PHYSICAL GEOGRAPHY.—I. GEOLOGY.
GEOGRAPHY AND GEOLOGY—THE EARTH’S CRUST—ORIGIN OF THE EARTH—DENUDATION
AND EXCAVATION BY WATER—ROCKS, GRAVEL, AND SAND—CLASSES OF ROCKS.
[Illustration: Fig. 636.—Cliffs worn by the sea.]
When we were at school, and learnt the various countries of Europe, we
had maps showing us the several divisions of one realm from another;
the mountains, lakes, and other prominent features of the continent
were learned and repeated, but we, maybe, seldom, perhaps never,
bestowed a thought upon the formations of the mountains, and the manner
in which rivers ran down into and through lakes to the ocean. There
were the mountains, there were the lakes and rivers, and capes, and
headlands, and there they are still, to all intents and purposes, the
same to see, to climb up, to sail down, as the case may be. But the map
of Europe has undergone a visible change. Territory has changed hands.
Germany has gripped France, England has got Cyprus, Turkey has been
dismembered, and Austria is annexing territory. This study is called
Geography,—POLITICAL GEOGRAPHY,—for it marks the political boundaries.
The knowledge of the formation of hills, headlands, lakes, rivers,
seas, their causes, constitution, and effects, how they rose, how
they exist and wax or wane during the course of centuries is PHYSICAL
GEOGRAPHY, which we propose to consider.
This tree of knowledge includes some very important branches, almost
parent stems. As a magnificent oak spreads forth its brawny arms, with
smaller branches and twigs, each of these great branches being as large
as an ordinary tree, so Physical Geography includes other arms such
as Geology, Meteorology, Botany, and Physiology—even Astronomy in its
comprehensive embrace. We find it is a difficult task to separate these
kindred sciences from the great tree. We may have therefore to refer to
earth, air, and water, and their various forms in hills and mountains,
wind, vapour, rain, glaciers, and sea. We must learn how this earth has
been gradually cooled, and what the various stages of its growth have
done. We must consider plant and animal life upon our planet, and how
the atmosphere affects them. All this is Physical Geography, and its
satellite sciences of Geology, Meteorology, “Climatology,” Botany, and
Physiology.
[Illustration: Fig. 637.—Disintegrated granite.]
“Everything must have a beginning,” and the earth must have had a
beginning, although the actual manner of the physical creation of the
planet is a disputed fact. We are not about to discuss the religious
side of the question, although we should undoubtedly find that Biblical
and Geologic teaching run side by side towards the same end, and the
testimony of the earth and sky bears witness to the Divine hand that
created the universe, which we can trace back to the dim and distant
ages when “the earth was without form and void, and darkness was upon
the face of the deep.”
With this brief preface, let us consider some of these aspects, and
pick up interesting facts from the ground.
GEOLOGY.
In the chemical and mineral sections of this volume we have heard
something concerning the formation of the globe and its composition,
its clays, rocks, etc. With these internal arrangements Chemistry and
Mineralogy have dealt. Geology tells us about the external surface
of the earth, its stones and rocks, and how they were formed, and
generally something about the conformation of the crust of the earth,
and its history.
When we speak of “crust” of the earth, we do not simply mean the
exterior layer of gravel, clay, or stone. The crust is a thick one,
and our crust extends just so far as we can cut into it. The surface
of the sea can scarcely be termed a crust, but we must penetrate that
ever-moving liquid boundary, and touch upon (and examine) what lies
down below far beyond the “full fathom five” of the lead line. In this
study we must not forget our Book of Nature, which is always open and
inviting us to read. We shall see how things are produced, and how
our physical surroundings will continue to be produced until the age
of miracles returns, and Providence sees fit to interfere with the
otherwise immutable laws which He in His wisdom has laid down for the
universe.
It will be of no use to go back into space and imagine the world a
red-hot fragment of matter, whirling through the heaven around the sun
which, as a larger aggregation of burning atoms, kept it, as it now
keeps it, in its place. The earth was a globe of liquid fire, or in
gaseous state, and the atoms gradually cooled on the surface; the fire
is still under our feet. The outer part would by degrees lose all its
heat, while the interior remained hot; the planet must then have been
surrounded by a steamy atmosphere, and enveloped in vapours condensed
from the air through which no light of the sun could by any appreciable
degree penetrate.
We can give an example of this, and it will be seen how the surface
of the earth gradually became formed from the vaporous condition. If
any one will take the pains to evaporate any saline solution in a
capsule till it is about to crystallise, and observe attentively the
pellicle of salt as it forms on the surface; first a partial film will
show itself in a few places, floating about and joining with others,
then when nearly the whole surface is coated, it will break up in some
places and sink into the liquid beneath, another pellicle will form and
join with the remains of the first, and as this thickens it will push
up ridges and inequalities of the surface from openings and fissures
in which little jets of steam and fluid will escape; these little
ridges are chains of mountains, the little jets of steam those volcanic
eruptions which were at that period so frequent, the surface of the
capsule is the surface of the earth, and the five minutes which the
observer has contemplated it, a million years.
The next effect of the cooling of the earth would be the gradual
condensation of the vapour of water with which it was surrounded; this
falling upon the earth formed seas and oceans, leaving only the higher
portions exposed above its level. The clearing up of the dense dark
clouds for the first time let in upon the earth’s surface the glorious
and vivifying rays of the sun, and this great effect possibly accords
with the earliest record in the Bible of the acts of creation: “And God
said, let there be light, and there was light.”
This clearing up of the vapours and the subsequent rain no doubt gave
rise to terribly grand electrical phenomena—thunderings and lightnings.
By degrees the waters got their own way, and then many changes took
place, land and water fighting, as it were, for the mastery, as they
are fighting to this day.
But perhaps some reader may not think that the land and water of our
earth are thus engaged. A very few minutes’ reflection will suffice
to confirm our assertion. Look at the lofty crags in the Alps, for
instance; what has shattered those peaks, and sent the masses toppling
down in stone avalanches to the lower slopes, and then into the
valleys?—Water. Water has been in the crevices, and was frozen there;
in freezing it expanded and loosened the crags, which, forced asunder,
gave an opening to more snow and ice, and so this powerful leverage,
aided by the wind and storm, is disintegrating our mountains.
[Illustration: Fig. 638.—Breakers on the coast of Cornwall.]
It is the same by the seashore; the cliffs are wearing away, and the
sea approaches; at other places the sea recedes from the land, as
coral formation and embryo chalk cliffs are rising under the surface
of the ocean. Lakes dry up, and the meadow or farm arises on the site,
while other old spots are submerged. No rest, no change of idea, but
ever changing in physical appearance, Nature goes on her wondrous way,
working now as steadily, as harmoniously, and as surely as she did
before time was, and as she will continue to do when time shall be no
more!
In our investigations into Geology we cannot enter into many technical
details. Our object in these pages is recreation; but we shall, even
under these circumstances, find plenty to interest, and sufficient
to lead any one who wishes to pursue the study for himself. We will
endeavour so to put the features of the stones and rocks before him
that they will be recognized by the passer by. We will try to show how
the earth has been built up, and how the great and terrible changes
through which our little globe has passed have been effected. In our
own islands, Great Britain and Ireland, we shall find traces of all
the materials likely to be useful to us in our quest, As has been well
said, “Geology is the Physical Geography of the past.”
CONSTITUTION OF THE EARTH.
[Illustration: Fig. 639.—Shells in chalk.]
The descent from rocks to stones, from stones to gravel, from gravel
to sand, is evident to everyone, so we need not insist upon the fact
that sand is powdered rock, and that an aggregation of sand particles
makes stones. We have heard in the _Mineralogy_ section that there are
certain “earths”—silica, alumina, lime, etc. Of these “earths,” the two
former constitute the greater portion of the ROCKS. Lime, also, is very
evident, and in limestones fossils or organic remains are abundant. Now
we must entirely put away from our minds the old idea that the earth
we live on was created at once, or as it appeared to the first human
beings. Our planet was prepared for man by degrees during millions
of years. We conclude that the earth was originally composed of
certain elements, and we find the same elements in the sun. Therefore,
supposing (as is supposed) that the earth came from the sun, we have
all the material of the globe in a fused state. As the earth cooled,
rocks were formed by pressure, and then water came, and now we can read
“books in the running brooks, and sermons in stones” at our leisure.
[Illustration: Fig. 640.—The streamlet.]
Perhaps as someone reads this he may be walking by the seashore kicking
the pebbles or seated upon the sands, the grains of which are so very
tiny. He will probably find sand, shingle, and gravel within reach, and
perhaps the curious-looking “pudding stone.” Now what can we learn from
these stones or sand grains or that curious bit of _conglomerate_?
Perhaps the reader may be at Ramsgate or Margate or another place where
the “white cliffs of Albion” glisten in the sun. Take up a piece of
chalk and examine it. It is soft and soils your hands, and you will
throw it away, perhaps—but don’t. Take it home and put it under the
microscope or a good magnifying glass. What do you see?
[Illustration: Fig. 641.—Cliffs showing strata.]
You will find the _remains of animals_—that is, shells and tiny bits
of coral packed close together. Under the microscope they will become
more separated, and the grains will be distinct fragments of shells,
etc. If this little bit of chalk be composed of marine animals’ shells,
of course the whole cliff is composed of the same kind of material.
But how did the shells get into the chalk? Shells are chalk—carbonate
of lime; lime was deposited at the bottom of the sea, and the infinite
millions of minute animals formed themselves shells, and left them to
be piled up by Nature’s forces into cliffs during countless ages.
[Illustration: Fig. 642.—Limestone with encrinites.]
Yes, but how did the lime get into the water to make the shells? We
will endeavour to explain. Rain, when falling, takes some carbonic
acid from the air, which we know contains it. This acts upon the lime
in the rocks (lime is oxide of calcium, and calcium is an element
in the earth), so we get a bi-carbonate of lime (soluble in water),
which rises from the rocks in springs. These springs and their streams
deposit lime, as we can see in caverns where we find stalactites and
stalagmites. The lime is transmitted to the ocean, and absorbed by the
crinoideans and molluscs which produced shells. These shells hardened
and crystallized became limestone, and whole mountains are formed of
this “organic” rock, which is used for so many purposes.
We have spoken of ORGANIC ROCKS, but there are others, and we ought,
perhaps, to have spoken of that kind before the chalk put them aside.
Let us go back to our sandy shore again and look at the SEDIMENTARY
ROCKS, which are the very first formation. We have all seen sandstone,
and visitors to the South Devon Coast will remember the red cliffs near
Dawlish and Teignmouth. These are red sandstone—not the very “old red”
so pleasantly written of by Hugh Miller, but at any rate sandstone, and
composed of grains of sand. When we were at Dawlish last year a piece
of the sandstone had fallen on to the beach, and when the waves came up
that stone was no doubt gradually washed away into sand, and then fell
to the earth as _sediment_.
[Illustration: Fig. 643.—Chalk cliff.]
We said something a few pages back about the wear and tear which
is always going on: the mountain is worn away—a mass falls, it is
broken into smaller pieces; these are carried by a river; the mud is
deposited, and the finer particles are ground and rounded into gravel,
and finally sand. Beneath the current of the river, and at the bottom
of a lake or sea, these sediments (mud, etc.) accumulate one on the top
of the other in regular series called _strata_, and then the weight and
pressure acting with the soluble mineral deposits always washing down,
consolidate and bind the loose sand-grains into stone, which, in the
course of ages, hardens. The stones thus formed from sediments such as
gravel, mud, and sand, are termed _Sedimentary Rocks_; they have become
rocks by enormous and continuous pressure. Thus:—
Sands have become “Sandstones”;
Gravel has become “Pudding-stone” (Conglomerate);
Mud and clay have become “Shale”;
Calcareous deposits have become “Limestone”;
Vegetable deposits have become “Coal.”
So we have sandy, clayey, limy, flinty, and corally rocks under long
names respectively—Arenarious, Argillaceous, Calcareous, Silicious; and
we may add Bitumenous and Ferruginous—Irony Rocks—to the list.
Speaking of sediments, it is curious to note the different colours of
the Arve and the Rhone which meet near Geneva. The white sedimentary
Arve can be traced for a long distance beside, not mixing with, the
blue Rhone. The same effect can be traced where the latter river
enters the Lake of Geneva. So the land is being perpetually carried
away and deposited; and where water gains on land there is somewhere
else always a corresponding elevation to compensate it. Thus places
disappear, and the sea washes over the site, as on the Kentish Coast,
where Earl Godwin’s land was inundated, and new land is reclaimed or is
elevated from the sea to make up the balance.
“There rolls the deep, where grew the tree;
Oh, earth, what changes hast thou seen!
There, where the long street roars, has been
The stillness of the central sea.”
We have spoken of sedimentary and organic rocks. There is yet another
kind called igneous, or fiery rocks—those upraised by volcanic action.
Of the igneous rocks the crystalline have been evidently in a fused
condition. Granite is an example; lava or basalt is the usual term
for volcanic rock, and the basaltic caves of Staffa and the Giants’
Causeway bear testimony to the igneous or volcanic origin of the
surroundings. The pillars and fantastic rocks of Ireland and Scotland
which are so remarkable, are simply lava, which was erupted in a molten
state, now cooled and contracted into blocks of curious regularity of
form.
[Illustration: Fig. 644.—Trap rock (Staffa).]
Granite, already referred to, is another igneous rock, and must have
been forced upwards; for as an igneous rock granite has cooled beneath
the crust of the earth, throwing out arms, while melting, into other
formations, and frequently being found in mountains. There is another
kind of igneous rocks formed by the continuous accumulation of the
ashes, etc., vomited forth from volcanoes. Masses of mountain are thus
produced in the course of years, and the material thus formed is called
_tufa_, or tuff, when consolidated; and this (now solidified) is what
caused the destruction of Pompeii and Herculaneum.
[Illustration: Fig. 645.—Eruption of granite.]
So we have two classes of IGNEOUS ROCKS, the Crystalline and
Fragmental (or the “Plutonic” and “Volcanic”), including basalts,
pitchstones, pumice, trachytes, granite, syenite, etc.; and, on the
other hand, tuff and “volcanic” breccia, with felstones, porphyries,
etc., which have been classed as intermediary.
Of these three classes of rocks, the sedimentary and the organic
compose the greater portion of the earth. We will now glance at the
crust of the earth and its various formations.
[Illustration: Volcanic eruption.]