impossible, to navigate the course of subterranean rivers to any
considerable distance, even at the time the water is low. When it
is high, the liquid mass, detained by the partitions, rises to a
very high level in the large interior cavities, and often reaches
the roof above. Sometimes when, through the clefts of the rocks, a
communication exists between the cave and some hollow above, the
surplus water from the subterranean streams makes its appearance
there. Thus the Recca, which flows beneath the adjacent plateau
of Trieste, does not always find space enough to flow freely in
its lower channels, and Schmidt has seen it ascend in the chasms
of Trebich to a height of 341 feet. It may be understood that the
pressure of such a column of water often shatters enormous pieces of
rock, and thus modifies the course of underground streams.
When the water, impelled by force of gravitation, seeks a new bed in
the cavernous depths of the earth, and disappears from its former
channels, these are at first much easier of access than they formerly
were; but ere long, in most caves, a new agent intervenes, which
seeks to contract or even completely obstruct them. This agent is
the snow-water, or rain, which percolates, drop by drop, through
the enormous filter of the upper strata. In passing through the
calcareous mass, each one of these drops dissolves a certain quantity
of carbonate of lime, which is afterward set free on the arch or
the sides of the cave. When the drop of water falls, it leaves
attached to the stone a small ring of a whitish substance; this is
the commencement of a stalactite. Another drop trickles down, and,
trembling on this ring, lengthens it slightly by adding to its edges
a thin circular deposit of lime, and then falls. Thus drop succeeds
drop in an infinite series, each depositing the particles of lime
which it contains, and forming ultimately a number of frail tubes,
round which the calcareous deposit slowly accumulates. But the water
which drops from the stalactites has not yet lost all the lime which
it held in solution; it still retains sufficient to enable it to
elevate the stalagmites and all the mammillated concretions which
roughen or cover the floor of the grotto. It is well known what
fairy-like decorations some caverns owe to this continuous oozing
through the vaults of their roofs. There are few sights in the world
more astonishing than that of these subterranean galleries, with
their dead-white columns, their innumerable pendants and multiform
groups, like veiled statues, all yet unstained by the smoke of the
visitor’s torch.
When the action of the water is not disturbed, the needles and
other deposits of the calcareous sediment continue to increase with
considerable regularity. In some cases each new layer which is
added to the concretions may be studied as a kind of time-measurer,
indicating the date when the running water abandoned the cave. At
length, however, the soft concentric layers disappear, and are
replaced by forms of a more or less crystalline character; for
in every case where solid particles exist, subject to constant
conditions of imbibition by water, crystals are readily produced.
Sooner or later, the stalactites, increasing gradually in a downward
direction, meet and unite with the needles rising from the surface of
the ground, and, forming by their number a kind of barrier, obstruct
the narrower passages and close up the defiles separating the cavern
into distinct chambers.
One of these Kentucky caves, called the “Mammoth Cave,” is the
largest which is at present known. The whole of its extent has
not been as yet fully explored, for it may be almost called a
subterranean world, having a system of lakes and rivers, and a
network of galleries and passages without number, which cross and
recross one another, going down to an immense depth. From the
chief entrance to the further recesses of the cave, the distance
is reckoned to be not less than 9¼ miles, and the whole length of
the two hundred alleys that have been traced out in this enormous
labyrinth is 217 miles in extent. This “Mammoth Cave” once served as
a retreat for savage tribes, for skeletons of men of an unknown race
have been found buried in it under layers of stalactite.
The district which is the most remarkable among all the calcareous
countries of Europe for its caves, its subterranean streams, and
its abysses is unquestionably the region of the Carniolan and
Istrian Alps, which extends to the east of the Adriatic, between
Laibach and Fiume. The whole surface of the country, as in certain
plateaus of the Jura in France, is everywhere pierced with deep
boat-shaped cavities, at the bottom of which the water forms a kind
of whirlpool, like the water flowing out of the hold of a stranded
ship. Many mountains are penetrated in every direction with caverns
and passages, just as if the whole rocky mass was nothing more than
an accumulation of cells. On one steep cliff-side may be noticed
all kinds of perforations at different heights--arched portals and
orifices of fantastic shape; on another there are numbers of springs
of blue water gushing from the caves, or from the rocks heaped up at
the foot of the cliff, and forming rivulets which disappear a little
further on in the fissures of the ground, as if through the holes of
a sieve. The whole surface of the plateaus, whether bare or covered
with forests, is scattered over with wells, or funnel-shaped holes
communicating with subterranean reservoirs.
One of the Istrian rivers, the subterranean course of which, although
still unknown as regards a great number of points, has given rise to
a most continuous course of investigations, is the celebrated Timavus
(Timavo), which falls into the sea near Duino, about twelve miles
to the north of Trieste. Virgil’s description no longer applies to
the mouths of the Timavo; at present they do not reach the number
of nine, because the extermination of the woods of the Carso has
diminished the mass of the water, or the action of the stream and
the alluvium of the delta have modified the form of the shore. But
still it is a magnificent spectacle to see the outlet of the three
principal torrents of water which rush foaming out of the heart of
the rocks, and are navigable from their mouths to their very source.
A river of this importance must certainly receive the drainage of
a vast basin, and yet all the neighboring valleys seem perfectly
devoid of rivulets, and their surface presents little else but the
bare rock; in fact, the whole of the rain and snow-water runs away
through underground caverns.
The most remarkable network of caverns in this region of the Alps is
that which spreads out from the southwest to the northeast across
the Adelsberg group of mountains, between Fiume and Laibach. The
principal cave is especially curious on account of its size, the
variety of its calcareous concretions, and the torrent which runs
roaring through it.
North of the town of Adelsberg the traveler passes along the base of
a hill with steep and bare sides, bringing into view the sharp edges
of its highly pitched calcareous beds. On the right the stream of
the Poik winds peaceably in the valley; and then, its course being
arrested by a headland, turning suddenly, it flows into the interior
of the mountain through a kind of high portal, opening between two
parallel beds of rocks. Unless the water in the stream is very low,
it is impossible to follow it over the accumulation of rocks upon its
bed; but on the right, at a height of a few yards, there is another
entry, through which the traveler may descend dry-shod into a vast
cavity or chamber, where the Poik again appears issuing from its
narrow passage of rocks.
At this point the cave divides; on the north the stream, the depth
of which varies, according to the season, from a few inches to 30 or
33 feet, buries itself in a winding avenue, which has been traversed
in a boat as far as a point 1,027 yards from the entrance; on the
northeast, a higher avenue, discovered only in 1818, pushes its
way far into the heart of the mountain, branching out in various
directions into narrow passages and wide compartments. This portion
of the grotto, which appears to have been the former bed of the Poik,
is the most curious part of the Adelsberg labyrinth; it affords
wonderful groups of stalactites, especially in the Salle du Calvaire,
the vaulted roof of which, having the enormous span of 210 yards,
has dropped upon a hillock of débris a perfect forest of stalagmitic
columns and white needles. The full length of the principal cave is
not less than 2,575 yards; but very probably some other and still
longer avenues may yet be discovered.
Although it is impossible to go in a boat along the subterranean
portion of the Poik for a greater distance than 1,027 yards,
by traversing the surface of the calcareous plateaus we can at
all events trace out the subterranean stream by means of the
funnel-shaped holes which open above its course. One of these gulfs,
the Piuka-Jama, is situated about a mile and a half to the north of
the entrance of the Adelsberg caves; the only way to descend into
this is by clinging to the branches of the shrubs and sliding down by
the assistance of a cord fastened to the top of the rocks. By these
means the entrance to a kind of air-hole may be reached, from which
the Poik is visible foaming over its bed of rocks, and only a slope
of débris is to be descended to reach the edge of the stream. It can
only be followed in the downstream direction for about 275 yards;
but it can easily be ascended for a distance of 495 yards by passing
under a high portal with lofty pillars, and in this way a point can
be reached which is less than a mile from the place where the stream
disappeared in the cave of Adelsberg.
Further down the stream the Poik is not visible again until it
emerges from the mountain, where it is known under the name of the
Planina; it rushes out through a circular arch at the base of a
perpendicular bluff crowned with fir-trees. It really is the Poik,
as is proved by the equal temperature of water and the sudden
increase of its liquid mass after a storm has burst at Adelsberg;
but the stream always issues from the cave much more considerable
in bulk than it is when it enters, owing to the tributaries which
pour into it on both sides during its subterranean course of five
to six miles. One of these rivulets, which comes down from the
plateaus of Kaltenfeld, joins the Poik at a little distance from its
outlet. Above the confluence the principal stream can be ascended
in a boat to a distance of more than 3,500 yards, which, with the
other explored parts of the subterranean river, makes about three
miles. Below the point of outlet the stream is partially lost in the
fissures of its bed, and then, joining the Unz, goes on and empties
itself into the Danubian Save.
About a dozen miles to the southeast of the Adelsberg and Planina
caves extends a large plain surrounded on all sides by high
calcareous cliffs, at the base of which nestle seven villages. In
this hollow, the most elevated portion of which is under cultivation,
the remainder being covered with rushes and other marsh-plants, there
are to be found more than 400 funnel-shaped holes resembling those
in other parts of Carniola. These _dolinas_, the average depth of
which is from 40 to 60 feet, have each their special name, such as
the “_Grand Crible_” (great sieve), the “_Crible-à-froment_” (corn
sieve), the “_Tambour_” (drum), the “_Cuve_” (tub), the “_Tonneau_”
(cask), pointing out the form or some remarkable peculiarity of
each abyss. During extremely dry seasons there is only one of these
cavities which contains any water; but after continuous and heavy
rain, the water of a stream which is swallowed up in the rocks a
little above the plain rises with a roaring noise in each of these
wells. Torrents escaping from all these open “_cribles_” form in the
wide space hemmed in by the cliffs a sea of blue and transparent
water. This is the lake of Jessero or Zirknitz, the _lacus Lugens_ of
the Romans. The surface of the sheet of water extends over an area of
14,826 acres; at the time of great inundations, this extraordinary
temporary lake, thus vomited out by the underground river, is not
less than 24,711 acres. The water runs away through a subterranean
channel, and, further on, empties itself into the Unz, below the
Planina.
Lacustrine basins of this sort, first emitted, and then again
absorbed by a subterranean water-course, are rather rare; there are,
however, some other remarkable instances of them in Europe. Thus,
in the Oriental Hartz, in the midst of a beautiful spot surrounded
by fir-trees, the charming lake called Bauerngraben (Peasants’
Ditch), or sometimes Hungersee (Lake of Famine), sometimes makes its
appearance; but when this mass of blue water has filled but for a few
days its basin of gypsum rock, it is suddenly swallowed up, and flows
away by subterranean channels into the stream of the Helme. The
celebrated lake of Copaïs, in Bœotia, may likewise be compared to the
Zirknitz lake, at least as regards certain portions of its basin.
RIVERS
--A. KEITH JOHNSTON
Rivers are the result of the natural tendency of water, as of all
other bodies, to obey the law of gravitation by moving downward
to the lowest position it can reach. The supply of water for the
formation of rivers, though apparently derived from various sources,
as from rain-clouds, springs, lakes, or from the melting of snow, is
really due only to atmospheric precipitation; for springs are merely
collections of rain-water; lakes are collections of rain or spring
water in natural hollows, and snow is merely rain in a state of
congelation. The rills issuing from springs and from surface-drainage
unite during their downward course with other streams, forming
_rivulets_; these, after a further course, unite to form _rivers_,
which, receiving fresh accessions in their course from _tributaries_
(subordinate rivers or rivulets) and their _feeders_ (the tributaries
of tributaries), sweep onward through ravines, and over precipices,
or crawl with almost imperceptible motion across wide, flat plains,
till they reach their lowest level in ocean, sea, or lake. The
path of a river is called its _course_; the hollow channel along
which it flows, its _bed_; and the tract of country from which it
and its subordinates draw their supplies of water, its _basin_, or
_drainage-area_. The basin of a river is bounded by an elevated
ridge, part of which is generally mountainous, the crest forming
the watershed; and the size of the basin, and the altitude of its
watershed, determine, _cæteris paribus_, the volume of the river. The
greater or less degree of uniformity in the volume of a river in the
course of a year is one of its chief physical features, and depends
very much on the mode in which its supply of water is obtained.
In temperate regions, where the mountains do not reach the limit
of perpetual snow, the rivers depend for their increase wholly on
the rains, which, occurring frequently, and at no fixed periods,
and discharging only comparatively small quantities of water at a
time, preserve a moderate degree of uniformity in the volume of the
rivers--a uniformity which is aided by the circumstance that in these
ones only about one-third of the rainfall finds its way directly over
the surface to the rivers; the remaining two-thirds sinking into
the ground, and finding its way to spring-reservoirs, or gradually
oozing through at a lower level in little rills which continue to
flow till the saturated soil becomes drained of its surplus moisture,
a process which continues for weeks, and helps greatly to maintain
the volume of the river till the next rainfall. This process, it
is evident, is only possible where the temperature is mild, the
climate moist, evaporation small, and the soil sufficiently porous;
and under these circumstances great fluctuations can only occur
from long-continued and excessive rains or droughts. In the hotter
tracts of the temperate zones, where little rain falls in summer,
we occasionally find small rivers and mountain torrents becoming
completely exhausted; such is often the case in Spain, Italy, Greece,
and with the Orange, one of the largest rivers of South Africa.
In tropical and semi-tropical countries, on the other hand, the year
is divisible into one dry and one wet season; and in consequence the
rivers have also a periodicity of rise and fall, the former taking
place first near the source, and, on account of the great length of
course of some of the tropical rivers, and the excessive evaporation
to which they are subjected (which has necessarily most effect where
the current is slow), not making itself felt in the lower part of
their course till a considerable time afterward. Thus, the rise of
the Nile occurs in Abyssinia in April, and is not observed at Cairo
till about mid-summer. The fluctuations of this river were a subject
of perpetual wonderment to the ancient civilized world, and were
of course attributed to superhuman agency; but modern travel and
investigation have not only laid bare the reason of this phenomenon,
but discovered other instances of it, before which this one shrinks
into insignificance.
The maximum rise of the Nile, which is about 40 feet, floods 2,100
square miles of ground; while that of the Orinoco, in Guiana, which
is from 30 to 36 feet, lays 45,000 square miles of savannah under
water; the Brahmaputra at flood covers the whole of Upper Assam to a
depth of 10 feet, and the mighty Amazon converts a great portion of
its 500,000 square miles of selvas into one extensive lake. But the
fluctuations in the rise of the flood-waters are surpassed by some
of the comparatively small rivers of Australia, one of which, the
Hawkesbury, has been known to rise 100 feet above its usual level.
This, however, is owing to the river-beds being hemmed in by lofty
abrupt cliffs, which resist the free passage of a swollen stream.
The increase from the melting of snow in summer most frequently
occurs during the rainy season, so that it is somewhat difficult
to determine, with anything like accuracy, the share of each
in producing the floods; but in some rivers, as the Ganges and
Brahmaputra, the increase from this cause is distinctly observable,
as it occurs some time after the rains have commenced, while in the
case of the Indus it is the principal source of flood. When the
increase from melted snow does not occur during the rainy season, we
have the phenomenon of flooding occurring twice a year, as in the
case of the Tigris, Euphrates, Mississippi, and others; but in most
of these cases the grand flood is that due to the melting of the snow
or ice about the source.
The advantages of this periodical flooding in bringing down abundance
of rich fertile silt--the Nile bringing down, it is said, no less
than 140 millions of tons, and the Irrawadi 110 millions of tons
annually--are too well known to need exposition here. Islands are
thus frequently formed, especially at a river’s mouth. Permanent and
capacious lakes in a river’s course have a modifying effect owing to
their acting as reservoirs, as is seen in the St. Lawrence; while
the Red River (North) and others in the same tract inundate the
districts surrounding their banks for miles. In tropical countries,
owing to the powerful action of the sun, all rivers whose source is
in the regions of perpetual snow experience a daily augmentation
of their volume; while some in Peru and Chili, being fed only by
snow-water, are dried up regularly during the night.
The course of a river is necessarily the line of lowest level from
its starting-point, and as most rivers have their sources high up a
mountain slope the velocity of their current is much greater at the
commencement. The courses of rivers seem to be partially regulated
by geological conditions of the country, as in the case of the San
Francisco of Brazil, which forms with the most perfect accuracy the
boundary-line between the granitic and the tertiary and alluvial
formations in that country; and many instances are known of rivers
changing their course from the action of earthquakes, as well as from
the silting up of the old bed. The inclination of a river’s course
is also connected with the geological character of the country; in
primary and transition formations, the streams are bold and rapid,
with deep channels, frequent waterfalls and rapids, and pure waters,
while secondary and alluvial districts present slow and powerful
currents, sloping banks, winding courses, and tinted waters; the
incline of a river is, however, in general very gentle--the average
inclination of the Amazon throughout its whole course being estimated
at little more than six inches per mile, that of the Lower Nile less
than seven inches, and of the Lower Ganges about four inches per mile.
The average slope of the Mississippi throughout its whole length is
more than seventeen inches per mile, while the Rhone is, with the
exception of some much smaller rivers and torrents, the most rapid
river in the world, its fall from Geneva to Lyons being eighty inches
per mile, and thirty-two inches from Lyons to its mouth.
The velocity of rivers does not depend wholly on their slope; much is
owing to their depth and volume (the latter being fully proved by the
fact that the beds of many rivers remain unaltered in size and slope
after their streams have received considerable accessions, owing to
the greater rapidity with which the water runs off); while bends in
the course, jutting peaks of rock or other obstacles, whether at the
sides or bottom, and even the friction of the aqueous particles,
which, though slight, is productive of perceptible effect, are
retarding agencies. In consequence, the water of a river flows with
different velocities at different parts of its bed; it moves slower
at the bottom than at the surface, and at the sides than the middle.
The line of quickest velocity is the line drawn along the centre of
the current, and in cases where this line is free from sudden bends
or sharp turns, it also represents the deepest part of the channel.
The average velocity of a river may be estimated approximately by
finding the surface-velocity in the centre of the current by means of
a float which swims just below the surface, and taking four-fifths
of this quantity as a mean. If the mean velocity in feet per minute
be multiplied by the area of the transverse section of the stream in
square feet, the product is the amount of water discharged in cubic
feet per minute. According to Sir Charles Lyell, a velocity of 40
feet per minute will sweep along coarse sand; one of 60 feet, fine
gravel; one of 120 feet, rounded pebbles; one of 180 feet (a little
more than two miles per hour), angular stones the size of an egg.
“Rivers are the irrigators of the earth’s surface, adding alike to
the beauty of the landscape and the fertility of the soil; they
carry off impurities and every sort of waste débris; and when of
sufficient volume, they form the most available of all channels of
communication with the interior of continents.... They have ever been
things of vitality and beauty to the poet, silent monitors to the
moralist, and agents of comfort and civilization to all mankind.” By
far the greater portion of them find their way to the ocean, either
directly or by means of semi-lacustrine seas; but others, as the
Volga, Sir-Daria (Jaxartes), Amu-Daria (Oxus), and Kur (Araxes), pour
their waters into inland seas; while many in the interior of Asia
and Africa--as the Murghab in Turkestan, and the Gir in the south of
Morocco--“lose themselves in the sands,” partly, doubtless, owing
to the porous nature of their bed, but much more to the excessive
evaporation which goes on in those regions.
SWAMPS AND MARSHES
--ÉLISÉE RECLUS