PHYSICAL GEOGRAPHY.
IGNEOUS ROCKS—LAND AND WATER—SPRINGS, WELLS, AND GEYSERS—SNOW AND
ICE—THEIR EFFECTS.
In the foregoing pages we have chiefly considered the stratified
rocks, but we are now approaching another branch of our subject—viz.,
“Physiography,” which, as distinguished from the usual so-called
Physical Geography, will deal with the phenomena of the earth, air, and
water, thus leading us to Meteorology as a conclusion.
We have arrived at a certain knowledge concerning the Earth as a
planet, her place in the universe, and the composition of the “Crust,”
as it is termed. We have examined the stratified rocks, which include
sand and gravel, stones, and boulders equally. To a geologist they
are all “rocks.” We must now examine the igneous rocks, which bear an
important part in the structure of the Earth, whose surface we have
now more minutely to examine. It has already been stated (p. 571) that
igneous rocks have been upheaved while in a state of fusion—that is,
while in a melted condition. These igneous, or fire-produced rocks,
are divided into classes, just as the unstratified rocks are, and the
divisions are called the VOLCANIC and PLUTONIC, including “Basic” and
“Acidic,” according as they are possessed of less silica or more.
Sometimes the igneous rocks are classed as volcanic, trappean (from
_trappa_, a stair, such as in the Giant’s Causeway), and granitic.
The volcanic in such case being the modern or upper rocks, such as
lava, scoria, etc., which, having been cast up by volcanoes, are of
comparatively recent formation.
The VOLCANIC rocks, then, are of recent date, _comparatively_ speaking;
they form the constituent portions of the volcanoes of the present day,
and are found as basaltic formations. They are traced as far back as
the Tertiary period of the globe. Amongst the volcanic rocks we find
basalt, augite, porphyry, serpentine, pumice, pitchstone, felspar, etc.
But no doubt volcanic action has been going on ever since the beginning
of the world as it is now, and will continue to do. It is somewhat
curious that the very old igneous rocks should not be more evident.
The PLUTONIC rocks do not differ essentially from the foregoing. There
is less quartz and more hornblende; and if the ages during which these
formations have been existent in the earth-depths after they became
solidified be considered, the differences will be fully accounted for.
The greenstones and syenites are prominent amongst the plutonic series.
These plutonic and volcanic rocks are separated into basic and acidic,
as already remarked, but the line cannot be drawn very distinctly.
Granite is the chief plutonic (acidic) rock, and we frequently find it
forced upwards into other strata, its essentially eruptive character
being thus decided. That granite must have taken an immense time to
solidify and crystallize is evident, for no new granites are ever
found. We find granite in all the _old_ mountain chains—such as the
Grampians in Scotland, and the Wicklow mountains. Our chief European
(active) volcanoes are, so to speak, modern, as may be supposed when
their constituents are known. It may be said that granite was first
deposited as sediment heated by subterranean fire, and forced up by
thermal action of water to the mountains, where it is uncovered by a
slow process of denudation and surface washings of the earth.
Now without at present going any farther into the causes of volcanoes
we can see at a glance that the eruption of the igneous rocks must have
created a marked and essential difference in the physical geography
of the globe. It is to these eruptions that the dislocation and
disturbance of the stratified formations are due. The igneous rocks
present ridges in the mountains; sometimes they are rounded at the
summits, while the aqueous and metamorphic rocks are disposed in layers.
[Illustration: Fig. 689.—Crater of Popocatapetl.]
These two classes in their varieties form the land and the crust of the
earth, which is ever being acted upon by air and water. The ice, again,
polishes and scratches the valleys in which it moves. The loosened
boulders that tumble from the mountains are carried down by the ice,
and deposited in the glacier moraine, whence flows a stream. By degrees
the stone is ground up, and carried away in the water to form sediment
in a “delta” at the _embouchure_, or to lie beneath the surface and
form rock once more. The igneous rocks, composed of lavas and ashes,
are volcanic rocks, deposited deep down, and then after the lapse of
ages disclosed by the action of air and water.
The consideration of the land and water upon the globe shows us that
they are distributed over the earth very unequally. There is nearly
three times as much water in our planet as there is land, and these
proportions could not be altered without giving rise to phenomena, the
results of which cannot be properly estimated. Our earth has an area
of 197,000,000 of square miles; about 52,000,000 of this is land, and
about 145,000,000 of it water; so about three-quarters of the globe is
made up of water. The first portion of our subject therefore should
be directed to the examination and consideration of water, and the
phenomena which arise from its presence upon the earth.
[Illustration: Fig. 690.—Distribution of land and water.]
We need not go into details which every geography indicates. We will
try to trace the _sources_, not the _plain effects_, which all can
afterwards study from special books. In a preceding portion of this
volume we have explained the chemical composition of water, and we
showed by experiment that it is a fluid composed of oxygen and hydrogen
gases, in the proportions of one to two volumes respectively. No matter
in what form water may appear,—as water, as ice, or as steam,—these
proportions never vary in pure water (_see_ p. 352). But water on
the earth is seldom, or never, pure. We know the difficulty we have
to procure good drinking water, and though it may be filtered, there
will remain natural salts, which are found in different degrees in
all water upon the globe. We know the rain, which is perfectly pure
when condensed from the clouds, absorbs carbonic acid, etc., from the
atmosphere. We have shown how this water as soon as it comes upon the
earth attacks the rocks, and as it progresses carries away lime. After
descending deep down, it rises again in the form of SPRINGS.
Now what are these springs? They are the result of percolation of
rain-water through certain strata. When water falls it is absorbed
into the ground, unless it happens to rest upon an impermeable rock,
in which case it becomes a rivulet. But it can penetrate between the
atoms of many rocks, and thus falls through sand and harder rocks, till
it reaches a stratum which will not receive it—like clay. We then find
that it will flow away in a spring, or if tapped will be an Artesian
well. These water-wells are of very ancient date, but the name is more
modern.[31] The springs flow out, and develop, with the assistance of
tributaries, into rivers. These again receive more tributaries, which
swell the volume of their waters, and widen out, carrying millions of
gallons hourly to the sea with sediment and gravel and stone.
[Illustration: Fig. 691.—Distribution of land and water.]
Water has enormous power of disintegration. We have only to cast our
eyes upon the illustrations in any volume of continental travel in
Europe or America to perceive the gorges and cañons worn out by the
resistless and frequently gently-flowing river to estimate the part
which water plays in Physical Geography and Meteorology.
But springs occur not only in the case mentioned; there are mineral
springs, hot springs, and oil springs, all following the same rules
of nature. The Artesian well has been mentioned. The Geysers of
Iceland have often been portrayed, and are amongst the most wonderful
phenomena of nature. These will serve as a type of the other thermal
springs, of which the districts of the Yellowstone in North America
afford perhaps the most extraordinary instances. These are intermittent
springs, and the water rises to a great height, at intervals of about
an hour and a half; and after many successive attempts, or trials, as
it were, the geyser shoots up to a great height enveloped in steam.
The cause of these well-known phenomena have been explained by Bunsen,
and it has already been referred to. We know that at a certain
air-pressure water boils at 212° (Fahr.), but on mountains at less
pressure it will boil before that degree, because the air is rarefied.
So conversely, under the ground, it may reach 212° without boiling. So
the surface (warm) water falls, and reaches a high temperature before
it is converted into steam. When it is so converted, the vapour is
formed very rapidly, and the expansive force is tremendous, shooting up
the water and all the contents of the tube with terrific violence, and
with a beautiful effect. Pressure therefore alters the boiling point of
water.
[Illustration: Fig. 692.—Geyser of the Yellowstone.]
The mineral springs of Bath and many continental towns owe their
properties to the solvent power of water, which assimilates the mineral
atoms and gases. They arise just in the same way as the ordinary
spring, the taste and smell depending upon the soil and strata. Perhaps
the oil wells are the most curious phenomena of this kind. They are
excavated upon the Artesian principle. The petroleum is bored for,
as we bore for water, and the oil rushes up with great force, and in
enormous quantities. Gas wells are also to be found in Pennsylvania,
and have supplied towns with gas for years. Both these Artesian
wells are caused by the decay of vegetation. The gas is in the coal
formation, and the oil has been pressed out from vegetable deposit,
and as anthracite is a stony coal, petroleum is a kind of coal-tar, of
natural formation.
We have alluded to the river, which emerges from the spring, which has
fallen as rain. But there is another, and, to many minds, a much more
interesting form of the universal fluid we call water. This is ice.
Familiar as ice is, either to the stay-at-home invalid, the skater,
and the traveller, there is a great deal to be said about it. It is a
subject we would dwell upon had we space, for the remembrance of many
a pleasant hour passed upon snow and glacier call upon us to go back
again, even though only in imagination. No one who has not climbed
the glacier—even the Mer de Glace to the _Jardin_, now such a common
excursion—can fail to be struck with the beauty and grandeur of the
scene presented to him, and to carry away a fond recollection of the
icy regions he penetrated.
[Illustration: Fig. 693.—Colorado Cañon (effects of water erosion).]
For the ordinary hard-working man there is no change, no rest so truly
beneficial as a trip amongst the mountains and snowfields of Europe.
He need not be a climber; that is, a climber like Tyndall or Whymper,
those giants of the Alpine Club. But a stroll up to the Bel Alp, the
Æggishhorn, the Riffel, the Montanvert, or the Grimsel, will give the
average pedestrian some of the finest glacier scenery in Europe, and
which may, we believe, compare with any in the world for beauty. These
glaciers—ice-rivers—we will now consider briefly. We may take the _Mer
de Glace_ as an example (_see_ the illustration, p. 596). That gives
us a very fair idea of the ice-river, but the cut below is a good
specimen of a glacier.
Suppose we start up from Chamouni, or come across from Argentières, we
shall reach the Montanvert by ascending through the wood, or by the
“Chapeau,” across the ice-sea. As we take the former course, we walk
alongside a white-flowing and rapid river, the Arve, which unites with
the Rhone below Geneva. This river divides, and if we keep alongside
one (the right or south branch), we shall reach the moraine and the
icy grotto, from which the water issues. It is in this way many large
rivers are born. The Rhine, the Rhone, the Aar, the Ticino, have all of
them their sources in the ice. The Visp and the Sass waters are other
almost equally well-known examples.
[Illustration: Fig. 694.—Source of the Rhine.]
There used to be a grotto or cavern, into which the tourist could enter
at the source of the Arveiron, and here the beautiful blue of the ice
could be studied. From this place the Chapeau is reached, up a stony
path amid the trees, and from the top outside the hut we can see
the Mer de Glace all broken and contorted. The frequently occurring
roar of a falling rock which heat has deprived of its icy support,
or the cracking and tumbling of ice-blocks, may be seen and heard in
the forenoon. But the grandeur and majesty of the ice and snow-clad
mountains is best enjoyed by moonlight.
On the ice we shall see huge stones and gravel and grit, which have
been carried down by the ever-moving glacier, which is denuded in its
course, and worn down upon the surface as it slides, scraping and
grinding the valley through which it flows. By passing along a path
now made easy by irons, but formerly without supports or guards, the
surface of the glacier will be reached, and a man with a hatchet will
cut steps for the timid traveller. We are now upon the deep ice-river,
which has its springs in the snowy regions of the Col de Géant, in the
snow which is continually falling upon the heights, and draining away
to water again to form a river.
Thus the circle of events is completed,—snow, _névé_, ice (glacier),
water, which last is again absorbed into the atmosphere, and again
descends as rain or snow. And this is always going on by the action of
the sun. It may here fairly be asked how snow becomes ice. Why does
not the snow turn into ice at once, and form a glacier at the top of
the mountain as well as at the bottom? We will endeavour to make this
clear. Snow is composed of crystals, which assume certain definite
forms, and when first the flakes fall they are soft and powdery. By
degrees they melt a little, and when unconsolidated form what is termed
_névé_, the border line between ice and snow. This semi-icy snow
descends under pressure, and, as it increases, the glacier is formed
by huge blocks and masses being pressed together on the steep slopes
of the mountains. Thus the glacier descends, rounding off rocks, and
scouring as it goes, moving at a certain estimated rate daily,—about
twenty inches on the average,—carrying stones and _débris_ which form
the moraine, and finally when the high temperature in the valley melts
the ice, it issues forth as a river into the plain, or bounds down the
mountain side in a cascade. An excursion—and one by no means dangerous
if a guide be taken—to the _Jardin_, near Chamouni, will reveal
many interesting features of glacier formation, and of the glaciers
themselves.
[Illustration: Fig. 695.—Glacier table.]
Physical Geography is therefore very much indebted to the action of
water as a fluid or as a solid. In the former condition it erodes the
rocks, carries down the stones and gravel and sand, forms deltas at
the mouths of rivers, and elevates plains by overflowing its banks
and depositing sediment. Water gives beautiful scenery, and the
ever-changing features of the landscape are due to it. From the time
the spring emerges to the time when it has developed into a river,
bearing fine ships upon its restless waters, the universal fluid is
always at its work of destruction and benefit combined. From the limpid
stream we pass to the salt ocean, the reservoir of all the waters of
the globe.
[Illustration: Fig. 696.—Life under water.]
We have in this chapter briefly considered two very important forces
which have much to do with the varying conformation of the earth—viz.,
fire and water in their results of volcanic action and erosion. The sea
will tell us something more.
FOOTNOTES:
[31] From Artois, where the first European well of this kind existed.