HOW THE MATERIALS WERE BROUGHT TOGETHER.
These changes in the heavens, though slow, produce
Like change on sea and land.
MILTON
Probably every mountain climber, resting for a brief space on a loose
boulder, or seeking the shade of some overhanging piece of rock, has
often asked himself, "How were all these rocks made?" The question
must occur again and again to any intelligent person on visiting a
mountain for the first time, or even on seeing a mountain-range in
the distance. He may well ask his companions how these great ramparts
of the earth were built up. But unless he possesses some knowledge
of the science of geology, which tells of the manifold changes which
in former ages have taken place on the earth, or unless, in the
absence of such knowledge, he chance to meet with a geologist, his
question probably remains unanswered. Such questions, however, can
be very satisfactorily answered,--thanks to the labours of zealous
seekers after truth, who have given the best part of their lives
to studying the rocks which are found everywhere on the surface of
the earth, and the changes they undergo. Geology is a truly English
science; and Englishmen may well cherish gratefully the memories of
its pioneers,--Hutton, Playfair, Lyell, and others, who have made the
way so clear for future explorers.
The story of the hills as written on their own rocky tablets and on
the very boulders lying loose on their sloping sides, and interpreted
by geologists, is a long one; for it takes us far back into the dim
ages of the past, and like the fashionable novel, may be divided into
three parts, or volumes. To those who follow the stony science it
is quite as fascinating as a modern romance, and a great deal more
wonderful, thus illustrating the force of the old saying, "Truth is
stranger than fiction."
The three parts of our story may be best expressed by the three
following inquiries:
I. How were the materials of which mountains are built up
brought together and made into hard rock?
II. How were they raised up into the elevated positions in
which we now find them?
III. How were they carved out into all their wonderful and
beautiful features of crag and precipice, peaks and passes?
A mountain group, with its central peak or spire, its long ridges,
steep walls, towers, buttresses, dark hollows, and carved pinnacles
standing out against the sky, has well been compared to a great
and stately building such as a cathedral or a temple. Mountains
are indeed "a great and noble architecture, giving first shelter,
comfort, and rest, but covered also with mighty sculpture and painted
legend;" and to many they are Nature's shrines, where men may offer
their humble praises and prayers to the great Architect who reared
them for His children. We have introduced this illustration because
it will help us in our inquiry. Suppose we were standing in front of
some great cathedral, such as Milan, with all its marble pinnacles,
or Notre Dame, with its stately towers, or the minsters of York or
Durham in our own country, and trying to picture to ourselves how
it was built. No one has lived long enough to watch the completion
of one of these great buildings; but for all that, we know pretty
well how it was made, even by watching the builder's operations for
a short time, or by following, as we often may, the various stages
in the construction of a small house. So it is with Nature's work.
We cannot, in our little lives, witness the rearing of a great
mountain-chain, or even the carving of a single hill; but we can
observe for ourselves the slow and continuous operations which in the
course of thousands and thousands of years produce such stupendous
results. We may learn how the building operations are conducted,
though the final results will only be manifested in the far-distant
future.
But to return to our cathedral. If we try to picture to ourselves
the long years during which it was covered with scaffolding and
surrounded by a busy army of workers, we shall soon perceive that
the operations may be broadly divided into three heads. _First_, we
must inquire how the separate stones of which it is composed were
brought together into one place, and we shall at once picture to
ourselves groups of men working in stone-quarries,--perhaps a long
way off,--busy with their crowbars and hammers, breaking off large
blocks of stone, and following the natural divisions of the rock
that their rough labour may be lessened; for all rocks will split
more easily along certain lines than along others. Sometimes it is
easier to follow the "bedding," or natural layers in which the rock
was formed; at other times the "joints," or cracks subsequently
formed as the rocky materials hardened and contracted in bulk, afford
easier lines for the workmen to follow. Others are busily engaged in
placing the stony blocks on trollies drawn by horses, that they may
be borne along the roads leading from the quarry to the site of the
future cathedral. And so, taking a bird's-eye view, we seem to see
horses and carts slowly moving on from many a distant quarry, but
all converging like the branches of a river to one main channel, and
finally depositing their burdens in the stone-yard where the masons
are at work. Perhaps bricks are partly employed, in which case we can
easily picture to ourselves the brickyards, where some are digging
out the soft clay, others moulding it into bricks with wooden moulds,
while others again lay them down in rows on the ground to dry, before
they are baked in the ovens. And when the bricks are ready for
use, the same means of transportation are employed; and cart-loads
of them are borne along the country roads until they so reach their
destination.
Now, all this may be summed up in the one word "transportation;"
and we shall presently inquire how the rocky matter of which the
mountains are built was transported.
_Secondly._ We have to inquire how the bricks and stones were raised
up. The analogy is not quite perfect in this case; for the mountains
were raised up _en bloc_, not bit by bit and stone by stone, as in
the case of the cathedral. Still they have been raised somehow.
Analogies are seldom complete in every detail; but for all that, our
illustration serves well enough, and will help us in following the
various processes of mountain building. In these days, the raising of
the stones is mostly effected by steam-power applied to big cranes
and pulleys. In old days they used cranes and pulleys, but the ropes
were pulled by hand-power. In either case the work proceeds slowly;
and we can easily picture to ourselves the daily raising of the
stones of which the cathedral is composed. "What were the forces
at work which slowly raised the mountains?" This question we will
endeavour to answer later on (see next chapter). This work may be
included in the one word, "elevation."
_And lastly._ We must inquire how the carving of the stately building
was effected, how its pinnacles received their shape, and how all
those lovely details received their final forms; how the intricate
traceries of its windows were made, and the statues carved which
adorn its solemn portals. This question is easily answered, for we
are all more or less familiar with what goes on in a stone-mason's
yard. Under those wooden sheds we see a number of skilled labourers
at work, busy with their chisels and mallets, cutting out, according
to the patterns made from the architect's detailed drawings, the
portions of tracery for windows, or the finials, crockets, and other
features of the future building. In another part of the yard may be
seen the stone-cutters, working in pairs and slowly pulling backwards
and forwards those long saws which, with the help of water and sand,
in time cut through the biggest blocks. All this work then may be
summed up under the one word, "ornamentation," for it includes the
cutting and carving of the stone.
Our three lines of inquiry may now be summed up in these three words,
which are easily remembered:--
_Transportation_,
_Elevation_,
_Ornamentation_.
Taking the first of these subjects for consideration in the present
chapter, we have now to inquire into the nature of the materials of
which mountains are composed and the means by which they have been
brought together and compacted into hard rock.
First, with regard to the nature of the materials which Mother Earth
uses to build her rocky ramparts: they are the same as the ordinary
rocks of which the earth's crust is composed; and the greater part of
them have been formed by the action of water. These are the ordinary
"stratified" rocks, which in one form or another meet us almost
everywhere, and may be said to be aqueous deposits, or sediments
formed in seas and inland lakes. They are always arranged in layers,
known to geologists as "strata," because they have been gently laid
down, or strewn (Latin, _stratum_), at the bottom of some large body
of water. There were pauses in the deposition of the materials,
during which each layer had time to harden a little before the next
one was formed. This accounts for the stratification. In this way
great deposits of sandstone, clay, and limestone, with their numerous
varieties, have been in the course of ages gradually piled up, till
they have attained to enormous thickness, which at first sight seem
almost incredible; but the bed of the seas in which they formed was
probably undergoing a slow sinking process that kept pace with the
growth of these deposits, otherwise the sea might have been more or
less filled up.
And these processes are still going on. In fact, it is entirely by
watching what goes on now that geologists are able to explain what
took place a very long time ago when there were no human beings on
the earth to record the events that took place. And so we argue
from the present to the past, from the known to the unknown. In
other words, geology is based upon physical geography, which tells
us of the changes now in progress on the earth. Thus, sandstone,
as frequently met with in different parts of Great Britain, and
largely used for building purposes, such as the familiar old red
sandstone[20] of South Wales, Hereford, and the north of England and
different parts of Scotland, was once soft sand in no way at all
different from the sand of the seashore at the present day, or of the
sandy bed of the North Sea. In process of time it became hardened,
and acquired its characteristic red colour, which is due to oxide of
iron. In some places numerous fossil fishes have been discovered in
this interesting formation, so intimately associated with the name
of Hugh Miller, who first thoroughly explored it; these and other
remains entombed therein tell us of the strange forms of life which
flourished on the earth during that very old-fashioned period of
the world's history; and by putting together all kinds of evidences
derived from the rock itself, geologists are able to form a very good
idea of the way in which this rock-deposit was accumulated, always,
however, basing their conclusions on a thorough knowledge of what
goes on at the present day in seas, rivers, and inland lakes.
[20] The reader will find an account of the old red sandstone in
the writer's "Autobiography of the Earth" (Edward Stanford, 1890).
In the great series of stratified rocks forming what is commonly
called the crust of the earth (an unfortunate term which has survived
from the time when the interior of the earth was generally believed
to be in a fiery molten condition, and covered by a thin coating of
solid rock at the surface), there are besides the sandstones, of
which we have just spoken, great deposits of dark-coloured clays,
shales, and slates. All these can be accounted for by the geologist.
They are simply different states of what was once soft mud. The
slates tell us that they have been subjected to very severe pressure,
which squeezed their particles till they were elongated and all
arranged in one direction, and this is the reason why they split up
into thin sheets.
Others, again, represent vast deposits of carbonate of lime,
thousands of feet thick and now occupying hundreds of square miles
of the earth's surface. Limestone rocks are as abundant in our own
country as the sandstones, shales, or slates. The chalk of which
the North and South Downs are composed is a familiar example. It is
seen again forming Salisbury Plain, in Hampshire and the Isle of
Wight, and then it may be traced running up the country in a long
band through the counties of Oxford, Cambridge, Lincoln, until it
reaches the coast at Flamborough Head in Yorkshire. Then we have the
Bath Oölites so much used in building, for they form an admirable
"freestone" that can be easily carved and cut in any direction (hence
the term "freestone"); and lastly, the great mountain limestone so
well developed in South Wales, Yorkshire, and the Lake country. All
these were slowly built up at the bottom of the seas which existed
in past ages; great beds of gravel formed at the mouths of rivers,
and long banks of pebbles and rounded stones collected on the shore
of primeval seas, and were ground against each other as now by
the action of the waves, until all their corners were rubbed off.
Pebble-beds, called by geologists conglomerates, are met with among
the stratified rocks; and their story is easily read by studying
what takes place at the present day on our seashores.
[Illustration: CHALK ROCKS, FLAMBOROUGH HEAD. FROM A PHOTOGRAPH
BY G. W. WILSON.]
Now, the sandstones, clays, gravels, and pebble-beds all represent,
as will presently be explained, so much material worn away from the
surface of the land and swept into the ocean (or in some cases into
inland seas and lakes) by streams and rivers, which are the great
transporting agents of the world. Hence such deposits of débris,
supplied by the constant wear and tear of all rocks exposed to the
atmosphere, are truly sedimentary and have a purely mechanical
origin. But it is not so with the limestones. The latter were never
transported, but grew at the bottom of the sea in very wonderful
ways. They have nothing to do with the wear and tear of the land to
which the others owe their existence, but represent vast quantities
of carbonate of lime extracted from sea water. Sea water contains
a certain amount of this substance in a dissolved state, or "in
solution," as a chemist would say; and the way in which this is
extracted by the agency of various creatures, such as coral polypes
and little microscopic creatures that build their shells of
carbonate of lime, of great beauty, forms one of the most interesting
subjects presented to the student of physical geography. Hence,
since limestone can only be accounted for by the agency of living
organisms,[21] it is rightly termed an _organic deposit_, and the
others are said to be _mechanical deposits_. But both are called
"aqueous rocks," because they are formed under water. It is important
to distinguish clearly between these two very different methods of
rock-formation.
[21] The flints usually found in limestone are also of organic
origin.
But although water plays such a very important part in the making of
the common rocks around us, yet there are others which have quite a
different origin,--rocks which have come up from below the surface of
the earth in a heated and molten condition, such as the lavas that
flow from volcanoes in active eruptions and the showers of ashes
and fine volcanic dust which often attend such eruptions (see chap.
viii., pp. 271-272). Some highly heated rocks, though they never rise
to the surface to form lava-flows, are forced up with overwhelming
pressure from below, and wedge themselves into the sedimentary rocks
that overlie them, thus forming what are known as volcanic dykes, and
intrusive masses or sheets of once molten rock. In this category we
include such rocks as basalt, felstone, pitchstone, and other rocks
of fiery origin that have flowed from volcanoes as lava, as well as
those like granite, which have cooled and become solid _below_ the
surface, and are Plutonic, or deep-seated, igneous rocks. Granite
may be exposed to the surface of the earth when the rocks which once
overlaid it have been worn away or "denuded." It is frequently seen
in the central regions of mountain-chains, where a vast amount of
erosion has been effected. Thus we see that heat has played its part
in the making of rocks; and for this reason such rocks as we have
just mentioned are called _igneous_. Fire and water are therefore
very important geological agents; but we should say heat rather than
fire, because the latter word might convey a false impression. No
rocks can be burned except coal, which may be considered rather as a
mineral deposit than as a rock. Some rocks may be heated, and undergo
many and various changes in their mineral composition; but they are
not capable of combustion.
So far, then, we have learned that the rocks exposed to view on
the surface of the earth may be divided into two classes; that is,
aqueous and igneous. There is yet a third class, which, though of
aqueous origin, has in course of time suffered considerable from
the internal heat of the earth and the enormous pressure due to the
weight of overlying rocks. Such rocks have been greatly changed
from their original condition, both in appearance and in mineral
composition, and are said to be "metamorphic," a word which implies
change. Thus chalk, or other limestone rock, has been metamorphosed
into marble; shales and slates into various kinds of "schists,"[22]
such as mica-schist, and even into gneiss, which closely resembles
granite. And it is quite possible that even granite may in some cases
be the result of the melting and consolidation under great pressure
of certain familiar stratified rocks. It is quite conceivable that
slate might be converted into granite, for their chemical composition
is similar, only the minerals of which it is composed would require
to be rearranged and grouped into new compounds. This would seem
quite possible; but at present we have no direct proof of such a
change having taken place. Even igneous rocks are found in some
places to have suffered very considerable change.
[22] Schists are so named from their property of splitting into
thin layers. Their structure is crystalline; and the layers, or
folia, consist usually of two or more minerals, but sometimes
of only one. Thus mica-schist consists of quartz and mica, each
arranged in many folia, but it splits along the layers of mica.
In some inland seas, like the Caspian Sea, deposits of rock salt and
gypsum may be formed by chemical precipitation, owing to evaporation
from the surface.
The various kinds of rock known to geologists may be conveniently
arranged as follows:
{ { Clay, shale, slate, etc.
{ I. Sedimentary. { Sandstones.
{ { Conglomerates.
{
Rocks of { { Limestones.
aqueous { II. Organic. { Flint.
origin. { { Coal.
{
{ III. Chemical. { Rock salt.
{ { Gypsum, etc.
{ I. Volcanic. { Lavas.
Rocks of { { Volcanic ashes, etc.
igneous origin. {
{ II. Plutonic. { Basalt.
{ { Granite.
Metamorphic rocks { Marbles.
of aqueous and { Various kinds of schists.
igneous origin. { Gneiss, etc.
So far we have only attempted to state very briefly the different
kinds of rocks, and to point out that they were formed in various
ways. We must now consider the question of rock-making more closely,
and see what we can learn about the wonderful ways in which rocks are
made; and it may be instructive to glance at the conflicting opinions
on this subject which learned men held not very long ago.
At the end of the last century a great controversy took place on
the question of the origin of rocks, and the learned men of the day
were divided into two parties. One of these parties, following the
teaching of Werner, professor of mining at Freyburg, who inspired
great enthusiasm among his disciples, declared that all rocks were
formed by the agency of water. This was a very sweeping and of course
rash conclusion. But whenever they examined rocks, they found so many
clear evidences of the action of water that a powerful impression
of the importance of this agency was naturally made on their minds.
They found rocks uniformly arranged in great layers which extended
for long distances, and containing the remains of animals which must
undoubtedly have lived in the seas or estuaries. These layers were
further divided into smaller layers, such as clearly were formed
by the slow settling down of sand and mud. Others again contained
gravels and rounded pebbles, testifying in no uncertain way to
the action of water. Even the little grains of sand are obviously
water-worn. This teaching was quite sound so long as they confined
their attention to clays, sandstones, and limestones; but when they
came to basalt and granite, a blind adherence to the views of their
master caused them to shut their eyes to the clear evidences of the
action of heat, presented by such rocks. The crystalline structure
of such rocks; their irregular arrangement, often so different
from the uniform disposition of the stratified rocks (although it
must be admitted that ancient lava-flows often lie very evenly
between aqueous rocks), and the way in which they burst through
overlying rocks, thus proving their former molten condition; the
signs of alteration exhibited in the aqueous rocks into which they
intruded themselves (changes which are obviously due to the action
of heat),--these and other evidences were entirely overlooked, and
Werner declared that basalt had been found as a sediment under water.
This school of geologists, believing so strongly in the all-powerful
influence of Father Neptune, received the not inappropriate title of
"Neptunists."
On the other hand, the party who happened to be in districts where
granite, basalt, and such igneous rocks abounded were equally
impressed with the importance of the powerful agency of heat. To
them nearly every rock they met with seemed to show some signs of
its action. And since Pluto was the classical deity of the lower
regions, and the earth shows evidences in places of greater heat
below the surface, this party received the title of "Plutonists;"
and so the battle raged hotly for some time between the Neptunists,
with their claims for cold water, and the fiery Plutonists of the
rival school of Edinburgh, with their subterranean heat. Fire and
water are never likely to agree; and they did not do so in this case.
But now that the battle is over, and both sides are found to have
been partly right and partly wrong,--though the Neptunists have the
advantage,--we can afford to smile at the fierceness of the contest,
and wonder how it was that each side thought they were so entirely in
the right.
Let us now consider the aqueous rocks, and see if we can gain a clear
idea of the ways in which they were formed; and first, we will take
those of a purely sedimentary origin,--the sandstones, pebble-beds,
gravels, and clays. These, as the reader has already probably
guessed, have all been transported by means of streams and rivers,
and settled down quietly in seas at the mouths of rivers or in inland
lakes. There is no trace of the action of heat in the forming of
these rocks, though they often show signs of having suffered more or
less change from contact with highly heated igneous rocks of later
date which forcibly intruded themselves from below; and if the change
thus effected were considerable, we should call the rocks so altered
metamorphic. But we are now dealing with their original state and how
they were made; and of that there is no possible doubt whatever. So
for the time being we may call ourselves Neptunists.
Streams and rivers are the great transporting agents whereby the
never-failing supply of débris from the waste of the land is
unceasingly brought down from the mountains and hills, through the
broad valleys and along the great plains, until finally it is flung
into the sea. The sea is the workshop where all the sedimentary
rocks are slowly manufactured from the raw material brought to it
by the rivers. But for the present we must confine our attention to
the question of transport. Referring back to our illustration of the
cathedral, we may say that streams and rivers play the part of cart
and horses. They bring the materials down from the quarry to the
scene of action,--the workshop where they are wanted. The quarries,
in this case, may be said to be almost everywhere. For wherever rocks
and soil are exposed to the action of wind and weather, there is
certain to be more or less decay and crumbling away. But it is among
the hills and in the higher parts of the mountains that the forces
of destruction are most active. How this is brought about will be
discussed in the seventh chapter, on the carving of the hills. The
frequent slopes covered with loose stones are sufficient evidence of
the continual destruction that takes place in these regions.
The transporting powers of rivers are truly prodigious. Looking at
a stream or river after heavy rain, we see its waters heavily laden
with mud and sand; but it is difficult to realise from a casual
glance the vast amount of material that is thus brought down to lower
levels. If we could trace the sediment to its source, we must seek it
among the rocks of mountains far away. Step by step we may trace it
up along the higher courses of the river, then along mountain streams
rushing over their rocky beds, tumbling in cascades over broken
rocks, or leaping in waterfalls over higher projections of rock,
until we come to the deep furrows on the sides of mountains along
which loose fragments of rock come tumbling down with the cascades of
water that run along these steep channels after heavy rain, leaving
at the base of the mountain great fan-shaped heaps of stones.
"Oft both slope and hill are torn
Where wintry torrents down have borne,
And heaped upon the cumbered land
Its wreck of gravel, rocks, and sand."
These accumulations are gradually carried away by the larger mountain
streams, which in hurrying them along cause a vast amount of wear
and tear; so that their corners are worn off, and they get further
and further reduced in size, becoming mere round pebbles lining
the bed of the stream, and finally by the time they reach the large
slow-moving rivers of the plains are mainly reduced to tiny specks
of mud or grains of sand. So then the rivers and streams not only
transport sediment, but they manufacture it as they go along. And
thus they may be considered as great grinding-mills, where large
pieces of stone go in at one end, and only fine sand and mud come out
at the other.
The amount of land débris thus transported depends partly on the
carrying power of rivers, which varies with the seasons and the
annual rainfall; partly on the size of the area drained by a river;
and again, partly on the nature of the rocks of which that area is
composed.
A stream, moving along at the rate of about half a mile (880 yards)
an hour, which is a slow, rate, can carry along ordinary sandy soil
suspended in a cloud-like fashion in the water; when moving at the
rate of two thirds of a mile (about 1,173 yards) an hour, it can roll
fine gravel along its bed; but when the rate increases to a yard
in a second, or a little more than two miles an hour, it can sweep
along angular stones as large as an egg. But streams often flow much
faster than this, and so do rivers when swollen by heavy rain.
A rapid torrent often flows at the rate of eighteen or twenty miles
an hour, and then we may hear the stones rattling against each other
as they are irresistibly rolled onward; and during very heavy floods,
huge masses of rock as large as a house have been known to be moved.
These are the two principal ways in which streams and rivers act as
transporting agents: they carry the finer materials in a suspended
state (though partly drifting it along their beds); and they push
the coarser materials, such as gravel, bodily along. But there
is one other way in which they carry on the important work of
transportation, which, being unseen, might easily escape our notice.
Every spring is busily employed in bringing up to the surface mineral
substances which the water has dissolved out of the underground
rocks. This invisible material finds its way, as the springs do, to
the rivers, and so finally is brought into that great reservoir, the
sea. Rain and river water also dissolve a certain amount of mineral
matter from rocks lying on the surface of the earth. Now, the
material which is most easily dissolved is carbonate of lime. Hence
if you take a small quantity of spring or river water and boil it
until the whole is evaporated, you will find that it leaves behind
a certain amount of deposit. This, when analysed by the chemist,
proves to be chiefly carbonate of lime; but it also contains minute
quantities of other minerals, such as common salt, potash, soda,
oxide of iron, and silica, or flint. All these and other minerals are
found to be present in sea water.
The waters of some of the great rivers of the world have been
carefully examined at different times, in order to form some idea of
the amount of solid matter which they contain, both dissolved and
suspended; and the results are extremely important and interesting,
for they enable us to form definite conclusions with regard to their
capacity for transport. This subject has been investigated with great
skill by eminent men of science. The problem is a very complicated
one; but it is easy to see that if we know roughly the number of
gallons of water annually discharged into the sea by a big river,
and the average amount of solid matter contained in such a gallon
of water, we have the means of calculating, by a simple process
of multiplication, the amount of solid matter annually brought
down to the sea by that river. But we must also add the amount of
sand, gravel, and stones pushed along its bed. This may be roughly
estimated and allowed for. These are some of the results:
The amount of solid matter discharged every year by that great river,
the Mississippi, if piled up on a single square mile of the bed of
the sea,--say, in the Gulf of Mexico, where that river discharges
itself,--would make a great square-shaped pile 268 feet high. But the
Gulf Stream, sweeping through this gulf, carries the materials for
many and many a mile away; so that in course of time it gradually
sinks and spreads itself as a fine film or layer over part of the
great Atlantic Ocean. The mud brought down by the great river
Amazon spreads so far into the Atlantic Ocean as to discolour the
water even at a distance of three hundred miles. The Ganges and the
Brahmapootra, flowing into the Bay of Bengal, discharge every year
into that part of the Indian Ocean 6,368,000,000 cubic feet of solid
matter. This material would in one year raise a space of fifteen
square miles one foot in height. The weight of mud, etc., that these
rivers bring down is sixty times that of the Great Pyramid of Egypt,
or about six million tons.
Or, to put the matter in another way, if a fleet of more than eighty
"Indiamen," each with a cargo of fourteen hundred tons of solid
matter, sailed down every hour, night and day, for four months, and
discharged their burdens into the waters of the Indian Ocean, they
would only do what the mighty Ganges does quietly and easily in the
four months of the flood season.
It is probable that even the Thames, a small river compared to
those just mentioned, manages to bring down, in one way or another,
fourteen million cubic feet of solid matter. These few figures may
suffice to give the reader some idea of the enormous amount of
rock-forming materials brought down to the seas at the present day.
Of course they are spread out far and wide by the numerous ocean
currents, some of which flow for hundreds of miles; and so the bed of
the sea can only be very slowly raised by their accumulation. Still
the geologist can allow plenty of time, for there is no doubt that
the world is immensely old; and if we allow thousands of years, we
may easily comprehend that deposits of very considerable thickness
may in this way accumulate on the floors of the oceans. Also the
coasts of continents and islands suffer continual wear and tear at
the hands of sea waves; and thus the supply of sediment is increased.
When the geologist comes to study the great rock-masses--hundreds,
and even thousands, of feet in thickness--of which mountain-ranges
are composed, he finds all those kinds of rock which we have
just been considering,--sandstones, shales (or hardened clays),
pebble-beds, and limestones,--and endeavours to picture to himself
their gradual growth in the ways we have described. In so doing, he
is driven to the conclusion that many thousands of years must have
been occupied in their construction.
We must now say a few words about those other aqueous rocks which
have an organic origin, of which limestone is the chief. It is indeed
a startling conclusion that deposits of great thickness, and ranging
for very many miles over the earth's surface, have been slowly built
up through the agency of marine animals extracting carbonate of lime
from the sea. Yet such is undoubtedly the case. Of this important
process of rock-building coral reefs are the most familiar example.
The great barrier reef along the northeast coast of Australia is
about 1,250 miles long, from ten to ninety miles in width, and rises
at its seaward edge from depths which in some places certainly exceed
eighteen hundred feet. It may be likened to a great submarine wall.
Now, all this solid masonry is the work of humble coral polypes (not
"insects"), building up their own internal framework or skeleton
by extracting carbonate of lime from sea water. Then the breakers
dashing against coral reefs produce, by their grinding action, a
great deal of fine "coral-sand" and calcareous mud, which covers the
surrounding bed of the sea for many miles.
Now, geologists find that some limestone formations met with in the
stratified rocks have certainly been formed in this way; for example,
certain parts of the great "mountain limestone." This is proved by
the fossil corals it contains, and by tracing the old coral reefs;
but it is also largely formed by the remains of other graceful
calcareous creatures known as encrinites, or "sea-lilies," with long
branching arms that waved in the clear water. Such creatures still
exist in some deeper parts of the sea, and look more like plants
than animals. In former ages they existed in great abundance, and so
played an important part as rock-formers,--for their stems, branches,
and all are made of little plates of carbonate of lime, beautifully
fitting together like the separate bones, or vertebræ, composing
the backbone of a fish; and when the creatures died, these little
plates no longer held together, but were scattered on the floor
of the sea-bed. Shell-fish abounded too, and their shelly remains
accumulated into regular shell-beds in some places. But at times mud
and sand would come and cover over all these organic deposits.
But of all rocks that have an organic origin, chalk is the most
interesting. Geologists were for a long time puzzled to know how this
rock could have been formed; but some soundings made in the Atlantic
Ocean previous to the laying of the first Atlantic cable led to a
very important discovery, which at once threw a flood of light on
the question. Samples of the mud lying on the bed of this ocean at
considerable distances from the European and American coasts, and
at depths varying from one thousand to three thousand fathoms, were
brought up by sounding apparatus.
Little was it thought that the dull grey ooze covering a large part
of the Atlantic bed would bring a message from the depths of the sea,
and furnish the answer to a great geological problem. Yet such was
the case; for under the microscope this mud was seen to be chiefly
composed of very minute and very beautiful shells, now known as
_foraminifera_, and much prized by microscopists. These tiny shells
are found at or near the surface of the sea; and after the death of
the creatures that inhabit them (which are only lumps of protoplasm
with no organs of any kind), the shells slowly sink down to the bed
of the ocean. Now, these creatures multiply at so inconceivable a
rate that a continuous shower of dead shells seems to be taking
place, and the result is the slow accumulation over vast areas of the
Atlantic and Pacific oceans of a great deposit of calcareous ooze,
which if raised above the sea-level would harden into a rock very
similar to chalk.
[Illustration: MICROPHOTOGRAPHS ILLUSTRATING ROCK FORMATION.
I. Foraminifera. II. Section of Granite. III. Nummulitic
Limestone.]
But this process only takes place in the deeper parts of our seas,
far removed from land, where the supply of land-derived materials
fails,--for even the finest mud supplied by rivers probably all
settles down before travelling two or three hundred miles from its
native shores.
Thus we learn that when one agency fails, Nature makes use of another
to take up the important work of rock-building. How the other rocks
which we mentioned in our list were formed,--such as granite, basalt,
and the metamorphic rocks,--we must explain in a future chapter
dealing with volcanoes and their work.