaskan coast at the foot of Mount St.
Elias (18,092 feet), they receive the name of Piedmont
(or Mountain-foot) glaciers ; when they largely, if not
wholly, cover even the inequalities of the uplands,
they are called Ice-sheets. Such may be seen in Arctic
and Antarctic regions.

Rock debris detached from peaks and precipices
falls upon the surface of a glacier and is carried along
by it. Most of that from the crags on either side comes
to rest on or near the edge, and thus forms a kind of
stony selvedge. This is called a lateral moraine. Where
the ice-streams from two valleys are united to form a
single glacier, the moraine on the left bank of one joins
that on the right bank of the other, thus producing a
single moraine which, as it is now more or less in the
middle of the ice-stream, is called a medial moraine.
Small fragments of rock, by absorbing heat from the
sun, tend to sink into the ice, but large accumulations,
like those in a moraine, have a protective effect, so that
the ice underneath becomes higher than that exposed
on either side. Thus a medial moraine takes the form
of a mound, the lower parts being ice and the upper a
mass of broken rock and grit, which at a distance pre-
sents a rude resemblance to a railway embankment.
But when the glacier is badly broken this regluarity
of form soon disappears, much of the debris being
engulfed in the crevasses and the rest scattered over the
surface. Large isolated boulders, by acting as parasols,
protect the ice immediately beneath them, and thus,
in course of time, become supported by pedestals of it
a yard or so high. These are called glacier tables. The
blocks and the grit, which continue to travel on the
surface, are ultimately dropped at the end of the glacier,
where they also form a stony mound, which is called

58 THE STRUCTURE OF THE EARTH

a terminal moraine ; and if the ice has anywhere made
a long halt this mound may reach a great size.1 In
the same way a lateral moraine may be stranded by the
retreating ice, and may also form a mound on the slope,
if that be not too steep, which runs parallel with the
bed of the valley. If, however, a glacier advance after
a halt, it will push part of the moraine before it and
" override " the rest, but to this phase in its history
we will return.

Large boulders which have become isolated from a
moraine are sometimes dropped by the retreating ice
on the sides or bed of a valley. These " erratics " are
often common, and when poised in rather unstable
positions are called " perched blocks." They are
scattered over the parts of England which, as said
above, have been in some way exposed to the action
of ice,2 and are frequent in the Alps, whence they may
be traced over the adjacent lowlands. They show how
much more extensive were the glaciers of this chain
during the Ice-Age, for boulders from the northern
slopes of the Pennines and the southern of the Oberland
may be traced down the course of the Rhone to within
a few miles of Lyons. Some of these are very large,
such as the Pierre-a-bot, near Neuchatel ; the Pierre
des Marmites, above Monthey ; and the Blauenstein,
near the Mattmark See (Saas-thal) ; the least of which
must exceed 40,000 cubic feet.

The debris swallowed up by the shallower crevasses
may be disgorged after a time upon the surface of the
glacier, but a considerable quantity is engulfed by the
deeper, carried down to the bottom, and then pushed
along by the moving ice. Thus the latter, which of
itself could only act as a burnisher, is converted into
a file, wearing away the rocky floor, smoothing off

1 Those deposited by the ancient glacier from the Dora Baltea
valley (Piedmont) are like lines of hills, some of them rising at
least 1000 feet above the lowland.

2 In England instances occur of enormous blocks of chalk and
other rock which have been in some way or other transported by
the action of ice (the author, Presidential Address to the British
Association, 1910, p. 19).

THE WORK OF SNOW AND ICE 59

projections, and replacing an angular by a rounded
surface, which sometimes may even be polished, but
is often also scratched and scored by the stony teeth.

Thus the rounded surfaces, called from their peculiar
outlines roches moutonnees, more poetically compared
by Ruskin to the backs of plunging dolphins, and the
"• handwriting on the wall," exposed by the retreating
ice are unmistakable, and may be traced in many
mountain regions, like the Alps, Scandinavia, and parts
of our own country, far below the level of existing
glaciers, or even in districts from which they have
completely disappeared. The debris overridden by
advancing ice may not only be pushed along beneath
it, but also entangled in its lower part, and this happens
to a greater extent in polar regions, where the ice-streams,
in consequence of the oblique incidence of the sun's
rays in summer, terminate in cliffs rather than in
slopes. Besides this, the ice-file, as already stated,
wears away a certain amount of debris, which also is
transported, and if not swept away by subglacial streams
is ultimately left, together with the other material.
This, which as a whole contains a larger amount of
" rock-flour " and finer debris than the ordinary ter-
minal moraine, has been called " ground moraine."
Its total amount and its ratio to that carried on the
surface depends on local circumstances, and is no doubt
larger in the case of ice-sheets than of ordinary valley
glaciers,1 and the proportion will increase with the area
covered by ice, because the fewer and smaller the
projecting crags, the less will be the quantity of ordinary
subaerial moraine. At one time geologists were apt
to exaggerate the amount of ground moraine, but it is
sometimes a factor which must not be altogether
neglected.

We must now glance at the material called boulder

1 The manner in which the debris becomes entangled in such a
country as Spitzbergen is well described by E. J. Garwood and J. W.
Gregory (Quart. Jour. Geol. Soc., vol. liv. (1898), p. 197). See also
T. C. Chamberlin, Glacial Studies in Greenland, Parts i.-x. ; Jour.
Geol., 1894-7.

60 THE STRUCTURE OF THE EARTH

clay. This, so far as it can be precisely defined, con-
sists of a clay, sometimes more or less sandy, in which
larger fragments of rock are embedded. For instance,
these, in the boulder clay of the eastern counties of
England, consist of chalk, generally in well-worn
pebbles, with pieces, often more angular, of other lime-
stones, flints, sandstones, and crystalline rock, in a
matrix which has obviously been largely derived from
the Oxford or the Kimeridge Clay. Many of the frag-
ments must have come from rocks which outcrop in
the northern counties or in Scotland, but some must
have travelled from Norway. The origin of these
boulder clays, whether they have been mainly formed
by great ice-sheets creeping over hill and dale, or by
ice floating in the sea, as will presently be described,
has been for many years a subject of controversy, on
which we have not space to enter, and must content
ourselves with saying that it is not yet so completely
settled as some partisans of the former appear to
believe.

In polar regions the sea rapidly freezes over at the
coming of winter to a depth of from two to three yards.
When a cake of ice forms against the shore (an " ice-
foot"), its base encloses beach shingle, which as the
frozen mass moves up and down with the tide is ground
against that frozen to the land. Where the latter ends
in cliffs, large masses of rock are detached from these by
sudden frosts, which fall or slide down upon the ice,
and this, which at a greater distance from land is called
floe-ice, breaks up at the coming of spring into huge
cakes, which float away with their cargoes of debris
towards lower latitudes. These ice-floes and the bergs
from glaciers are often of great size, especially hi the
southern hemisphere, and must distribute their burdens
as they gradually melt away over large areas of the
sea-bed, to within 35° to 40° of the equator, coming
nearer to it in this than in the northern quarter.

A few words must be said before quitting this subject
about the melting away of glaciers. As one of them
descends below the snow-line its surface is melted by

THE WORK OF SNOW AND ICE 61

the sun, especially in summer. The water thus formed
quickly gathers into streams which carve for themselves
channels in the ice as they would do in ordinary rock,
but with this difference, that any one of them is engulfed
when a crevasse opens across its path. It plunges
down to the bottom of the glacier, wearing for itself a
sort of shaft, called a mouhn, and as stones are often
carried down by the falling water and " churned "
about by it, the rock beneath is presently excavated
and " potholes " are formed. Some very fine examples
of these " giants' kettles " (Riesen-topfe or Marmites
de Ge'ants) as they are called, measuring sometimes half
a dozen yards in depth and width, may be seen in the
sandstone over which ice once passed, at a place called
the Glacier Garden, near the Lion Monument at Lucerne.
In some of them the large rounded boulders which have
aided in the work of excavation are still lying.

It is therefore obvious that ice, where it is formed, is
an important agent in modifying the surface of the
globe. Indirectly glaciers do much by feeding large
rivers and preventing them from drying up during the
heat of summer, by supplying their waters with abundant
sand and mud for transport to distant regions, and with
gravel, often extremely coarse, which is deposited nearer
to the mountains. It is obvious, from what has been
already said, that glaciers must lower then* beds , but
to what extent they have done it has been, during
the last hah* century, a much-disputed question. Sir
Andrew Ramsay in 1862 claimed that the larger
Alpine lakes (and a fortiori those of smaller size in
that and similar mountain regions) had been excavated
by ice. This hypothesis commended itself to not a few
geologists, but it has been criticised by many observers,
not less well acquainted than its distinguished author
with glaciated regions. These, while admitting that
under certain circumstances mountain tarns and some
small lakes may have been thus eroded, maintained the
difficulties against claiming such an origin for the larger
Alpine lakes to be very serious. The hypothesis has of
late years been carried to an extreme, against which

62 THE STRUCTURE OF THE EARTH

Ramsay himself would probably have protested, and
the glaciers of the Alps have been credited with having
deepened their valleys during the ice-age, sometimes by
at least a thousand feet. It is needless to say that
this hypothesis is yet more vigorously repudiated by
the opponents of lake- excavation, who deny that any
proof can be produced of ice, though an agent of some
importance in abrasion, being able to do much in
erosion.1 Time will show which of the two schools has
most accurately interpreted this chapter in the more
recent history of the earth, and whether the work of
snow and ice is nearly so important in sculpturing and
transporting as that of rain and rivers.

CHAPTER VI

THE WORK OF THE SEA

THE sea, like the rain and the rivers, is an agent of
denudation, transport, and deposition, and they are in
reality dependent on it. From the sea they come,
drawn up into the atmosphere by the sun, and to it
they return. Its waters cover rather more than seven-
tenths of the surface of the globe,2 and if this were
perfectly smooth they would form an outer shell a
little less than two miles in thickness, or, if separated,
a ball nearly 850 miles in diameter. The winds raise
waves on the surface of the sea which, though powerless
in its greater depths, wear away, as will be described,
the margin of the land. Differences in the amount of

1 Ramsay's paper on the origin of lakes appeared in Quart. Jour.
Oeol. Soc., xviii. (1862), p. 185. It was criticised by the present
writer in vol. xxvii. 312 ; xxix. 382 ; and xxx. 479. The valley-
deepening hypothesis is advocated by Penck and Bruckner, Die
Alpen in Eiszeitalter (1909) ; and criticised by the author, ut supra,
vol. Iviii. 690, and Presidential Address to the British Association,
1910. The literature connected with the subject is voluminous.

8 The proportion usually given is 0*71.

THE WORK OF THE SEA 63

heat received from the sun set up currents ; the action
of those, however, must be more or less superficial.
Those in shallower waters produce denudation like
rivers ; but hi the great depths of the ocean, though
here also movements initiated by differences of tempera-
ture must continue, these in all probability are so
feeble that a very slow accumulation is the only change.

The action of the sun and moon, as described in books
on astronomy, causes tides 1 or periodic fluctuations in
the ocean level. Since the difference between high and
low water increases as the water becomes shallower,
the ebb and flow may considerably augment the denud-
ing and transporting power of the water. The currents
also of the larger rivers can still be detected at con-
siderable distances — that of the Amazon perhaps so
much as 300 miles from land — so that they must thus
carry to long distances some of the finer mud.

The ocean bed is irregular in form, but for a descrip-
tion of this we must refer to books on physical geo-
graphy, and be content to say that sometimes it descends
very gradually, as it does in the neighbourhood of the
British Islands, where no part of the North Sea, except
a submerged channel off the Norway coast, exceeds
100 fathoms in depth, and that contour line runs about
35 miles west of Valentia and more than 200 miles in
the same direction from the Land's End. The edge of
this submarine plateau is within a shorter distance of
the Biscayan and Iberian coasts, but after passing it
the descent generally becomes steeper, though, as we
have said, if the Atlantic were dried up it would be
possible to drive from the Land's End to Newfound-
land, and that is true of other oceans. Much of
that ocean exceeds 2000 fathoms, and some parts even
3000 fathoms ; but it only once (to the north of the
West Indies) attains 4660 fathoms in depth. In the

1 In the open ocean the rise and fall of the tide is not more than
3 or 4 feet, but the difference increases as the water shallows, and
it may amount in certain gulfs or estuaries to from 10 to over 20
yards. In such cases the movement up and down, twice in the day,
of a large body of water must produce important effects.

64 THE STRUCTURE OF THE EARTH

Pacific, however, there are hollows still more profound,
soundings between 4000 and 5000 fathoms being com-
paratively numerous, and the greatest known depth (to
the east of the Kermadec Islands) 5155 fathoms. This
distinctly exceeds the greatest height of the land,
Mount Everest (slightly over 29,000 feet) ; and there is
also this difference, that, if a cast of the earth were
made, the mountains would be represented by furrows,
but the ocean depths by plateaux.

The waves act upon the land like great water-hammers
or battering-rams. Their power on exposed coasts is
always great, and becomes tremendous during storms.
At Skerryvore lighthouse, some 10 miles away from
Tiree, the average pressure of the waves in the summer
months is estimated at 611 Ibs. on the square foot,
and during the winter at 2086 Ibs., while on one
occasion (March 25, 1845) this rose to 6083 Ibs., or
2 tons 14 cwt. on the same area. It is not then sur-
prising to read that at Whalsay, in the Shetland Islands,
blocks weighing from 6 to 13 tons have been detached
during storms from their places on cliffs fully 70 feet
above the sea, and others nearly 300 cubic feet in
volume have been torn from a rocky shore and thrust
up its acclivity for a distance of 40 or 50 yards. Thus
the waves carve a rocky coast into crags and skerries,
as we can see in many parts of our coasts, not only in
the softer rocks like the chalk of Kent and Yorkshire,
but also at the Lizard, the Land's End, and " Tintagel
Castle by the Cornish sea." The Stag's Leap at Fresh-
water Gate, Old Harry north of Swanage, the Parson
and Clark at Dawlish, St. Michael's Mount, and other
insulated masses farther west, are all remnants of land
which has been eaten up by the sea. Where the rocks
are still less capable of resistance, its inroads, even in
comparatively sheltered regions like the North Sea, are
often formidable. It is rapidly encroaching on the York-
shire coast in the neighbourhood of the Humber, and
on those of Norfolk, Suffolk, and Kent, notwithstanding
the efforts made in many places to check its ravages.
The site of Roman Cromer is said to lie some two miles

THE WORK OF THE SEA 65

out at sea ; Dunwich which was an important place
in the reign of Edward I, has been reduced to a village
sheltering itself in a valley running into the land. Some
distance up this one small church is still in safety ; of
the eleven others, the ruins of the last, the eastern end
of which twenty years ago was more than five yards
from the edge of the cliff, have now lost two or three
bays. Great inroads have been made at Southwold
and Pakefield ; in fact it is estimated that the annual
loss in some places on the Norfolk and Suffolk coasts
averages two feet.1

Not only do the waves break off fragments from the
rocks, but they also undermine the cliffs, causing further
falls. The harder materials are banged and ground
one against another by the waves, and thus converted
into pebbles, which they sweep along the shore, here
piling them up as shingle banks, when they may have
a protective influence, there carrying them into deeper
water. But these heavier materials rarely travel far.
At the depth of a few fathoms the waves almost always,
and the currents generally, are incapable of moving
more than sand and still lighter materials. So much
depends on local circumstances, such as the nature of
the sea-bed and of the coast, that it is impossible to
be precise in any brief statement, but we may say that
the deposits, as we recede from the land, should be,
and generally are, in the following succession : first
shingle, next gravel, then sand, and lastly mud, beyond
which comes a broad area in which terrestrial debris,
as we have already implied, plays but a small part.
Here the remains of organisms, generally minute, accu-
mulate in the deep and undisturbed bed of the sea.
These are mostly calcareous, small algae and f oraminifera,
with occasional contributions from corals and molluscs,
but are also siliceous, such as diatoms, radiolarians,
and spicules of sponges. This globigerina ooze, as it is
called from a foraminifer usually abundant in it, were
it upraised, would much resemble the chalk of our

1 See, for instances, W. H. Wheeler, The Sea Coast (1902), chapter
vii.

66 THE STRUCTURE OF THE EARTH

English hills. It extends to a depth sometimes as
much as 2900 fathoms ; more often, however, when well
beyond 2000 fathoms it passes into an amorphous
reddish clay, the only organisms in which are some
annelids and radiolarians. Here and there before reach-
ing this last area oozes are found consisting of diatoms
or pteropods, or containing a mineral called glauconite
(a hydrous silicate of alumina, iron, and potash), which
is precipitated, as on the Agulhas Bank, within the
f oraminif ers, thus forming a rock which would resemble
the Upper Green Sand of South-east England. The
origin of the Bed Clay, in which concretions of man-
ganese oxide are formed, has been disputed. Some
have regarded it as consisting of the finest variety of
mud, drifted from the land, which, however, is improb-
able ; others as a chemical precipitate in the f ora-
minif eral tests, which have been afterwards dissolved
by the sea water; while others consider it to be the
detritus of pumice, which, after floating on the ocean
surface, has become waterlogged and sunk to the bottom,
where it has been joined by meteoric dust. Be this as
it may, the depths are being filled up, but as a rule
very slowly ; and though here and there fairly marked
inequalities may be noted in the ocean's bed, resembling
submerged cliffs or river channels, these are generally
not far from the land.

The ocean level has obviously altered much since
the geological record began. One cause of this may
be a sinking of the land, another a rise of the sea.
These, it may be remarked, are not always convertible
terms, though both are due to a change in the form
of the earth, for a part of the land may have its distance
from the centre diminished or increased, and the sea
may do the same in consequence of an alteration in the
shape of its bed, besides being affected by one or two
other, though minor, causes. Obviously, then, the
zone of marine denudation advances or recedes, and
that of deposition, whether mechanical, chemical, or
organic, must be correspondingly affected.

As regards the former, the main difference between

THE WORK OF THE SEA 67

the sea and a river as a denuding agent is that the
one planes and the other furrows. Stages in the latter
are sometimes marked by steps, or terraces, more or
less distinct, on the sides of a valley ; in the former
by the same along the side of the land. As this is
rising from the sea, the waves at each pause forthwith
proceed to cut a groove or cliff or slope, depending on
the coherency of the materials. These, together with
the beaches at their foot, may sometimes be found
some distance, perhaps a few hundred feet, above the
sea-level. On the coast of Chili they are said to occur
in places as high as nearly 1300 feet, and changes of
this kind must have happened in comparatively recent
times along a large part of the western coast of South
America. On the same side of Scotland a terrace-like
raised beach is conspicuous, in many places 25 feet
above the sea, and signs of others may be detected at
a higher level. The former also may be traced rather
nearer to the sea along several parts of the British
coast.

From what has been said, it follows that the tendency
of the sea, whether in denuding or depositing, is to
produce level surfaces on a large scale. In the former
case it may meet with masses of rock harder or more
capable of resisting attack than their neighbours, so
that they may be left as islands, from which the sea
may have to retreat before it can reduce them to the
general level. This, for instance, would be the con-
dition of the Channel Islands, if the sea-bed were ele-
vated by much less than a hundred fathoms ; but all
the new land surface around them would form, as a
whole, a gently shelving plain, in which the rivers
flowing over the present land surface would proceed to
carve channels. Again, if a corresponding portion of
the sea-bed within the zone of deposition were up-
raised, it would present a similar or even more uniform
contour. The plains of Holland, though here it is
rather the sea which has been excluded than the land
which has been upraised, may serve to illustrate the
effects of submarine deposition.

68 THE STRUCTURE OF THE EARTH
CHAPTER VII

VOLCANOES AND THEIR LESSONS

A VOLCANO is formed by the discharge, more or less
explosively, of material, most of which is or has been
in a molten condition, from an opening in the ground.
Around this that material is piled up to form a hill,
sometimes comparatively low, called a cone, at the
top of which is a bowl-like hollow — the crater. The
cone is often wholly built up of slaggy or rough frag-
ments— named volcanic ash, or scoria, or lapilli, or
pumice if very cellular, siliceous, and light in colour —
though occasionally it consists only of overflowed molten
material called lava, but very commonly (especially in
the case of large volcanoes) of a mixture of the two,
lava either flowing in streams from the crater, or more
frequently breaking out from some fissure in the side
or at the base of the cone. Thus volcanoes afford
several varieties of form and structure, as will presently
be indicated, and may illustrate every phase, to use a
metaphor, from the activity of life to the rigidity of
death, in which case the corpse may exhibit all stages
of dissection.

In the British Islands no example of an active volcano
can be found, for, though during past geological ages
eruptions were far from rare in one part or another,
they have so long ceased that some of their most obvious
features have been destroyed. Cones and craters are
better preserved in the Eifel district of Germany and
in that of Auvergne in Central France, but no volcano,
still active, can be found nearer than Southern Italy or
the neighbouring islands. We may find it convenient
to select, as the first example, a volcano which is com-
paratively small, has well-defined boundaries, and
generally shows some signs of activity. The conditions
are fulfilled by Stromboli, one of the Lipari Islands,

VOLCANOES AND THEIR LESSONS 69

about thirty-eight miles from the Calabrian coast,
which is often called " the weather-glass of the Mediter-
ranean." The highest point in its rim is 3090 feet
above the sea, and, so far as can be ascertained, it is
wholly built up of scoria and lava. In one respect,
however, the volcano is not quite normal, for it dis-
charges at present not from the centre of its original
crater ring, but from three or four small orifices, very
near to each other, on its north-western side, where
the original rock-wall has been shattered by later ex-
plosions. From these marginal craters a cloud of steam,
blackened with volcanic dust and scoria, is ejected to
heights sometimes as much as 400 feet, and the debris
comes raining down over an area surrounding the centre
of discharge. This habit of frequent but comparatively
innocuous explosion is called a Strombolian phase of
volcanic activity. But even here, now and again, it
is interrupted by one of greater violence, when large
quantities of dust, scoria, and even blocks up to a
yard or more in diameter are discharged, together with
splashes of liquid lava which solidify in falling — the so-
called volcanic bombs. These have formed a black slope
— the Sciarra — leading from the mouth of the craters
to the sea, down which they may sometimes be seen
rolling like red-hot cannon-balls. On ascending the
mountain or examining from a boat the low crags at
its margin, we find no rocks other than volcanic ; and
as the island slopes down for nearly 600 fathoms to
the general level of the sea-floor, it has probably been
built up from that depth in a cone well over 5000 feet
high. The other islands of the Lipari group have a
similar origin, but only one of them, by name Vulcano,
is occasionally active.

Vesuvius, generally an attraction, but sometimes a
terror, to Naples, is much better known than Stromboli
to the world at large, but its boundaries are less definite,
for on the more western side it is closely connected with
a great group of minor cones — the Phlegrsean Fields —
and on the east and south-east merges with a moun-
tainous region, composed of sedimentary rocks — part

70 THE STRUCTURE OF THE EARTH

of the Apennines. Vesuvius, like Stromboli, retains
a large fragment of an ancient crater — Monte Somma —
the highest point of which is 3640 feet above sea-level,
but here the vent still active is more nearly central in
position, and is rather higher than the other.1

Vesuvius has a very interesting history, because, in
the year 79 of our era, it suddenly awoke one August
day after a slumber so prolonged that not even tradi-
tion had preserved the memory of an eruption, and it
celebrated its renewal of work by a destructive orgie,
of which the younger Pliny has given such a graphic
account. In less than a day the volcano had blown
away about half the crater ring of Somma, had buried
part of Stabisa and the whole of Pompeii beneath vol-
canic ash, had overwhelmed Herculaneum beneath a
stream of mud, which afterwards set like a cement, had
destroyed an immense amount of property and many
hundreds — the exact number is not recorded — of lives.
Since that date eruptions, sometimes hardly less severe,
have occurred at uncertain and occasionally rather long
intervals, during which perhaps nothing more than a
little steam escaped from the crater. That of December
1631 was noteworthy for the emission of a large quantity
of lava, which broke out at nearly 3000 feet above the
Mediterranean, and flowed down the slopes in no less
than seven streams, one of them taking its course to
the sea through Torre del Greco, about two-thirds of
which it destroyed, together with some 18,000 lives,
there or ekawhere. Since that time there have been
several severe eruptions, the record of the later part
of the eighteenth century being particularly bad. Such
eruptions, more or less violent after rather long intervals
of repose, are called the Vesuvian type.

Of this a remarkable variety has recently attracted
special attention. The islands of St. Vincent and
Martinique in the West Indies are both formed of

1 Exceptionally violent eruptions have more than once reduced
the height of the cone by more than 400 feet, after which quieter
discharges have again built it up to at least this distance above
Somma.

VOLCANOES AND THEIR LESSONS 71

volcanic materials, and each is crowned by a fairly
large crater ; that in the former island being named
the Soufriere, and that in the latter one Mont Pelee.
Both had been at rest for not much less than a cen-
tury, and lakes had formed in each — no uncommon
thing in ancient craters. On May 7, 1902, the Sou-
friere, after some warnings, broke out into violent
eruption, and on the next day Mont Pelee followed its
example. In both islands large areas were buried
beneath scoria, dust, and mud (no lava was emitted,
so far as is known), but the most terrible feature was the
sudden discharge of an enormous quantity of extremely
hot dust, which was more fatal than any plague of
Egypt to herb and tree, to beast and man. The loss
of human life in St. Vincent is estimated at nearly 1600,
but in Martinique the glowing avalanche swept down
upon St. Pierre, when the people were thronging to
church on Ascension Day morning, and in a few minutes
a flourishing city was a heap of rums, and some 28,000
of its inhabitants had perished. Only a small part lay
outside that path of destruction, within which hardly
any living person escaped, the only one unhurt being
a prisoner, who was so closely immured that the burn-
ing dust failed to make its way into his cell. A similar
explosion, though on a rather smaller scale, was wit-
nessed, on the evening of July 9th, by Dr. Tempest
Anderson and Dr. J. S. Flett from the deck of a vessel
moored ofi St. Pierre, and they only just managed to
escape its path. It started from a gap in the lip of
the crater, rushed like a glowing avalanche down the
upper slopes of the mountain, and then drifted above
them as a dark cloud, showering down dust and scoria,
still hot. Such is now called a Pelean type of eruption.
Mont Pelee, however, not content with its discharge
of incandescent dust, afterwards exhibited a pheno-
menon which, so far as known, is without a parallel.
In July 1902 a mass of solid lava began to be protruded
slowly from the crater, like a cork about to be dis-
charged from a bottle of soda-water. This, however,
did not happen, for though " the spine " continued

72 THE STRUCTURE OF THE EARTH

to rise till it had reached a height of nearly 2000 feet,
it rapidly scaled away, until in the spring of 1907 it
had become little more than a heap of ruins at the
top of a dome of broken rock.

The fine dust from these volcanoes has often travelled
for long distances. That from the Soufriere on May 7th
fell thickly at Barbadoes, 120 miles away, and it re-
peated the effort in October 1902 and March 1903, as
it had already done in May 1812. Cotopaxi, on July 3,
1880, when the late E. Whymper was making his
second ascent of Chimborazo, suddenly ejected a black
cloud of dust to a height of about four miles, which
was carried by the winds across the sixty miles' interval
between the summits, and began to settle down upon
the latter a short time after he had arrived upon it.
The dust of Vesuvius has fallen in Montenegro, at
Tripoli, and even at Constantinople.1 Krakatoa, in
the Strait of Sunda, which in 1883 broke a long silence,
not only covered the adjacent sea with pumice, and
sent quantities of dust to Batavia, ninety-four miles
away, and six miles farther to Buitenzorg, but also is
supposed to have shot the finest material to a height
exceeding twenty-five miles, from which it so slowly
settled down as to make the circuit of the globe at least
once, and to produce the wonderful sunset glows which
attracted so much attention in the late autumn of
that year.

The lava solidifying in the throat of a volcano is
a common feature in its closing days. It may be said
to die from an obstruction of the gullet. The elements
then begin their destructive work : they tear down
the crater, and sweep away the materials of the cone,
till at last the plug forms the highest part of the moun-
tain. That is the case with Aconcagua, the culminating
summit in the whole chain of the Andes, and with more
than one lofty volcanic peak in its immediate neigh-
bourhood or in the Ecuador group. In Auvergne,
though cones and well-preserved craters are common,

1 The dates and authorities are mentioned in the Encyclopaedia
Britannica, art. Volcano (llth edition).

VOLCANOES AND THEIR LESSONS 73

the Pic de Sancy (the highest summit in the district,
for it is more than 6000 feet above sea-level) has lost
all trace of a crater, and in the more southern part of
Scotland volcanic necks, as they are called, the more
or less dilapidated ruins of cones, generally small, are
very far from rare. North Berwick Law is one of these,
Arthur's Seat is another, though its precise history is a
mater of controversy, and many can be seen in all
stages of dissection, either in the crags or on the beach
of Fifeshire.

As was said above, lava, in struggling to reach the
surface of the earth, makes its way along fissures, often
nearly vertical, in which ultimately it becomes solid.
Sometimes, when these are numerous and the neigh-
bourhood suitable, it wells forth from them in great
sheets many hundred square miles hi extent. That has
happened in Idaho and adjoining districts in the United
States, where an area, said to be hardly less extensive
than France and Great Britain together, has been
buried under vast sheets of basalt, sometimes to a depth
of over two thousand feet. During a distinctly more
remote geological period similar discharges, though on
a rather less gigantic scale, occurred in Antrim, the
Inner Hebrides, and part of the adjacent mainland,
and these outbreaks affected, though sometimes only
locally, a district estimated at about 40,000 square
miles. To this date belongs a group of important
dykes l in the North of England, one of which, the
Cleveland dyke, runs for some ninety miles across
country 2 from Armathwaite almost to the sea near
Maybecks in Yorkshire. Sometimes also the molten
material thrusts itself horizontally between the beds
of stratified rock, and after it has become solid, may
be compared to a paper-knife pushed between the
pages of a book. Such intrusions are called sills, and

1 Wall-like masses of igneous rocks, which do not always reach
the surface, are called dykes when the fissure runs evenly and
is nearly vertical, and veins when it branches.

* This assumes three dykes which are not continuous to be, as
is highly probable, really identical.

74 THE STRUCTURE OF THE EARTH

often are not at first sight easily distinguished from
sheets of lava which formerly have flowed upon the
surface and subsequently been covered by sedimentary
deposits. In another mode of intrusion, generally
more limited in extent, the molten material lifts the
overlying strata in the form of a low-crowned arch,
thus taking a shape something like that of a mush-
room. Intrusive masses of this kind, called laccolites,
were first noticed in the Henry Mountains of the United
States, and they have been detected since then in
other countries, including Great Britain. It is possible,
indeed, that some of the larger masses of granite, like
those of Dartmoor, may really be laccolites on a large
scale instead of being boss-like in shape, enlarging rather
than contracting in extent in a downward direction.
The earth's crust, in fact, sometimes is not only partly
built up of horizontal masses of once molten rock, but
also is traversed, pierced, studded, and strengthened
by others, which vary, as has been described, in texture
and chemical composition.

CHAPTER VIII

MOVEMENTS OF LAND AND THEIR RESULTS

PROCESSES of denudation, whether by streams or by
the sea, tend, as we have shown, to lower the level
of the land, so that, if time enough were given, all its
irregularities would be worn away. But this does not
altogether happen ; there are stratified rocks in many
regions on the earth's surface which are proved by
their fossil contents to have been laid down below,
perhaps very far below, the surface of the sea, but
which are now high above it. The shells of marine mol-
luscs now extinct have been dug out of the so-called
London Clay on the slopes of Hampstead and Highgate.
The chalk of the North and the South Downs consists
of organisms which lived in the sea, and was probably

MOVEMENTS OF LAND 75

formed at a greater depth from the surface than it is
now above it. Fossil shells are found high up on
mountain ranges, as on the top of the Diablerets (10,650
feet) in the Western Alps, and over 16,000 feet in the
Himalayas. Hence one of two things must have
happened : either the surface of the sea must have sunk,
or that of the land must have risen. Possibly both may
have occurred, but to what extent the change may be
attributed to the one cause or the other can be more
easily determined after a brief review of the facts.

Evidence of upheaval must be more common than
that of depression. We can trace a bed by its fossils
from the present sea-level to a height of many hundred
feet above it, but we can only ascertain what lies below
that level by boring or by mining. Coal, for instance,
is formed of plants which, as a rule, must have lived
in fresh-water marshes, and thus must have grown a
little, though it may not have been much, above sea-
level. Now coal seams are sometimes worked at least
3000 feet below it. Not seldom marine and fresh-
water deposits are found to alternate. For instance, in
the South-east of England the so-called oolites — marine
in origin — pass up through estuarine deposits (which
themselves indicate an oscillation between sea and
land) into a great succession of fresh-water beds, which
sometimes exceed 2000 feet in thickness, the well-known
Hastings Sands and Weald Clays. These are followed
by a group of marine sands and clays, which in England
are called the Lower Greensand, and to this succeeds
the blue clays of the Gault and the Upper Greensand
(also marine), which are followed by the soft white lime-
stone of the Chalk. This succession proves that an area,
once occupied by the sea, must have been for a long
time sufficiently raised above it to become the delta of a
great river, and have afterwards been again submerged
to an even greater depth than before. These facts may
suffice, but the story might be continued to prove the
occurrence of similar changes during still later chapters
of the earth's history, and it is repeated in almost every
part of the earth and through all the ages of geology,

76 THE STRUCTURE OF THE EARTH

so that the words of Tennyson 1 are no poet's dream but
an expression of a scientific fact :

" There rolls the deep where grew the tree.
O earth, what changes hast thou seen !
There, where the long street roars, hath been
The stillness of the central sea."

These changes in the relative level of the earth and
sea have not only occurred in the past but also are
probably still in progress. Now and again, after an
earthquake, the land is found to have been lifted up or
dropped by a few inches or feet, and sometimes variably
on either side of a fissure, as, for instance, in Japan
or New Zealand. There is a well-known case, often
quoted, near Pozzuoli in the Bay of Naples. A short
distance from the sea are the ruins of a building, gener-
ally called the Temple of Serapis, of which three columns,
made from a Greek marble called cipoUino, are still
standing. Their bases are now very slightly below
sea-level ; their shafts, for the next 12 feet, are yet
smooth, but for the next 9 feet are pierced with boring
molluscs. It is known that the building was intact
during the third century of our era, for it received new
decorations from the Emperor Alexander Severus, that
it probably fell into ruins during or soon after the fifth
century, and that, prior to 1530, the sea washed the
base of the cliff which rises some little distance inland,
though it had then begun to retreat. Since that time
there has been a much greater upward movement, and
now one in the opposite direction has apparently begun.
Here, however, the area affected may not be large, and
be connected with the neighbouring volcanoes, but it
seems to have been now established that there is a
slow but unequal rise of the land in Southern Sweden
and a similar subsidence on the coasts of Newfoundland
and Labrador.2 In tropical seas not a few islands show
that coral-reefs have been raised, often from 20 to 80 feet,

1 "In Memoriam," cxxiii.

2 The evidence is quoted in Sir A. Geikie's Text-Book of Geology
(1903), p. 380.

MOVEMENTS OF LAND 77

above the water, and sometimes, as in Cuba, for quite
1000 feet. Yet at Funafuti, one of the Ellice Islands,
a boring was put down for rather more than 1100 feet,
which indicated a depression about equal in amount to
the upheaval in Cuba.1 Again, rocks bored by litho-
domous molluscs, incrusted by barnacles, serpulse, and
corallines, or worn and grooved by the action of the
waves, may be found at considerable heights above sea-
level, and raised-beaches — beds of pebbles and sand,
containing occasionally marine organisms, and identical
with those which can be found where the waves are still
breaking— are common on many of our coasts, from Corn-
wall to the North of Scotland, at various heights up to
at least 25 feet above sea-level. In the latter country
similar raised beaches may be seen at about twice and
four times this height. Platforms and caves, worn by
the waves at the foot of cliffs, can often be noticed on
the western coast and islands of Scotland. At several
places on the estuary of the St. Lawrence sea-shells,
differing little from those still living nearer to its mouth,
occur at various elevations up to quite 500 feet, and
the bones of whales, with other marine creatures, have
been found in the neighbourhood of Smith Sound at
heights up to rather more than 1000 feet above the sea.
By examining the relations of the stratified rocks
over larger areas we are able to infer the nature of the
movements to which they have been subjected. In the
south-eastern part of England, to which reference has
already been made, we find, in travelling from London
to Brighton, the chalk of the North Downs dipping
northwards.2 So do the underlying strata — the Upper
Greensand, the Gault, the Lower Greensand, and the
Weald Clay, till we come to the Hastings Sands. In
the last the beds for a time follow the same rule, then
they bend over in a kind of arch and are inclined toward

1 Keef-building corals, as a rule, do not flourish at a depth
exceeding 150 feet.

2 The dip of an inclined stratum is measured by the angle which
it makes with the plane of the horizon. Its line of intersection
with that plane is called the strike, and the one with the surface
of the ground, the outcrop.

78 THE STRUCTURE OF THE EARTH

the south, after which we find the same strata, but in
reversed order, dipping in that direction. Hence as
these strata must have been deposited one above the
other, almost, if not quite horizontally, they must have
been subsequently bent into a low-crowned arch, during
or after which process — probably to some extent in both
— they have been subjected to great denudation, which
has removed huge masses of rock, so that the widely
separated chalk hills of the North and South Downs
remain like the abutments of a broken arch. But this
is not all. The chalk of the North Downs, after dis-
appearing beneath the sands and clays of later date,
which underlie London, rises again in the Essex and
Hertfordshire hills, and is usually struck, if a boring be
put down, at the depth of less than 150 feet beneath
the metropolis. Thus the anticline of the Weald, which
can be traced for some distance on the eastern side of
the Strait of Dover, is succeeded by the syncline of the
London basin.1

The Pennine range, which extends into Derbyshire
from the northernmost part of England, proves the
occurrence of movements on a yet larger scale and with
rather more complication. In the hSl district of that
county a mass of grey limestone forms a kind of saddle,
dipping to the east on one side of the crest, to the west
on the other. On each of these it is succeeded by a
thick series of shales and sandstones, which is followed
by another one containing important deposits of coal.
The original continuity and horizontality of these beds
becomes plain on examination, so that here also the
crust of the earth has been bent. Similar movements,
accompanied sometimes with important fractures and
displacements, called faults, can be shown to occur in
many places.

But the movements of the earth's crust, especially
in mountain chains, are sometimes more complicated
than those which we have been describing. Places
may be found where a portion of it, in outline a broad

1 Beds which dip in opposite directions from a central axis are
called anticlinal, and if towards it, synclinal.

MOVEMENTS OF LAND 79

strip, sometimes more than a hundred miles in length,
has evidently undergone great compression, which
must be the result of lateral thrusts, though the exact
cause of these cannot always be readily ascertained.
The effect may be illustrated by supposing a number
of rather stiff rugs to be laid one above another on
the ground between two boards, one of which is steadily
impelled towards the other. These rugs will pucker
up in folds, which will become sharper as the process
is continued, and in some cases one fold might become
doubled back upon the other. If the material of these
layers were less flexible than carpet, they might at
last be unable to bear the strain, and a rupture might
occur near the crest of a fold, after which one part might
be pushed forward over the other. In many mountain
ranges cases of overfolding and thrust-faulting, as these
are called, are far from rare, and they have sometimes
produced an apparent sequence in the strata which is
quite illusory. Examples of these may be found in
many parts of the Alps and, nearer home, in the High-
lands of North-west Scotland. In the latter it was for
a long time supposed that a group of comparatively
unaltered strata, some of which contained fossils, were
overlain by another which had undergone very im-
portant mineral changes. Had that been true, the
beds which lay at the top must have been affected by
subterranean heat and other agents, producing altera-
tion much more than in those beneath them ; while the
real fact was that a group of more crystalline rocks
had been thrust over another much later in date, and
the perplexities had been increased by certain modi-
fications during the process.

These corrugations, fractures, and slidings of wedges
in the earth's crust, one above the other, are far com-
moner in mountain regions than was formerly supposed,
and the failure to recognise them often led to very
erroneous ideas as to the thickness of deposits and the
possibilities of metamorphism. For instance, it was
supposed that the stratified rocks in the southern
uplands of Scotland occurred in an orderly upward

80 THE STRUCTURE OF THE EARTH

succession, and attained a thickness of fully 14,000 feet.
But Professor Lapworth demonstrated, nearly forty
years ago, and his work has since been confirmed by
the more detailed investigations of the Geological
Survey, that the same beds are repeated several times
in closely compressed folds, thus reducing the total
thickness to a few hundred feet.1 Again, in that mighty
wall of rock which forms the northern face of the Bernese
Oberland, gneiss apparently alternates with limestones
or shales of Secondary age, but in reality great wedge-
like masses of the older rock have been forced through
the broken folds of the newer. Even apart from these
complications a little study of the Alpine rock-masses
proves them often to exhibit folding on a gigantic
scale. In the district just mentioned the grand preci-
pices of the Wetterhorn, together with its northern
peak, consist of limestones, but the middle and southern
peaks are formed of the ancient crystalline rock ; and
this is also the case with their neighbours, slightly farther
south, the Schreckhorn and the Finster Aarhorn, in
which that rock, though formerly buried beneath the
above-named sediments, now overtops them in the latter
peak by a thousand feet. The range of Mont Blanc
afEords a yet more conspicuous instance of folding.
Its upper part is formed of ancient crystalline rock,
while the valleys of Chamonix and Courmayeur are
excavated in slaty beds of Secondary age. The one
rock rises some 15,700 feet above sea-level, the other
barely attains 7000 feet. A little study shows that in
the Mont Blanc Aiguilles and the ranges of the Brevent
and the Mont Chetfl, to the north and south respectively,
we can recognise the shattered cusps of three enormous
folds, while the slaty beds above-mentioned are the
remnants of their troughs.

Besides the conspicuous displacements, indicated by
great and often repeated flexures of the earth's crust,
large blocks of it are often either raised up or dropped

1 Folds which follow on such close succession that their cusps
point in the same direction and their sides are nearly parallel are
technically called isoclinal

MOVEMENTS OF LAND 81

down, without any crumpling. Such displacements
may be produced by an arching of the crust between
two positions rather far apart, the result being the
formation of one or more set of fractures, and a settling
down of the broken masses either into the underlying
void or on to more " pasty " material below, till they
again arrive at positions of equilibrium. When the
plane of fracture is either vertical or slopes down beneath
the dropped portion, the fault is called a normal one ;
but if the slope is in the contrary direction, it is said to
be reversed. The former obviously is more likely to
be the result of a strain and the latter of a thrust. When
the strips of crust formed by a number of parallel normal
faults are let down continuously more and more in
either direction, this is called step-faulting ; and a
modification of it, the dropping of a long strip of the
crust between two parallel faults or groups of faults,
is named a trough-fault. As faulting obviously brings
into juxtaposition two very different kinds of rock, it
has a great effect on scenery, and trough-faulting on
a large scale may give rise to valleys.

For instance, the Valley of the Jordan takes its
origin from two nearly parallel faults or groups of
faults which run southward from Lake Huleh (the
ancient Merom) to the Gulf of Akabah, whence they
may be traced southward towards the lake region of
Central Africa. In most parts of the area thus affected
careful study is needed to detect the displacements ;
but in others, according to Professor J. W. Gregory,1
these are so recent that the fault-face is comparatively
unmodified by weathering. To a striking instance of
this, west of Mount Kenya, he gave the name of the
Rift Valley, and that term has often been extended,
but improperly, to valleys which, like that of the
Jordan, would more correctly be called trough-fault
valleys.

Displacements along the planes of faults, when they
are at all sudden, produce tremors in the crust of the
earth, which are sometimes propagated through it to

1 The Great Rift Valley of Central Africa (1896), p. 220.

F

82 THE STRUCTURE OF THE EARTH

very great distances. Such tremors, which may also
be connected, though more locally, with volcanic ex-
plosions or the struggles of lava to reach the surface,
are called earthquakes. They may vary in their in-
tensity from a slight quivering of the ground, like that
caused by the passage of a heavy waggon, to a con-
cussion which is sometimes very destructive. Certain
regions suffer from such earthquakes more severely
than others, and these are observed to be, as a rule,
closely connected with regions of folding or faulting.
During one of them undulatory movements traverse
the ground, in brief but rapid succession ; waves, one
of which is often formidably great, are started when
the shock originates beneath the sea ; chasms open in
the earth, and landslips may occur ; buildings are
shattered and thrown down, often with great loss of
life. In North America, Charleston and San Francisco
have suffered severely more than once, and both in
comparatively recent years. On November 1, 1775,
the greater part of Lisbon was destroyed, with a loss
of more than 30,000 lives. The Calabrian coast, with
the immediately adjacent part of Sicily, has several
times suffered heavily. In the worst shock of a dis-
turbed period, which lasted from 1783 to 1786, Messina
and other towns were shattered, with an estimated loss
of about 40,000 persons. The calamities recurred in
1857 ; and the partial destruction of Messina in Sicily,
and Reggio in Calabria, about five o'clock in the morn-
ing of December 28, 1908, during which, according to
the official estimate, 77,285 persons perished, is still
fresh in memory. In the Rann of Cutch, on June 16,
1819, a large area of land sank beneath the sea during
an earthquake, while a smaller one was elevated ; and
in Japan, which might be called a land of earthquakes,
one on October 28, 1891, caused a crack to open in
the ground which ran for about seventy miles, crossing
almost the entire breadth of Nippon, and caused a
vertical displacement which hi some places amounted
to about twenty feet.

It would be easy to multiply examples, but these may

MOVEMENTS OF LAND 83

suffice to give some idea of the terrible destruction
which may be caused by earthquakes. Our own islands
have happily been almost immune from serious shocks,
probably because the movements caused by folding
practically ceased at a fairly distant epoch, and the
rocks afiected by them are buried in our lowland dis-
tricts beneath a thick covering of comparatively loose
and inelastic materials — which, if the former were shaken,
would act like a feather-bed.1 But where the older
rocks come to the surface, as in Scotland, minor shocks
are not infrequent, and they occasionally make them-
selves felt in different parts of England. As a rule only
a slight trembling of the ground is perceptible, and that
over a comparatively limited area, but now and again
some little damage has been done to buildings, such as
when the front of Lincoln Cathedral was cracked in the
year 1185, and when, in the East Anglian earthquake
of April 22, 1884, the cost of repairs amounted to several
thousand pounds. The concussions due to the more
severe earthquakes are often felt over very large areas
of the earth's surface, and can now be detected, where
they are far too slight to be otherwise noticed,2 and
the position of the centre of disturbance can even be
located, by the aid of delicate recording instruments,
called seismometers, which have added greatly to our
knowledge of the nature of the movements and have
supplied some indirect, but important, evidence in
regard to the internal constitution of the globe.

It is very difficult to determine the causes to which
these several forms of crust- disturbance are due. More
than one explanation has been ofiered. Some regard
them as a consequence of the secular cooling of our
planet, which, as already stated, must have formerly
been an incandescent mass. The loss of heat by radia-
tion and the consequent contraction of the zone beneath
the part which had already become solid would cause

1 The Lisbon earthquake was not noticed in England, but the
shock was perceived, though but slightly, in Scotland.

* It is estimated that 30,000 to 40,000 earthquakes occur
annually, the great majority being fortunately harmless.

84 THE STRUCTURE OF THE EARTH

the latter to wrinkle, like the skin of an apple when
it is drying. This contraction might sometimes make
a separation between the zone, which was already cold,
and that which was still plastic, the consequence of
which might be fracture and collapse under the action of
gravitation. Others suppose the strain produced by the
rotation of the globe acting upon a crust unequal in
strength ; while others, assigning the same cause, think
it must be attributed to efforts to assume a form of
perfect equilibrium. Even if this had ever been attained,
either on first cooling or at a subsequent time, the changes
which result from denudation and the transference of
material from one part to another would soon introduce
instability. This explanation is perhaps the one re-
garded with most favour at the present time, but we
may venture to doubt whether this cause, though it
must produce some effects, is adequate to account for
such foldings so remarkable as those in the Appalachians
and the Alps which are believed to indicate that a strip
of crust has been reduced in breadth in the one case by
46 miles, in the other by 74. The whole subject, how-
ever, together with that of earthquakes, is far too com-
plicated and difficult for discussion in these pages, so
that we must be content to leave it without further
notice as one of the problems in physical geology con-
cerning which, notwithstanding considerable accessions
to knowledge during the last twenty or thirty years,
we have still much to learn.

CHAPTER IX

THE LIFE HISTORY OP THE EARTH

THE history of living creatures shows a progressive
evolution, though races, like individuals, die out. It
illustrates the adaptation of forms to their environment,

LIFE HISTORY OF THE EARTH 85

with consequent modification and the destruction of
those incapable of further change. The earlier pages
of the record are so defective or blurred that they
cannot be read. They begin with the Cambrian period,
but we can see from some traces, generally obscure, of
things that have lived in still older deposits and from
the fact that the great divisions of the invertebrata are
represented very early in this period, that it must be
long subsequent to the beginning of life. In Britain
the records of the early Cambrian are scanty, com-
prising perhaps 200 species, but they are rather fuller
in other lands, especially America. Briefly stated, the
life of the Cambrian period is mainly made up of
brachiopods and trilobites. It also comprises a few
lamellibranchs, gastropods, and (in the uppermost divi-
sion) a cephalopod ; the crinoids or " sea-lilies " and the
star-fish already existed. In the Ordovician the trilo-
bites increase in number and diversity ; the other
organisms, named above, show a more gradual advance,
and the graptolites, animals rather distantly related to
the sea- firs (sertularidce) of our coasts, which appeared
at the end of the Cambrian, are very abundant and
valuable, in consequence of their rather restricted
vertical range, for indicating horizons. In the Silurian
the trilobites are dwindling, but a peculiar group of
rather large crustaceans makes its appearance, of which
the living limulus or king-crab is to some extent a
survivor. Crinoids, corals, and the molluscs generally
are much more largely and often profusely represented,
and, rather late in the period, the first vertebrate, a
fish, makes its appearance.

Fishes became abundant in the Devonian or Old Bed
Sandstone,1 and corals, with different orders of molluscs,
are plentiful, but trilobites are steadily declining. In
the Carboniferous system, molluscs, both marine and
fresh water, are abundant, and the oldest-known land-
shells appear ; insects were plentiful, while in beds of
the former origin corals, brachiopods, and crinoids are
very numerous. Fish often almost swarmed, and a
1 Much of the latter is believed to be a fresh-water deposit.

86 THE STRUCTURE OF THE EARTH

new class of the vertebrates — the amphibians — is rather
sparingly represented.

The Permian fauna is not well developed in Britain,
owing probably to local peculiarities, but in other
countries it inoUcates an alliance with that of Carboni-
ferous tunes with forerunners of the coming system,
among which a reptile is of special importance. That,
the Trias, is also abnormal in Britain, but from other
countries we can see that the Palaeozoic fauna had almost
disappeared and been replaced by that characteristic
of the Mesozoic era. Certain cephalopods, which range
throughout it, especially those called ammonites, make
their appearance ; the molluscs and crustaceans are
greatly changed, and show more resemblance to those
which are now living. Quite late in the Trias the first
mammal, small and with some reptilian affinities, has
been found. The Jurassic system is rich in lif e ; crinoids
on certain horizons, with almost everywhere molluscs of
all kinds, especially ammonites, belemnites (a sort of
cuttlefish), and brachiopods. Reptiles now become
abundant, and several attain to a great size. Some,
like the giant Diplodocus, which was about 80 feet in
length, were vegetarians ; but certain others, not so
big, but more active, must have been terrors to all
weaker creatures. Mammals, small and feeble, are
found, and the first bird, which exhibits characters
indicating a descent from reptiles. The Neocomian is
often imperfectly represented in England, and its lower
part is a fresh-water deposit in the south-east. The
fauna bears a general resemblance to that of the Jurassic,
with many indications of coming change. In Cretaceous
times a great part of Britain was gradually submerged,
while the pure-white chalk, characteristic of that
period, was deposited. Large reptiles still existed, but
are evidently declining, the most remarkable being
the Mosasaurus, which sometimes attained a length of
60 feet, and from its rather snake-like form might be
taken for the original sea-serpent. Among the molluscs
and other invertebrates, the genera characteristic of
the Mesozoic era continue, but a change is beginning

LIFE HISTORY OF THE EARTH 87

to be marked. Mammals are still few, but birds are
rather commoner, and nearer in structure to living
forms, though some of them were armed with teeth.

In England whole pages are torn out of the story-
book of life between the end of the Mesozoic and the
beginning of the Kainozoic, the first period of which,
the Eocene, shows a great change in the fauna. Brachio-
pods have become scarce ; the ammonites and allied
chambered cephalopods have disappeared ; the gastro-
pods and lameUibranchs mostly belong to existing
genera ; the great reptiles have died off ; birds are
commoner, and so are mammals, which rapidly increase
in size and variety. They now show signs of a rapid
evolution, so that before long large and strange-looking
representatives of this class take the place formerly
occupied by the great reptiles. New conditions have
begun to prevail, and the remainder of the Kainozoic
era shows a gradual approach to the forms of life which
now occupy the globe ; existing genera among the
vertebrates and species among the invertebrates gradu-
ally making their appearance, while those of older date
drop out of the race.

The plant-history of the globe shows a similar pro-
gress, but its more marked changes do not altogether
synchronise with those among the animals. At first
its record is very imperfect. In the Cambrian, remains
of plants are few and obscure. They are but little
better in the Ordovician, and not common in the Silurian.
They cease to be rare in the Devonian, and are, of course,
abundant in the Carboniferous, the vegetation of which
continued, though with changes, into the Permian.
The Palaeozoic flora, like its fauna, differs widely from
the present one. It is characterised by the absence of
dicotyledonous plants and the dominance of ferns,
horse -tails, and club -mosses, representatives of the
second (Calamites) attaining a large size, and those of
the third (Lepidodendron and Sigillaria) taking the
place of forest-trees. A few conifers, however, existed,
and perhaps may have been more abundant in hilly
districts, the flora of which is almost unrepresented. A

88 THE STRUCTURE OF THE EARTH

change sets in with the Mesozoic ; the older flora dis-
appears, and one more nearly resembling the present
begins, the characteristic of which is the dominance of
palms and cycads. Another change is initiated in the
Neocomian flora, and that of the Cretaceous assumes
an aspect distinctly modern, and the vegetation through-
out the Kainozoic era shows an increasingly close
approximation to that which is now in existence.

Great changes of climate must have occurred in the
Kainozoic era. During the Eocene it was much warmer
than now, and towards the middle of this was almost
tropical in the south-east of England. Then it became
gradually colder, till in the later part of the Pliocene it
must have been nearly the same as at the present time.
But soon afterwards it became more severe, and in the
Glacial Epoch (the beginning of the Pleistocene) the
climate of our islands may have been no less severe
than we now find in Spitzbergen. But even then there
may have been oscillations, as there probably were in
the transition to the time when history begins. It is
still a moot question when man first appeared in this
part of the world. He was certainly here soon after, per-
haps during, the Glacial Epoch. Of late years attempts
have been made to carry his arrival still farther back,
but in this controversy the final verdict may be in
favour of the sceptic.

BIBLIOGRAPHY

CHAPTER 1. — Interesting accounts of geological specula-
tions in early times are given in Sir C. LyelTs
Principles of Geology, Book I., chaps, ii.-v. (vol. i.
pp. 6-102, llth ed., 1872), and by Sir A. Geikie,
The Founders of Geology (1905). The latter carries
on the history almost to the present century. The
reception accorded to The Origin of Species, by
C. Darwin, is related by Professor Huxley in
F. Darwin's Life and Letters of C. Darwin (vol. ii.
chap, v., 1887). This may serve as an example of
the kind of opposition against which geologists had
to contend within the memory of people still living.

CHAPTER II. — On the Age and Figure of the Earth, see
Age of the Earth, by W. J. Sollas, Sees. I. and II. ;
also Sir A. Geikie, Text-Book of Geology (Books
I. and II., Part i., 1903); and articles on the Earth
in Lord Kelvin's Popular Lectures and Addresses
(Vol. ii., " Geology and General Physics ").

CHAPTER III. — This subject is treated more fully by
Sir A. Geikie (ut supra, Book III., Part ii. § 1), and
in A Text-Book of Geology, by P. Lake and R. H.
Rastall (chap. ii. pp. 33-7 and 69-74), not to mention
others of the larger text-books.

CHAPTERS IV. AND V. — These subjects are dealt with
fully by many authors — for instance, by Sir C.
Lyell (ut supra, chaps, xv.-xix.), Sir A. Geikie (ut
supra, Book III., Part ii. §§ 2-5), P. Lake and
R. H. Rastall (ut supra, chaps, iii.-vi.), J. E. Marr,
Scientific Study of Scenery.

90 BIBLIOGRAPHY

CHAPTER VI. — See for fuller information Sir C. Lyell
(ut supra, chaps, xx.-xxii.), Sir A. Geikie (ut supra,
Book III., Part ii. §§ 6, 7), P. Lake and R. H.
Rastall (ut supra, chaps, vii., viii.).

CHAPTER VII. — See for fuller information J. W. Judd,
Volcanoes (International Scientific Series), T. G.
Bonney, Volcanoes (Progressive Science Series, 3rd
ed.) ; also Sir A. Geikie (ut supra, Part vii. § 2),
P. Lake and R. H. Rastall (ut supra, chap. xii.).
Good photographic illustrations of the phenomena
of volcanoes, by Dr. Tempest Anderson, will be
found in his Volcanic Studies.

CHAPTER VIII. — See for fuller information C. E.
Button, Earthquakes (Progressive Science Series) ;
Sir A. Geikie (ut supra, Book III., Parts ii. and iii.) ;
P. Lake and R. H. Rastall (ut supra, chap. xi.).

CHAPTER IX. — The story of the general succession of
Life on the Earth is told hi almost every text-book
of geology. Good pictures of fossils are given by
Sir J. Prestwich (Geology, vol. ii.) and Sir A. Geikie
(ut supra, Book VI., Parts ii.-v.). For special
treatises on Palaeontology see A. S. Woodward,
Vertebrate Palaeontology (Cambridge Natural Science
Manuals) ; H. Woods, Palaeontology Invertebrate,
(Cambridge Biological Series) ; and A. C. Seward,
Fossil Plants, 2 vols. (Id.). The annual volumes
published by the Palseontographical Society (now
sixty-five in number) are devoted to the illustration
of British fossils. Good illustrations of the strange
vertebrates of past times are given in Extinct
Monsters, by H. N. Hutchinson (2nd ed.). For
stratigraphical information see the text-books
by Sir A. Geikie, and by Lake and Rastall, men-
tioned above; also The Building of the British Isles,
by A. J. Jukes-Browne (3rd ed.).

The author in writing the present volume has kept hi
mind, not students preparing for examinations, but

BIBLIOGRAPHY 91

persons of ordinary education who desire to acquire
some general knowledge concerning the earth and the
processes by which its surface is modified. Should
they wish for rather fuller and more formal information
they will obtain it hi such books as A Class Book of
Geology, by Sir A. Geikie ; The Student's Lyell, by J.
W. Judd ; and Intermediate Text-Book of Geology, by
C. Lapworth. Much valuable information on the
several subjects mentioned in the present book will
be found by referring to their special names in Chambers' s
Encyclopaedia or in the Encyclopaedia Britannica.

INDEX

ALPS, folding in, 80 ; contrac-
tion in, 84

Amphibians, first appearance of,
86

Archaean rocks, their nature, 32

Arrhenius on interior of earth,
20

Atlantic, depth of bed of, 14, 63

Avalanches, 54

BEACHES, raised, 67, 77
Boulder clay, 60
Bracklesham Bay, fossils at, 12

CALABKIA, earthquakes of, 82
Cayes, the making and instances

of, 43
Clarke, Colonel, on figure of

earth, 21
Climate, changes of, during

Kainozoic era, 88
Coal seams, 75
Cotopaxi, eruption and dust of,

72

Crevasses in glaciers, 55
Crust - disturbance, suggested

causes of, 83

DAB WIN, Sir G. H., on shape

and age of earth, 21, 25
Deltas, 47

Deluge, supposed effect of, 11
Dust, volcanic, 72
Dykes, 73

EARTH, shape and dimensions
of, 13, 21 ; grouping of lands
and seas on, 14 ; motions of,
15 ; early condition of, 16 ;
internal temperature of, 17 ;

condition of interior, 19, 20 ;
age of, 22-25 ; life history of,
84

Earth-pillars, 41

Earthquakes, causes and in-
stances of, 82; in Britain,
83

FAULTS, different kinds of, 79-

81 ; and earthquakes, 82
Fishes, first appearance of, 85
Folding, faulting, and over-
thrusting, 77-79
Fossiliferous systems, names of,

31

Fossils, lessons of, 9, 12, 85
Fossils, marine, present height
above sea, 75

GEOLOGICAL groups, table of,

31
Geology, problems and methods

of, 7 ; mistaken notions in,

11 ; uniformitarian views in,

22 ; nomenclature in, 30
Giants' kettles, 61
Glacial Epoch, temperature of,

88
Glaciers, 55 ; crevasses of, 55 ;

length of, 56 ; rocks rounded

by, 58 ; excavatory powers of,

61

Glacier-tables, 57
Glauconite, 66
Globigerina ooze, 65
Green, Lowthian, on figure of

earth, 21
Gregory, J. W., on Kift Valley,

81
Grouping of stratified rocks, 30

INDEX

HEADON Hill, fossils of, 9, 12

Highlands, North-west, over-
thrusts in, 79

Hills, lessons of, 8

Hutton, J., on geological time,
22

Huxley, Professor on classifica-
tion of strata, 30

ICE-FOOT, and transport by, 60
Ice-sheets, 57

Igneous rocks, composition and
nature of, 25-28

JEANS, Mr., on figure of earth,

21
Joly, Professor, on saltness of

sea and its meaning, 25
Jordan valley, origin of, 81

KELVIN, Lord, on interior of

earth, 19
Krakatoa, eruption and dust of,

72

LACCOLITES, 74

Lakes and ice excavation, 61

Lava, flows of, 70, 73

Level of land and sea, changes

in, 74-78

Life, earliest records of, 85
Limestone, origin of, 29, 65
Loess, 38
Lyell, Sir Charles, uniformi-

tarian views of, 22

MAMMALS, first appearance of,

86 ; dominance of, 87
Medway, the gravels of, 52,

56
Metamorphic rock, character of,

32
Metamorphism of rocks, 32 ;

early conditions of, 33
Moon, age of, 25
Moraines, 57 ; subglacial or

" ground," 59
Mountains, greatest heights of,

64
Mountains, structure of, 78-81

NAMES of groups of stratified
rocks, 31

OCEAN, volume of, 62; depths

of, 63 ; form of bed, id. ;

deposits in, 65 ; changes in

level of, 66, 75
Organisms, as rock formers, 29,

65 ; accumulation of, in sea, 65
Ovid, geological ideas of, 10

PEBBLES, lessons of, 8 ; trans-
port of, 46

Pelee, Mont, eruption of, 71 ;
spine of, id.

Perched-blocks, 58

Planetary system, 16

Plants, first occurrence of, 87 •
changes in character, id.

Pot-holes, 61

Pythagoras, geological ideas of,
10

RAIN, its distribution, 39;
excessive, 40 ; work of, id. ;
corrosive action of, 42

Ramsay, Sir A., on Alpine lakes,
61

Red clay, 66

Reptiles, first appearance of,
86 ; dominance of, id.

Rift valleys, 81

Rivers, composition of water,
44 ; transporting force, 46 ;
amount of material moved,
47 ; deltas of, id. ; buried
channels of, 50; trespass of, 52

Rocks, igneous, composition and
nature of, 25-28 ; metamor-
phism of, 33; effects of
pressure on, id.

Rocks, stratified, composition
of, 28 ; destruction of, 29 ;
unconformity in, 29 ; group-
ing of, 30 ; metamorphism of,
33

SAND, lessons «f, 8 ^ transport

and abrasive effect of, 37
Sand-dunes, 38
Sand-pipes, 42

INDEX

Sea, saltness of and geological
time, 25 ; volume of, 62 ; form
of bed, 63 ; depths of, 64 ;
encroachments of, id. ; dis-
tribution of materials in, 65

Sills, 73

Snow-line, the, 54

Soufriere, eruption of, 71 ; dust
from, 72

Springs, water of, 45

Step and trough faults, 81

Strata, breaks in groups of, 30 ;
changes in character of, id. ;
list of, 31

Stratified rocks, thickness of,
23 ; time of deposit, 24 ;
composition of, 28 ; grouping
of, 30 ; metamorphism of, 32

Stromboli, eruptions of, 69

Systems, geological, list of, 31

TEMPERATURE, variations of,
35 ; effects of these, id. ;
changes of, in ascending, 54

Time and geology, 22

Travertine, its making and uses,
45

Tufa, its making and uses, 45

UNCONFORMITY, its signifi-
cance, 29

Uniformitarian views in geology,
22

Upheaval of land, evidence of,
75-77

VALLEYS, the making and
forms of, 48 ; alluvial flats
in, 50 ; hidden beds of, id. ;
transverse and longitudinal,
53 ; of Jordan, 81 ; rift,
origin of, id.

Vesuvius, eruptions of, 70
Volcanoes, products of, 26 ;
materials of, 68 ; eruptions
of, 69-72 ; remnants of, 72

WAVES, force of, 64; ravages

by, id. ; work of, 65
Weald, the history of, 52;

structure of, 75
Whymper, E., on eruption of

Cotopaxi, 72
Winds, their work of transport,

36 ; of abrasion, 37

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*' We have nothing: but the highest praise for these
little books, and no one who examines them will have
anything else." — Westminster Gazette, 22nd June 1912.

THE PEOPLE'S BOOKS

THE FIRST NINETY VOLUMES

The volumes issued are marked with an asterisk

*2.

I

*7-
»8.

•£.

*n.

*I2.

;-3.
•S

16.

'•'&

•19.

Animal Life

Botany ; The Modern Study of Plants

Bacteriology

The Structure of the Earth

Evolution .

Darwin

Heredity .

Inorganic Chemistry

Organic Chemistry

The Principles of Electricity .

Radiation

The Science of the Stars .

Light, according to Modern Science

Weather-Science

Hypnotism

SCIENCE]

The Foundations of Science . . By W. C. D. Whetham, F.R.S.
Embryology— The Beginnings of Life By Prof. Gerald Leighton, M.D.
Biology— The Science of Life . . By Prof. W. D. Henderson, M.A

By Prof. E. W. Mac Bride, F.R.S.

By M. C. Slopes, D.Sc., Ph.D.

By W. E. Carnegie Dickson, M.D.

By the Rev. T. G. Bonney, F.R.S.

By E. S. Goodrich, M.A., F.R.S.

By Prof. W. Garstang, M.A., D.Sc.

ByJ. A. S. Watson, B.Sc.

By Prof. E. C. C. Balv, F.R.S.

By Prof. J. B. Cohen, B.Sc., F.R.S.

By Norman R. Campbell, M.A.

By P. Phillips, D.Sc.

By E. W. Maunder, F.R.A.S.

By P. Phillips, D.Sc.

By R. G. K. Lempfert, M.A.

By Alice Hutchison, M.D.
Mother's Book by a j By a University Woman.

Youth and Sex— Dangers and Safe- /By Mary Scharlieb.M.D., M.S., and
guards for Boys and Girls . . \ G. E. C. Pritchard, M.A., M.D.
Motherhood-A Wife's Handbook . By H. S. Davidson, F.R.C.S.E.

Lord Kelvin By A. Russell, M. A., D.Sc.

Huxley By Professor G. Leighton, M.D.

Sir W. Huggins and Spectroscopic/ By E.W. Maunder, F.R.A.S., of the

Astronomy \ Royal Observatory, Greenwich.

Practical Astronomy . . . By H. Macpherson, Jr., F.R.A.S.

/By Sydney F. Walker, R.N.,
\ M.I.E.E.

Navigation By Rev. W. Hall, R.N., B.A.

05. Pond Life By E. C. Ash, M.R.A.C.

•66. Dietetics By Alex. Bryce, M.D., D.P.H.

PHILOSOPHY AND RELIGION

23. The Meaning of Philosophy . . By Prof. A. E.Taylor, M. A., F. B.A.

•26. Henri Bergson By H. Wildon Carr.

27. Psychology By H. J. Watt, M.A., Ph.D.

28. Ethics By Canon Rashdall, D.Litt., F. B.A.

29. Kant's Philosophy By A. D. Lindsay, M.A.

30. The Teaching of Plato . By A. D. Lindsay, M.A.

*67. Aristotle . ... . By Prof. A. E. Taylor, M. A., F. B.A.

68. Nietzsche . . . . . By M. A. Mugge, Ph.D.

*69. Eucken .... . By A. J. Jones, M. A., B.Sc., Ph.D.

70. Beauty^an^ Essay in Experimental j By c w Valentine, B.A.

71. The Problem of Truth ' ! .' By H. Wildon Carr.

31. Buddhism J By Prot T.W.Rhys Davids, M.A.,

•„. Roman Catholicism . . . . { ** g; £ £££ P«** ^gr.
33. The Oxford Movement ... By Wilfrid P. Ward.

'21.
»««.

24.

*62.

•63. Aviation
*64.

PHILOSOPHY AND UUMlt»-«M«Mttfl

34. The Bible in the Light of the Higher/ By Rev. W. F. Adeney, M.A., and
„ £rit,iclTm ..... * Rev. Prof. W.H. Bennett, Litt.D.

35. Cardinal Newman ..... By Wilfrid Meynell.

•72. The Church of England ... By Rev. Canon Masterman.

73. Anglo-Catholicism . . . . By A. E. Manning Foster.
*74. The Free Churches .... By Rev. Edward Shillito, M.A.

75- Judaism ...... . By Ephraira Levine, B.A.

•76. Theosophy ...... .By Mrs. Annie Besant.

HISTORY

*36. The Growth of Freedom . . . By H. W. Nevinson.

37- Bismarck ... ... By Prof. F. M. Powicke, M.A.

•38. Oliver Cromwell . . . . .By Hilda Johnstone, M.A.

•39. Mary Queen of Scots . . . . By E. O'Neill, M.A.

40. Cecil Rhodes . . . » . . By Ian Colvin.
*4i. Julius Caesar ...... By Hilary Hardinge.

43. England in the Middle Ages . . By Mrs. E. O'Neill, M.A.

44. The Monarchy and the People . . By W. T. Waugh, M.A.

45. The Industrial Revolution . . By A. Jones, M.A.

46. Empire and Democracy . . . By G. S. Veitch, M.A.

*AT HnmP Rill*. / By L. G. Redmond Howard. Pre-

*6i. Home Rule ...... | face by Robert Harcourt, M. P.

77. Nelson ....... By H. W. Wilson.

78. Wellington and Waterloo ... By Major G. W. Redway.

SOCIAL AND ECONOMIC

*47. Women's Suffrage . . . . By M. G. Fawcett, LL.D.

1 M.A.

40. An Introduction to Economic Science By Prof. H. O. Meredith, M.A.

50. Socialism ....... By F. B. Kirkman, B.A.

70. Socialist Theories in the Middle Ages By Rev. B. Jarrett, O.P., M.A,

*8o. Syndicalism ...... By J. H. Harley, M.A.

81. Labour and Wages . . . . By H. M. Hallswortk, M.A., B.Sc.

* 82. Co-operation ...... By Joseph Clayton.

•83. Insurance as Investment . . . By W. A. Robertson, F.F. A.

•92. The Training of the Child . . . By G. Spiller.

LETTERS

*5i. Shakespeare ...... By Prof. C. H. Herford, Litt.D.

52. Wordsworth ..... By Miss Rosaline Masson.

*53. Pure Groid-A Choice of Lyrics and \By R c O'Neill.

*54. Francis Bacon . '.'.'.'. By Prof. A. R. Skemp, M.A.

*55. The Brontes ...... By Miss Flora Masson.

'56. Carlyle ....... By the Rev. L. Mac Lean Watt.

*t,7. Dante . By A. G. Ferrers Howell.

58. Ruskin ....... By A. Blyth Webster, M.A.

59. Common Faults in Writing English By Prof. A. R. Skemp, M.A.
*6o. A Dictionary of Synonyms ... By Austin K. Gray, B.A.

84. Classical Dictionary .... By Miss A. E. Stirling.

*8s. History of English Literature . . By A. Compton-Rickett.

86. Browning. . .... By Prof. A. R. Skemp, M.A.

Charles Lamb

88. Goethe .

89. Balzac

90. Rousseau.

By Miss Flora Masson.

By Prof. C. H. Herford, Litt.D.

By Frank Harris.

By F. B. Kirkman, B.A.

91. Ibsen. . . . . \ . . By Hilary Hardinge.
*93. Tennyson By Aaron Watson,

LONDON AND EDINBURGH : T. C. & E. C. JACK
NEW YORK: DODGE PUBLISHING CO.

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