basins of that region is to be sought in underground movements
which, according to Helmersen, are still in progress, but partly,
and, in the case of the smaller basins, probably chiefly in a
general diminution of the water supply all over Central Asia and the
neighboring regions. The rivers that flow from the north toward Lake
Balkash, and that once doubtless emptied into it, now lose themselves
in the wastes and are evaporated before reaching that sheet of water,
which is fed only from the mountains to the south. The channels of
the Amur Darya, Syr Darya, and other streams bear witness also to the
same general desiccation. At present, the amount of water supplied
by rivers to the Caspian Sea appears on the whole to balance that
removed by evaporation, though there are slight yearly or seasonal
fluctuations. In the Aral basin, however, there can be no doubt that
the waters are progressively diminishing.
Owing to the enormous volume of fresh water poured into it by its
rivers, the Caspian Sea is not as a whole so salt as the main
ocean, and still less so than the Mediterranean Sea. Nevertheless
the inevitable result of evaporation is there manifested. Along the
shallow pools which border this sea, a constant deposition of salt
is taking place, forming sometimes a pan or layer of rose-colored
crystals on the bottom, or gradually getting dry and covered with
drift-sand. This concentration of the water is particularly marked
in the great offshoot called the Karaboghaz, which is connected with
the middle basin of the Caspian Sea by a channel 150 yards wide and
5 feet deep. Through this narrow mouth there flows from the main sea
a constant current, which Von Baer estimated to carry daily into
the Karaboghaz 350,000 tons of salt. An appreciable increase of the
saltness of that gulf has been noticed; seals, which once frequented
it, have forsaken its barren shores. Layers of salt are gathering on
the mud at the bottom, where they have formed a salt bed of unknown
extent, and the sounding-line, when scarcely out of the water, is
covered with saline crystals.
The study of the precipitations which take place on the floors of
modern salt lakes is important in throwing light upon the history
of a number of chemically formed rocks. The salts in these waters
accumulate until their point of saturation is reached, or until
by chemical reactions they are thrown down. The least soluble are
naturally the first to appear, the water becoming progressively
more and more saline till it reaches a condition like that of the
mother-liquor of a salt work. Gypsum begins to be thrown down from
sea-water, when 37 per cent of water has been evaporated, but 93
per cent of water must be driven off before chloride of sodium can
begin to be deposited. Hence the concentration and evaporation of
the water of a salt lake having a composition like that of the sea
would give rise first to a layer or sole of gypsum, followed by one
of rock-salt. This has been found to be the normal order among the
various saliferous formations in the earth’s crust. But gypsum may be
precipitated without rock-salt, either because the water was diluted
before the point of saturation for rock-salt was reached, or because
the salt, if deposited, has been subsequently dissolved and removed.
In every case where an alternation of layers of gypsum and rock-salt
occurs, there must have been repeated renewals of the water-supply,
each gypsum zone marking the commencement of a new series of
precipitates.
But from what has now been adduced it is obvious that the composition
of many existing saline lakes is strikingly unlike that of the sea
in the proportions of the different constituents. Some of them
contain carbonate of sodium; in others the chloride of magnesium
is enormously in excess of the less soluble chloride of sodium.
These variations modify the effects of evaporation of additional
supplies of water now poured into the lakes. The presence of the
sodium-carbonate causes the decomposition of lime salts, with the
consequent precipitation of calcium-carbonate accompanied with a
slight admixture of magnesium-carbonate, while by further addition of
the sodium-carbonate a hydrated magnesium-carbonate may be eventually
precipitated. Hunt has shown that solutions of bicarbonate of lime
decompose sulphate of magnesia with the consequent precipitation of
gypsum, and eventually also of hydrated carbonate of magnesia, which,
mingling with carbonate of lime, may give rise to dolomite. By such
processes the marls or clays deposited on the floors of inland seas
and salt lakes may conceivably be impregnated and interstratified
with gypseous and dolomitic matter, though in the Trias and other
ancient formations which have been formed in inclosed saline waters,
the magnesium-chloride has probably been the chief agent in the
production of dolomite.
The Dead Sea, Elton Lake, and other very salt waters of the
Aralo-Caspian depression, are interesting examples of salt lakes
far advanced in the process of concentration. The great excess of
the magnesium-chloride shows, as Bischof pointed out, that the
waters of these basins are a kind of mother-liquor, from which most
of the sodium-chloride has already been deposited. The greater the
proportion of the magnesium-chloride, the less sodium-chloride can
be held in solution. Hence, as soon as the waters of the Jordan and
other streams enter the Dead Sea, their proportion of sodium-chloride
(which in the Jordan water amounts to from .0525 to .0603 per cent)
is at once precipitated. With it gypsum in crystals goes down,
also the carbonate of lime which, though present in the tributary
streams, is not found in the waters of the Dead Sea. In spring, the
rains bring large quantities of muddy water into this sea. Owing
to dilution and diminished evaporation, a check must be given to
the deposition of common salt, and a layer of mud is formed over
the bottom. As the summer advances and the supply of water and mud
decreases, while evaporation increases, the deposition of salt
and gypsum begins anew. As the level of the Dead Sea is liable to
variations, parts of the bottom are from time to time exposed, and
show a surface of bluish-gray clay or marl full of crystals of common
salt and gypsum. Beds of similar saliferous and gypsiferous clays,
with bands of gypsum, rise along the slopes for some height above
the present surface of the water, and mark the deposits left when
the Dead Sea covered a larger area than it now does. Save occasional
impressions of drifted terrestrial plants, these strata contain no
organic remains. Interesting details regarding saliferous deposits of
recent origin, on the site of the Bitter Lakes, were obtained during
the construction of the Suez Canal. Beds of salt, interleaved with
laminæ of clay and gypsum-crystals, were found to form a deposit
upward of 30 feet thick extending along 21 miles in length by about
8 miles in breadth. No fewer than 42 layers of salt, from 3 to 18
centimetres thick, could be counted in a depth of 2.46 metres. A
deposit of earthy gypsum and clay was ascertained to have a thickness
of 367 feet (112 metres), and another bed of nearly pure crumbling
gypsum to be about 230 feet (70 metres) deep.
The desiccated floors of the great saline lakes of Utah and Nevada
have revealed some interesting facts in the history of saliferous
deposits. The ancient terraces marking former levels of these lakes
are cemented by tufa, which appears to have been abundantly formed
along the shores where the brooks, on mingling with the lake,
immediately parted with their lime. Even at present, oolitic grains
of carbonate of lime are to be found in course of formation along
the margin of Great Salt Lake, though carbonate of lime has not been
detected in the water of the lake, being at once precipitated in the
saline solution. The site of the ancient salt lake which has been
termed Lake Lahontan displays areas several square miles in extent
covered with deposits of calcareous tufa, 20 to 60 and even 150
feet thick. This tufa, however, presents a remarkable peculiarity.
It is sometimes almost wholly composed of what have been determined
to be calcareous pseudo-morphs after gaylussite (a mineral composed
of carbonates of calcium and sodium with water)--the sodium of the
mineral having been replaced by calcium. When this variety of tufa,
distinguished by the name of _thïnolite_, was originally formed, the
waters of the vast lake must have been bitter, like those of the
little soda-lakes which now lie on its site--a dense solution in
which carbonate of soda predominated. On the margin of one of the
present Soda Lakes, crystals of gaylussite now form in the drier
season of the year. Yet no trace of carbonate of lime has been
detected in the water. The carbonate of lime in the crystals must
be derived from water which on entering the saline lakes is at once
deprived of its lime.
UNDERGROUND WATER: SPRINGS, CAVES, RIVERS, AND LAKES
--ÉLISÉE RECLUS
If all soils were absolutely impervious, there would be no springs,
and the whole of the liquid mass furnished by rain and snow would
flow away over the surface of the ground like the torrents and
flood-waters of the mountains. The greater part, however, of the
water which falls upon the ground sinks in the first place into the
depths of the earth. There it becomes more or less perfectly purified
from the foreign bodies with which it was charged, gradually rising
to the temperature of the strata through which it passes, and
becoming impregnated with the soluble salts which it meets with.
Ultimately, when the water, in sinking down, encounters impervious
beds, it can penetrate no further, and, flowing laterally to the
outcrop of the beds, makes its escape in the form of springs.
The absorption of the rain and melted snow takes place in various
ways, according to the nature of the soil. Ordinary vegetable
earth only allows the water to penetrate to a very slight depth,
especially when the rain falls in showers and the slope of the
ground is favorable for drainage. As mould is capable of absorbing
a very large quantity--indeed, more than half its own weight, it
prevents the strata beneath from receiving its due share of moisture,
retaining almost the whole of it for the use of the vegetation which
it nourishes. In fact, it requires an altogether exceptional rainfall
to saturate any ordinary arable soil to the extent of a yard below
the surface. Water passes with much more facility through sandy
and gravelly beds; but compact loams and clay will not allow it to
penetrate through them, retaining it in the form of pools or ponds on
the surface of the ground.
The action of vegetation is not confined merely to imbibing the water
falling from the clouds; it often, also, assists the superabundant
moisture in penetrating the interior of the ground. Trees, after
they have received the water upon their foliage, let it trickle down
drop by drop on the gradually softened earth, and thus facilitate
the gentle permeation of the moisture into the substratum; another
part of the rain-water, running down the trunk and along the roots,
at once finds its way to lower strata. On mountain slopes, the
mosses and the freshly growing carpet of Alpine plants swell like
sponges when they are watered with rain or melted snow, and retain
the moisture in the interstices of their leaves and stalks until the
vegetable mass is thoroughly saturated and the liquid surplus flows
away. Peat-mosses especially absorb a very considerable quantity of
water, and form great feeding reservoirs for the springs which gush
out at a lower level. The immense fields of peat which cover hundreds
and thousands of acres on the mountain slopes of Ireland and Scotland
may, notwithstanding their elevation and inclined position, be
considered as actual lacustrine basins containing millions of tons of
water dispersed among their innumerable leaflets. The superabundant
water of these tracts of peat-mosses issues forth in springs in the
plains below.
Rocks, like vegetable earth, also absorb water in greater or less
quantities, according to their fissures and the density of their
particles. If the soil is formed of volcanic scoriæ, or porous beds
of pebbles, gravel, or sand, the water rapidly descends toward the
underlying strata. Some of the harder rocks, especially certain kinds
of granite, absorb but a very small quantity of water, on account of
the small number of their clefts; others, on the contrary, as most
of the calcareous masses, imbibe every drop of water which falls on
their surface. There are some rocks which have their layers broken
and cracked to such an extent that they resemble enormous walls of
rubble-work; the rain instantly disappears on them as if it had
fallen into a sieve. But the greater part of the calcareous rocks
belonging to various geological periods are formed of thick and
regular strata, cleft at intervals by long vertical crevices. Below
the surface-beds, perhaps, are layers of soft marl, which the water
penetrates with difficulty, although it can soften and carry away its
particles. Here are formed, rill by rill, the subterranean rivulets
which ultimately spread all over the substratum of marl, following
the general slope of the bed. After a more or less considerable lapse
of time, the stratum of marl ultimately becomes saturated, and the
water then flows out through caverns which are variously modified by
subsidences--faults in the strata and the perpetual action of the
streams. The springs which proceed from calcareous rocks of this
nature are in general the most abundant, owing to the length of their
subterranean course. The water which falls on vast areas on the
surface of plateaus is ultimately united in one bed. A liquid mass of
this kind, which springs up suddenly into sight, just as if it merely
issued from the soil, drains perhaps an extent of country of many
hundreds or thousands of square miles.
Thus, according to the nature of the rock on which the rain
falls, the latter finds its way again to the surface, either at a
considerable distance from the spot where it fell, or else springs
out in little rivulets immediately below the place where its drops
were first gathered. On a great many mountains we are surprised to
meet with springs gushing out at a few yards from the summit. These
jets have, indeed, often been considered as the evidence of some
miraculous intervention. Among others, we may mention the “Sorcerers’
Spring,” which gushes out on one of the highest points of the
Brocken, the culminating peak in the Hartz Mountains.
The springs which cause the most astonishment are those which for a
time flow plentifully, and then all at once cease running, but, after
an uncertain lapse of time, again make their appearance. One might
almost fancy that some invisible hand alternately opened and shut the
secret flood-gate which gave an outlet to the subterranean stream.
The cause for this phenomenon of intermission is easily explained.
When the water brought by the underground stream is collected in a
capacious cavity in the rock, which communicates with the exterior
surface through a siphon-shaped channel, the liquid mass gradually
rises in the stone reservoir before it rushes out into the air. It is
necessary that the reservoir should be filled up to the level of the
siphon, in order that the latter should be primed, and that the water
should flow out as a spring into the external basin. If the water in
the reservoir is not replenished with sufficient rapidity, and is
unable to keep at least on a level with the external outlet, the jet
of water will immediately cease, and can not recommence until the
upper part of the liquid mass has again risen up to the highest point
of the siphon. After an indefinite period of repose, the spring then
enters on a new phase of activity.
There are many of these subterranean streams which, before they break
forth in springs, do not flow over beds continuously sloping in the
direction of their current, as in the case with the water-courses on
the surface of the ground. There are some indeed which first descend
into the bowels of the earth, either by a uniform declivity or by a
series of cascades or rapids, and ultimately reascend from the depths
toward the surface, or jet out vertically from the ground.
In obedience to the law which compels liquids to seek the same
level in all connected reservoirs, a rivulet of water will never
fail to dart forth as a spring as soon as it finds an outlet below
the caverns in which the water is collected from which it proceeds.
Likewise, if the spot where the gushing out takes place is on a much
lower level than that of the feeding reservoirs situated above, the
liquid jet must necessarily shoot up in a column above the surface
of the ground. This is the case at Châtagna, in the department of
the Jura, where a natural _jet d’eau_ springs up to a height of
10 or 12 feet. In the grotto of Male-Mort, near Saint-Etienne, in
Dauphiné, the jet of water is not less than 23 to 26 feet in height.
But the water of the fountains being always more or less charged with
sediment, the deposit accumulates in the form of a circular hillock
around the orifice, thus almost always ultimately raising it to the
level of the top of the liquid column. As an instance of these rising
fountains, we may mention the famous springs of Moses (_Aïn Musa_),
which gush out in a charming oasis not far from the shores of the
Gulf of Suez. These springs, the temperature of which varies from
70° to 84° (Fahr.), now flow from the top of several small cones of
sandy and slimy débris which they have gradually thrown up above the
level of the plain. They are also shaded by olive and tamarind trees.
In the innumerable multitude of springs, either cold or thermal,
which rise from the earth, we may observe the whole range of possible
temperatures from freezing-point up to the boiling-point. A spring
which flows from the side of the Hangerer, in the Oetzthal, at a
height of 6,742 feet, is only 1° warmer than ice. On the Alps, the
Pyrenees, and all the other chains of snow-clad mountains, near
the summits small rills of water are very frequently met with,
the temperature of which is scarcely higher than that of melting
snow. Even at the bases of mountains, and especially those of a
calcareous nature, a great number of springs are found which are
much colder than the surrounding soil. This is so because, in
addition to the water, the air also enters the subterranean channels
and circulates in all the network of clefts and crevices, and, by
incessantly gliding over the wet sides of the channels, produces a
rapid evaporation of moisture, and, in consequence, refrigerates the
surface of the rocks and even the stream itself. The temperature,
therefore, of springs which proceed from the interior of cavernous
mountains is always several degrees lower than the normal temperature
of the soil.
Springs which have a higher temperature than the soil are called
_thermal_ springs.
It is to be remarked that nearly all thermal springs which do not owe
their high temperature to the vicinity of volcanoes issue forth from
faults which open on the surface of masses of a crystalline nature,
and principally at the side of modern eruptive rocks which have been
thrust up through older strata.
The influence of rains and seasons has much less effect upon thermal
waters than upon cold springs which proceed from the upper layers
of the soil. A great number of warm springs, however, undergo
certain changes in their yield of water, which must be without
doubt attributable, at least partially, to the same causes as the
variations in the discharges of superficial streams. In Auvergne,
in the Pyrenees, and in Switzerland, several springs, perfectly
protected against any infiltration of rain-water, flow in much
greater abundance at the very same period when the adjacent torrents
become swollen. At Brig-Baden, in the Valais, the water, the mean
temperature of which is in autumn and winter from 71° to 72° (Fahr.),
rises to 113° and 122° (Fahr.) when the breath of spring melts the
ice on the Jungfrau.
Most thermal springs contain mineral substances in solution;
there are, however, a certain number which are almost as pure
as rain-water--such as, for instance, the celebrated waters of
Plombières, also that of Gastein, Pfeffers, Wildbad, and Badenweiler.
The springs of Chaudes-Aigues--those in France which have the highest
temperature, 158° to 176° (Fahr.)--contain only a small amount of
mineral substances. The inhabitants of Chaudes-Aigues use the water
to prepare their food, to wash their linen, and to warm their houses.
Wooden conduits, erected in all the streets of the town, supply, on
the ground floor of each house, a reservoir which serves to heat it
during cold weather, and thus dispenses with fires and chimneys.
Among the various substances which spring-water brings to the
surface, those which are most common proceed from the strata
which serve to constitute the very framework of the globe. Chalk,
especially, occurs in different proportions in most springs, either
under the form of sulphate of lime, or, more often, as carbonate of
lime. Water which contains carbonic acid in solution is charged with
calcareous matter dissolved away from the sides of the rocks through
which it passes; then, by means of evaporation, it redeposits the
stony substances which it previously held in solution. Hence arise
all those calcareous concretions which form around so many springs;
also the stalactites in caverns.
[Illustration: The Giant’s Causeway, Antrim, Ireland]
Nearly all countries of the world possess some of these curious
springs, which cover with a calcareous crust any object placed
in their waters. Among these incrusting springs, those of Saint
Allyre, near Clermont, Rivoli, and San Filippo, not far from Rome,
have justly become celebrated. These latter have, in a space of
twenty years, filled up a pond with a bed of travertin 30 feet
thick, and, in the neighborhood, entire strata of this same rock
may be seen having a depth of more than 328 feet. The springs of
Hammam-Mes-Khoutine, in the province of Constantine, are also
very remarkable on account of the considerable amount of their
deposits. This water, which rises at a temperature of 203° (Fahr.),
and from which a high column of steam always rises, is frequently
compelled to change its point of issue on account of the dense beds
of travertin which are gradually deposited upon the soil. Most of
these deposits are of a dazzling white hue, striped here and there
with bright colors, and are developed in mammillated strata; other
concretions, accumulating gradually round an orifice, have taken
the form of cones, and are like small craters near a volcano, some
of them rising to a height of as much as 33 feet; lastly, there are
masses of travertin which stretch out in a kind of wall below the
flow which deposits them. One of these walls, which is interrupted at
intervals by heaps of earth upon which large trees grow, is not less
than 4,921 feet long, 66 feet high, and, on an average, from 33 to 49
feet wide.