PHENOMENA OF SPRINGS.
Origin of Springs--Artesian wells--Borings at Paris--Distinct causes
by which mineral and thermal waters may be raised to the
surface--Their connection with volcanic agency--Calcareous
springs--Travertin of the Elsa--Baths of San Vignone and of San
Filippo, near Radicofani--Spheroidal structure in travertin--Lake of
the Solfatara, near Rome--Travertin at Cascade of Tivoli--Gypseous,
siliceous, and ferruginous springs--Brine springs--Carbonated
springs--Disintegration of granite in Auvergne--Petroleum
springs--Pitch lake of Trinidad.
_Origin of springs._--The action of running water on the surface of the
land having been considered, we may next turn our attention to what may
be termed "the subterranean drainage," or the phenomena of springs.
Every one is familiar with the fact, that certain porous soils, such as
loose sand and gravel, absorb water with rapidity, and that the ground
composed of them soon dries up after heavy showers. If a well be sunk in
such soils, we often penetrate to considerable depths before we meet
with water; but this is usually found on our approaching the lower parts
of the formation, where it rests on some impervious bed; for here the
water, unable to make its way downwards in a direct line, accumulates as
in a reservoir, and is ready to ooze out into any opening which may be
made, in the same manner as we see the salt water flow into, and fill,
any hollow which we dig in the sands of the shore at low tide.
The facility with which water can percolate loose and gravelly soils is
clearly illustrated by the effect of the tides in the Thames between
Richmond and London. The river, in this part of its course, flows
through a bed of gravel overlying clay, and the porous superstratum is
alternately saturated by the water of the Thames as the tide rises, and
then drained again to the distance of several hundred feet from the
banks when the tide falls, so that the wells in this tract regularly ebb
and flow.
If the transmission of water through a porous medium be so rapid, we
cannot be surprised that springs should be thrown out on the side of a
hill, where the upper set of strata consist of chalk, sand, or other
permeable substances, while the subjacent are composed of clay or other
retentive soils. The only difficulty, indeed, is to explain why the
water does not ooze out everywhere along the line of junction of the two
formations, so as to form one continuous land-soak, instead of a few
springs only, and these far distant from each other. The principal cause
of this concentration of the waters at a few points is, first, the
frequency of rents and fissures, which act as natural drains; secondly,
the existence of inequalities in the upper surface of the impermeable
stratum, which lead the water, as valleys do on the external surface of
a country, into certain low levels and channels.
That the generality of springs owe their supply to the atmosphere is
evident from this, that they become languid, or entirely cease to flow,
after long droughts, and are again replenished after a continuance of
rain. Many of them are probably indebted for the constancy and
uniformity of their volume to the great extent of the subterranean
reservoirs with which they communicate, and the time required for these
to empty themselves by percolation. Such a gradual and regulated
discharge is exhibited, though in a less perfect degree, in every great
lake which is not sensibly affected in its level by sudden showers, but
only slightly raised; so that its channel of efflux, instead of being
swollen suddenly like the bed of a torrent, is enabled to carry off the
surplus water gradually.
Much light has been thrown, of late years, on the theory of springs, by
the boring of what are called by the French "Artesian wells," because
the method has long been known and practised in Artois; and it is now
demonstrated that there are sheets, and in some places currents of fresh
water, at various depths in the earth. The instrument employed in
excavating these wells is a large augur, and the cavity bored is usually
from three to four inches in diameter. If a hard rock is met with, it is
first triturated by an iron rod, and the materials being thus reduced to
small fragments or powder, are readily extracted. To hinder the sides of
the well from falling in, as also to prevent the spreading of the
ascending water in the surrounding soil, a jointed pipe is introduced,
formed of wood in Artois, but in other countries more commonly of metal.
It frequently happens that, after passing through hundreds of feet of
retentive soils, a water-bearing stratum is at length pierced, when the
fluid immediately ascends to the surface, and flows over. The first rush
of the water up the tube is often violent, so that for a time the water
plays like a fountain, and then, sinking, continues to flow over
tranquilly, or sometimes remains stationary at a certain depth below the
orifice of the well. This spouting of the water in the first instance is
probably owing to the disengagement of air and carbonic acid gas, for
both of these have been seen to bubble up with the water.[304]
At Sheerness, at the mouth of the Thames, a well was bored on a low
tongue of land near the sea, through 300 feet of the blue clay of
London, below which a bed of sand and pebbles was entered, belonging,
doubtless, to the plastic clay formation; when this stratum was pierced,
the water burst up with impetuosity, and filled the well. By another
perforation at the same place, the water was found at the depth of 328
feet below the surface clay; it first rose rapidly to the height of 189
feet, and then, in the course of a few hours, ascended to an elevation
of eight feet above the level of the ground. In 1824 a well was dug at
Fulham, near the Thames, at the Bishop of London's, to the depth of 317
feet, which, after traversing the tertiary strata, was continued through
67 feet of chalk. The water immediately rose to the surface, and the
discharge was about 50 gallons per minute. In the garden of the
Horticultural Society at Chiswick, the borings passed through 19 feet of
gravel, 242-1/2 feet of clay and loam, and 67-1/2 feet of chalk, and the
water then rose to the surface from a depth of 329 feet.[305] At the
Duke of Northumberland's, above Chiswick, the borings were carried to
the extraordinary depth of 620 feet, so as to enter the chalk, when a
considerable volume of water was obtained, which rose four feet above
the surface of the ground. In a well of Mr. Brooks, at Hammersmith, the
rush of water from a depth of 360 feet was so great, as to inundate
several buildings and do considerable damage; and at Tooting, a
sufficient stream was obtained to turn a wheel, and raise the water to
the upper stories of the houses.[306] In 1838, the total supply obtained
from the chalk near London was estimated at six million gallons a day,
and, in 1851, at nearly double that amount, the increase being
accompanied by an average fall of no less than two feet a year in the
level to which the water rose. The water stood commonly, in 1822, at
high-water mark, and had sunk in 1851 to 45, and in some wells to 65
feet below high-water mark.[307] This fact shows the limited capacity of
the subterranean reservoir. In the last of three wells bored through the
chalk at Tours, to the depth of several hundred feet, the water rose 32
feet above the level of the soil, and the discharge amounted to 300
cubic yards of water every twenty-four hours.[308]
By way of experiment, the sinking of a well was commenced at Paris in
1834, which had reached, in November, 1839, a depth of more than 1600
English feet, and yet no water ascended to the surface. The government
were persuaded by M. Arago to persevere, if necessary, to the depth of
more than 2000 feet; but when they had descended above 1800 English feet
below the surface, the water rose through the tube (which was about ten
inches in diameter), so as to discharge half a million of gallons of
limpid water every twenty-four hours. The temperature of the water
increased at the rate of 1.8 degrees F. for every 101 English feet, as
they went down, the result agreeing very closely with the anticipations
of the scientific advisers of this most spirited undertaking.
Mr. Briggs, the British consul in Egypt, obtained water between Cairo
and Suez, in a calcareous sand, at the depth of thirty feet; but it did
not rise in the well.[309] But other borings in the same desert, of
variable depth, between 50 and 300 feet, and which passed through
alternations of sand, clay, and siliceous rock, yielded water at the
surface.[310]
The rise and overflow of the water in Artesian wells is generally
referred, and apparently with reason, to the same principle as the play
of an artificial fountain. Let the porous stratum or set of strata, _a_
_a_, rest on the impermeable rock _d_, and be covered by another mass of
an impermeable nature. The whole mass _a_ _a_ may easily, in such a
position, become saturated with water, which may descend from its higher
and exposed parts--a hilly region to which clouds are attracted, and
where rain falls in abundance. Suppose that at some point, as at _b_, an
opening be made, which gives a free passage upwards to the waters
confined in _a_ _a_, at so low a level that they are subjected to the
pressure of a considerable column of water collected in the more
elevated portion of the same stratum. The water will then rush out, just
as the liquid from a large barrel which is tapped, and it will rise to a
height corresponding to the level of its point of departure, or, rather,
to a height which balances the pressure previously exerted by the
confined waters against the roof and sides of the stratum or reservoir
_a_ _a_. In like manner, if there happen to be a natural fissure _c_, a
spring will be produced at the surface on precisely the same principle.
[Illustration: Fig. 20.]
Among the causes of the failure of Artesian wells, we may mention those
numerous rents and faults which abound in some rocks, and the deep
ravines and valleys by which many countries are traversed; for, when
these natural lines of drainage exist, there remains a small quantity
only of water to escape by artificial issues. We are also liable to be
baffled by the great thickness either of porous or impervious strata, or
by the dip of the beds, which may carry off the waters from the
adjoining high lands to some trough in an opposite direction, as when
the borings are made at the foot of an escarpment where the strata
incline inwards, or in a direction opposite to the face of the cliffs.
The mere distance of hills or mountains need not discourage us from
making trials; for the waters which fall on these higher lands readily
penetrate to great depths through highly inclined or vertical strata, or
through the fissures of shattered rocks, and after flowing for a great
distance, must often reascend and be brought up again by other fissures,
so as to approach the surface in the lower country. Here they may be
concealed beneath the covering of undisturbed horizontal beds, which it
may be necessary to pierce in order to reach them. It should be
remembered, that the course of waters flowing under ground bears but a
remote resemblance to that of rivers on the surface, there being, in the
one case, a constant descent from a higher to a lower level from the
source of the stream to the sea; whereas, in the other, the water may at
one time sink far below the level of the ocean, and afterwards rise
again high above it.
Among other curious facts ascertained by aid of the borer, it is proved
that in strata of different ages and compositions, there are often open
passages by which the subterranean waters circulate. Thus, at St. Ouen,
in France, five distinct sheets of water were intersected in a well, and
from each of these a supply obtained. In the third waterbearing stratum,
at the depth of 150 feet, a cavity was found in which the borer fell
suddenly about a foot, and thence the water ascended in great
volume.[311] The same falling of the instrument, as in a hollow space,
has been remarked in England and other countries. At Tours, in 1830, a
well was perforated quite through the chalk, when the water suddenly
brought up, from a depth of 364 feet, a great quantity of fine sand,
with much vegetable matter and shells. Branches of a thorn several
inches long, much blackened by their stay in the water, were recognized,
as also the stems of marsh plants, and some of their roots, which were
still white, together with the seeds of the same in a state of
preservation, which showed that they had not remained more than three or
four months in the water. Among the seeds were those of the marsh plant
_Galium uliginosum_; and among the shells, a freshwater species
(_Planorbis marginatus_), and some land species, as _Helix rotundata_,
and _H. striata_. M. Dujardin, who, with others, observed this
phenomenon, supposes that the waters had flowed from some valleys of
Auvergne or the Vivarais since the preceding autumn.[312]
An analogous phenomenon is recorded at Reimke, near Bochum in
Westphalia, where the water of an Artesian well brought up, from a
depth of 156 feet, several small fish, three or four inches long, the
nearest streams in the country being at a distance of some leagues.[313]
In both cases it is evident that water had penetrated to great depths,
not simply by filtering through a porous mass, for then it would have
left behind the shells, fish, and fragments of plants, but by flowing
through some open channels in the earth. Such examples may suggest the
idea that the leaky beds of rivers are often the feeders of springs.
MINERAL AND THERMAL SPRINGS.
Almost all springs, even those which we consider the purest, are
impregnated with some foreign ingredients, which, being in a state of
chemical solution, are so intimately blended with the water as not to
affect its clearness, while they render it, in general, more agreeable
to our taste, and more nutritious than simple rain-water. But the
springs called mineral contain an unusual abundance of earthy matter in
solution, and the substances with which they are impregnated correspond
remarkably with those evolved in a gaseous form by volcanoes. Many of
these springs are thermal, _i. e._, their temperature is above the mean
temperature of the place, and they rise up through all kinds of rock;
as, for example, through granite, gneiss, limestone, or lava, but are
most frequent in volcanic regions, or where violent earthquakes have
occurred at eras comparatively modern.
The water given out by hot springs is generally more voluminous and less
variable in quantity at different seasons than that proceeding from any
others. In many volcanic regions, jets of steam, called by the Italians
"stufas," issue from fissures, at a temperature high above the boiling
point, as in the neighborhood of Naples, and in the Lipari Isles, and
are disengaged unceasingly for ages. Now, if such columns of steam,
which are often mixed with other gases, should be condensed before
reaching the surface by coming in contact with strata filled with cold
water, they may give rise to thermal and mineral springs of every degree
of temperature. It is, indeed, by this means only, and not by
hydrostatic pressure, that we can account for the rise of such bodies of
water from great depths; nor can we hesitate to admit the adequacy of
the cause, if we suppose the expansion of the same elastic fluids to be
sufficient to raise columns of lava to the lofty summits of volcanic
mountains. Several gases, the carbonic acid in particular, are
disengaged in a free state from the soil in many districts, especially
in the regions of active or extinct volcanoes; and the same are found
more or less intimately combined with the waters of all mineral springs,
both cold and thermal. Dr. Daubeny and other writers have remarked, not
only that these springs are most abundant in volcanic regions, but that
when remote from them, their site usually coincides with the position of
some great derangement in the strata; a fault, for example, or great
fissure, indicating that a channel of communication has been opened with
the interior of the earth at some former period of local convulsion. It
is also ascertained that at great heights in the Pyrenees and Himalaya
mountains hot springs burst out from granitic rocks, and they are
abundant in the Alps also, these chains having all been disturbed and
dislocated at times comparatively modern, as can be shown by independent
geological evidence.
The small area of volcanic regions may appear, at first view, to present
an objection to these views, but not so when we include earthquakes
among the effects of igneous agency. A large proportion of the land
hitherto explored by geologists can be shown to have been rent or shaken
by subterranean movements since the oldest tertiary strata were formed.
It will also be seen, in the sequel, that new springs have burst out,
and others have had the volume of their waters augmented, and their
temperature suddenly raised after earthquakes, so that the description
of these springs might almost with equal propriety have been given under
the head of "igneous causes," as they are agents of a mixed nature,
being at once igneous and aqueous.
But how, it will be asked, can the regions of volcanic heat send forth
such inexhaustible supplies of water? The difficulty of solving this
problem would, in truth, be insurmountable, if we believed that all the
atmospheric waters found their way into the basin of the ocean; but in
boring near the shore we often meet with streams of fresh water at the
depth of several hundred feet below the sea level; and these probably
descend, in many cases, far beneath the bottom of the sea, when not
artificially intercepted in their course. Yet, how much greater may be
the quantity of salt water which sinks beneath the floor of the ocean,
through the porous strata of which it is often composed, or through
fissures rent in it by earthquakes. After penetrating to a considerable
depth, this water may encounter a heat of sufficient intensity to
convert it into vapor, even under the high pressure to which it would
then be subjected. This heat would probably be nearest the surface in
volcanic countries, and farthest from it in those districts which have
been longest free from eruptions or earthquakes.
It would follow from the views above explained, that there must be a
twofold circulation of terrestrial waters; one caused by solar heat, and
the other by heat generated in the interior of our planet. We know that
the land would be unfit for vegetation, if deprived of the waters raised
into the atmosphere by the sun; but it is also true that mineral springs
are powerful instruments in rendering the surface subservient to the
support of animal and vegetable life. Their heat is said to promote the
development of the aquatic tribes in many parts of the ocean, and the
substances which they carry up from the bowels of the earth to the
habitable surface, are of a nature and in a form which adapts them
peculiarly for the nutrition of animals and plants.
As these springs derive their chief importance to the geologist from
the quantity and quality of the earthy materials which, like volcanoes,
they convey from below upwards, they may properly be considered in
reference to the ingredients which they hold in solution. These consist
of a great variety of substances; but chiefly salts with bases of lime,
magnesia, alumine, and iron, combined with carbonic, sulphuric, and
muriatic acids. Muriate of soda, silica, and free carbonic acid are
frequently present; also springs of petroleum, or liquid bitumen, and of
naphtha.
_Calcareous springs._--Our first attention is naturally directed to
springs which are highly charged with calcareous matter, for these
produce a variety of phenomena of much interest in geology. It is known
that rain-water collecting carbonic acid from the atmosphere has the
property of dissolving the calcareous rocks over which it flows, and
thus, in the smallest ponds and rivulets, matter is often supplied for
the earthy secretions of testacea, and for the growth of certain plants
on which they feed. But many springs hold so much carbonic acid in
solution, that they are enabled to dissolve a much larger quantity of
calcareous matter than rain-water; and when the acid is dissipated in
the atmosphere, the mineral ingredients are thrown down, in the form of
porous tufa or of more compact travertin.[314]
_Auvergne._--Calcareous springs, although most abundant in limestone
districts, are by no means confined to them, but flow out
indiscriminately from all rock formations. In central France, a district
where the primary rocks are unusually destitute of limestone, springs
copiously charged with carbonate of lime rise up through the granite and
gneiss. Some of these are thermal, and probably derive their origin from
the deep source of volcanic heat, once so active in that region. One of
these springs, at the northern base of the hill upon which Claremont is
built, issues from volcanic peperino, which rests on granite. It has
formed, by its incrustations, an elevated mound of travertin, or white
concretionary limestone, 240 feet in length, and, at its termination,
sixteen feet high and twelve wide. Another encrusting spring in the same
department, situated at Chaluzet, near Pont Gibaud, rises in a gneiss
country, at the foot of a regular volcanic cone, at least twenty miles
from any calcareous rock. Some masses of tufaceous deposit, produced by
this spring, have an oolitic texture.
_Valley of the Elsa._--If we pass from the volcanic district of France
to that which skirts the Apennines in the Italian peninsula, we meet
with innumerable springs which have precipitated so much calcareous
matter, that the whole ground in some parts of Tuscany is coated over
with tufa and travertin, and sounds hollow beneath the foot.
In other places in the same country, compact rocks are seen descending
the slanting sides of hills, very much in the manner of lava currents,
except that they are of a white color and terminate abruptly when they
reach the course of a river. These consist of a calcareous precipitate
from springs, some of which are still flowing, while others have
disappeared or changed their position. Such masses are frequent on the
slope of the hills which bound the valley of the Elsa, one of the
tributaries of the Arno, which flows near Colle, through a valley
several hundred feet deep, shaped out of a lacustrine formation,
containing fossil shells of existing species. I observed here that the
travertin was unconformable to the lacustrine beds, its inclination
according with the slope of the sides of the valley. One of the finest
examples which I saw was at the Molino delle Caldane, near Colle. The
Sena, and several other small rivulets which feed the Elsa, have the
property of encrusting wood and herbs with calcareous stone. In the bed
of the Elsa itself, aquatic plants, such as Charae, which absorb large
quantities of carbonate of lime, are very abundant.
[Illustration: Fig. 21.
Section of travertin, San Vignone.]
_Baths of San Vignone._--Those persons who have merely seen the action
of petrifying waters in England, will not easily form an adequate
conception of the scale on which the same process is exhibited in those
regions which lie nearer to the active centres of volcanic disturbance.
One of the most striking examples of the rapid precipitation of
carbonate of lime from thermal waters, occurs in the hill of San Vignone
in Tuscany, at a short distance from Radicofani, and only a few hundred
yards from the high road between Sienna and Rome. The spring issues from
near the summit of a rocky hill, about 100 feet in height. The top of
the hill stretches in a gently inclined platform to the foot of Mount
Amiata, a lofty eminence, which consists in great part of volcanic
products. The fundamental rock, from which the spring issues, is a black
slate, with serpentine (_b_ _b_, fig. 21), belonging to the older
Apennine formation. The water is hot, has a strong taste, and, when not
in very small quantity, is of a bright green color. So rapid is the
deposition near the source, that in the bottom of a conduit-pipe for
carrying off the water to the baths, and which is inclined at an angle
of 30 degrees, half a foot of solid travertin is formed every year. A more
compact rock is produced where the water flows slowly; and the
precipitation in winter, when there is least evaporation, is said to be
more solid, but less in quantity by one-fourth, than in summer. The rock
is generally white; some parts of it are compact, and ring to the
hammer; others are cellular, and with such cavities as are seen in the
carious part of bone or the siliceous millstone of the Paris basin. A
portion of it also below the village of San Vignone consists of
incrustations of long vegetable tubes, and may be called tufa. Sometimes
the travertin assumes precisely the botryoidal and mammillary forms,
common to similar deposits in Auvergne, of a much older date; and, like
them, it often scales off in thin, slightly undulating layers.
A large mass of travertin (_c_, fig. 21) descends the hill from the
point where the spring issues, and reaches to the distance of about half
a mile east of San Vignone. The beds take the slope of the hill at about
an angle of 6 degrees, and the planes of stratification are perfectly
parallel. One stratum, composed of many layers, is of a compact nature,
and fifteen feet thick; it serves as an excellent building stone, and a
mass of fifteen feet in length was, in 1828, cut out for the new bridge
over the Orcia. Another branch of it (_a_, fig. 21) descends to the
west, for 250 feet in length, of varying thickness, but sometimes 200
feet deep; it is then cut off by the small river Orcia, as some glaciers
in Switzerland descend into a valley till their progress is suddenly
arrested by a transverse stream of water.
The abrupt termination of the mass of rock at the river, where its
thickness is undiminished, clearly shows that it would proceed much
farther if not arrested by the stream, over which it impends slightly.
But it cannot encroach upon the channel of the Orcia, being constantly
undermined, so that its solid fragments are seen strewed amongst the
alluvial gravel. However enormous, therefore, the mass of solid rock may
appear which has been given out by this single spring, we may feel
assured that it is insignificant in volume when compared to that which
has been carried to the sea since the time when it began to flow. What
may have been the length of that period of time we have no data for
conjecturing. In quarrying the travertin, Roman tiles have been
sometimes found at the depth of five or six feet.
_Baths of San Filippo._--On another hill, not many miles from that last
mentioned, and also connected with Mount Amiata, the summit of which is
about three miles distant, are the celebrated baths of San Filippo. The
subjacent rocks consist of alternations of black slate, limestone, and
serpentine. There are three warm springs containing carbonate and
sulphate of lime, and sulphate of magnesia. The water which supplies the
baths falls into a pond, where it has been known to deposit a solid mass
_thirty feet thick_ in about _twenty years_.[315] A manufactory of
medallions in basso-relievo is carried on at these baths. The water is
conducted by canals into several pits, in which it deposits travertin
and crystals of sulphate of lime. After being thus freed from its
grosser parts, it is conveyed by a tube to the summit of a small
chamber, and made to fall through a space of ten or twelve feet. The
current is broken in its descent by numerous crossed sticks, by which
the spray is dispersed around upon certain moulds, which are rubbed
lightly over with a solution of soap, and a deposition of solid matter
like marble is the result, yielding a beautiful cast of the figures
formed in the mould. The geologist may derive from these experiments
considerable light, in regard to the high slope of the strata at which
some semi-crystalline precipitations can be formed; for some of the
moulds are disposed almost perpendicularly, yet the deposition is nearly
equal in all parts.
A hard stratum of stone, about a foot in thickness, is obtained from the
waters of San Filippo in four months; and, as the springs are powerful,
and almost uniform in the quantity given out, we are at no loss to
comprehend the magnitude of the mass which descends the hill, which is a
mile and a quarter in length and the third of a mile in breadth, in some
places attaining a thickness of 250 feet at least. To what length it
might have reached it is impossible to conjecture, as it is cut off,
like the travertin of San Vignone, by a small stream, where it
terminates abruptly. The remainder of the matter held in solution is
carried on probably to the sea.
_Spheroidal structure in travertin._--But what renders this recent
limestone of peculiar interest to the geologist, is the spheroidal form
which it assumes, analogous to that of the cascade of Tivoli, afterwards
to be described. (See fig. 22, p. 244.) The lamination of some of the
concentric masses is so minute that sixty may be counted in the
thickness of an inch, yet, notwithstanding these marks of gradual and
successive deposition, sections are sometimes exhibited of what might
seem to be perfect spheres. This tendency to a mammillary and globular
structure arises from the facility with which the calcareous matter is
precipitated in nearly equal quantities on all sides of any fragment of
shell or wood or any inequality of the surface over which the mineral
water flows, the form of the nucleus being readily transmitted through
any number of successive envelopes. But these masses can never be
perfect spheres, although they often appear such when a transverse
section is made in any line not in the direction of the point of
attachment. There are, indeed, occasionally seen small oolitic and
pisolitic grains, of which the form is globular; for the nucleus, having
been for a time in motion in the water, has received fresh accessions of
matter on all sides.
In the same manner I have seen, on the vertical walls of large
steam-boilers, the heads of nails or rivets covered by a series of
enveloping crusts of calcareous matter, usually sulphate of lime; so
that a concretionary nodule is formed, preserving a nearly globular
shape, when increased to a mass several inches in diameter. In these, as
in many travertins, there is often a combination of the concentric and
radiated structure.
_Campagna di Roma._--The country around Rome, like many parts of the
Tuscan States already referred to, has been at some former period the
site of numerous volcanic eruptions; and the springs are still copiously
impregnated with lime, carbonic acid, and sulphuretted hydrogen. A hot
spring was discovered about 1827, near Civita Vecchia, by Signor
Riccioli, which deposits alternate beds of a yellowish travertin, and a
white granular rock, not distinguishable, in hand specimens, either in
grain, color, or composition, from statuary marble. There is a passage
between this and ordinary travertin. The mass accumulated near the
spring is in some places about six feet thick.
_Lake of the Solfatara._--In the Campagna, between Rome and Tivoli, is
the Lake of the Solfatara, called also Lago di Zolfo (lacus albula),
into which flows continually a stream of tepid water from a smaller
lake, situated a few yards above it. The water is a saturated solution
of carbonic acid gas, which escapes from it in such quantities in some
parts of its surface, that it has the appearance of being actually in
ebullition. "I have found by experiment," says Sir Humphry Davy, "that
the water taken from the most tranquil part of the lake, even after
being agitated and exposed to the air, contained in solution more than
its own volume of carbonic acid gas, with a very small quantity of
sulphuretted hydrogen. Its high temperature, which is pretty constant at
80 degrees of Fahr., and the quantity of carbonic acid that it contains,
render it peculiarly fitted to afford nourishment to vegetable life. The
banks of travertin are everywhere covered with reeds, lichen, confervae,
and various kinds of aquatic vegetables; and at the same time that the
process of vegetable life is going on, the crystallizations of the
calcareous matter, which is everywhere deposited, in consequence of the
escape of carbonic acid, likewise proceed. There is, I believe, no place
in the world where there is a more striking example of the opposition or
contrast of the laws of animate and inanimate nature, of the forces of
inorganic chemical affinity, and those of the powers of life."[316]
The same observer informs us that he fixed a stick in a mass of
travertin covered by the water in the month of May, and in April
following he had some difficulty in breaking, with a sharp-pointed
hammer, the mass which adhered to the stick, and which was several
inches in thickness. The upper part was a mixture of light tufa and the
leaves of confervae; below this was a darker and more solid travertin,
containing black and decomposed masses of confervae; in the inferior part
the travertin was more solid, and of a gray color, but with cavities
probably produced by the decomposition of vegetable matter.[317]
The stream which flows out of this lake fills a canal about nine feet
broad and four deep, and is conspicuous in the landscape by a line of
vapor which rises from it. It deposits calcareous tufa in this channel,
and the Tiber probably receives from it, as well as from numerous other
streams, much carbonate of lime in solution, which may contribute to the
rapid growth of its delta. A large proportion of the most splendid
edifices of ancient and modern Rome are built of travertin, derived from
the quarries of Ponte Lucano, where there has evidently been a lake at a
remote period, on the same plain as that already described.
[Illustration: Fig. 22.
Section of spheroidal concretionary Travertin under the Cascade of
Tivoli.]
_Travertin of Tivoli._--In the same neighborhood the calcareous waters
of the Anio incrust the reeds which grow on its banks, and the foam of
the cataract of Tivoli forms beautiful pendant stalactites. On the sides
of the deep chasm into which the cascade throws itself, there is seen an
extraordinary accumulation of horizontal beds of tufa and travertin,
from four to five hundred feet in thickness. The section immediately
under the temples of Vesta and the Sibyl, displays, in a precipice about
four hundred feet high, some spheroids which are from _six to eight feet
in diameter_, each concentric layer being about the eighth of an inch in
thickness. The preceding diagram exhibits about fourteen feet of this
immense mass, as seen in the path cut out of the rock in descending from
the temple of Vesta to the Grotto di Nettuno. I have not attempted to
express in this drawing the innumerable thin layers of which these
magnificent spheroids are composed, but the lines given mark some of the
natural divisions into which they are separated by minute variations in
the size or color of the laminae. The undulations also are much smaller
in proportion to the whole circumference than in the drawing. The beds
(_a_ _a_) are of hard travertin and soft tufa; below them is a pisolite
(_b_), the globules being of different sizes: underneath this appears a
mass of concretionary travertin (_c_ _c_), some of the spheroids being
of the above-mentioned extraordinary size. In some places (as at _d_)
there is a mass of amorphous limestone, or tufa, surrounded by
concentric layers. At the bottom is another bed of pisolite (_b_), in
which the small nodules are about the size and shape of beans, and some
of them of filberts, intermixed with some smaller oolitic grains. In the
tufaceous strata, wood is seen converted into a light tufa.
There can be little doubt that the whole of this deposit was formed in
an extensive lake which existed when the external configuration of this
country varied greatly from that now observed. The Anio throws itself
into a ravine excavated in the ancient travertin, and its waters give
rise to masses of calcareous stone, scarcely if at all distinguishable
from the older rock. I was shown, in 1828, in the upper part of the
travertin, the hollow left by a cart-wheel, in which the outer circle
and the spokes had been decomposed, and the spaces which they filled
left void. It seemed to me at the time impossible to explain the
position of this mould without supposing that the wheel was imbedded
before the lake was drained; but Sir R. Murchison suggests that it may
have been washed down by a flood into the gorge in modern times, and
then incrusted with calcareous tufa in the same manner as the wooden
beam of the church of St. Lucia was swept down in 1826, and stuck fast
in the Grotto of the Syren, where it still remains, and will eventually
be quite imbedded in travertin.
I have already endeavored to explain (p. 241), when speaking of the
travertin of San Filippo, how the spheroidal masses represented in
figure 22 may have been formed.
_Sulphureous and gypseus springs._--The quantity of other mineral
ingredients wherewith springs in general are impregnated, is
insignificant in comparison to lime, and this earth is most frequently
combined with carbonic acid. But as sulphuric acid, and sulphuretted
hydrogen are very frequently supplied by springs, gypsum may, perhaps,
be deposited largely in certain seas and lakes. Among other gypseous
precipitates at present known on the land, I may mention those of Baden,
near Vienna, which feed the public bath. Some of these supply singly
from 600 to 1000 cubic feet of water per hour, and deposit a fine
powder, composed of a mixture of sulphate of lime with sulphur and
muriate of lime.[318] The thermal waters of Aix, in Savoy, in passing
through strata of Jurassic limestone, turn them into gypsum or sulphate
of lime. In the Andes, at the Puenta del Inca, Lieutenant Brand found a
thermal spring at the temperature of 91 degrees Fahr., containing a large
proportion of gypsum with carbonate of lime and other ingredients.[319]
Many of the mineral springs of Iceland, says Mr. R. Bunsen, deposit
gypsum.[320] and sulphureous acid gas escapes plentifully from them as
from the volcanoes of the same island. It may, indeed, be laid down as a
general rule, that the mineral substances dissolved in hot springs agree
very closely with those which are disengaged in a gaseous form from the
craters of active volcanoes.
_Siliceous springs.--Azores._--In order that water should hold a very
large quantity of silica in solution, it seems necessary that it should
be raised to a high temperature.[321] The hot springs of the Valle das
Fernas, in the island of St. Michael, rising through volcanic rocks,
precipitate vast quantities of siliceous sinter. Around the circular
basin of the largest spring, which is between twenty and thirty feet in
diameter, alternate layers are seen of a coarser variety of sinter mixed
with clay, including grass, ferns, and reeds, in different states of
petrifaction. In some instances, alumina, which is likewise deposited
from the hot waters, is the mineralizing material. Branches of the same
ferns which now flourish in the island are found completely petrified,
preserving the same appearance as when vegetating, except that they
acquire an ash-gray color. Fragments of wood, and one entire bed from
three to five feet in depth, composed of reeds now common in the island,
have become completely mineralized.
The most abundant variety of siliceous sinter occurs in layers, from a
quarter to half an inch in thickness, accumulated on each other often to
the height of a foot and upwards, and constituting parallel, and for the
most part horizontal, strata many yards in extent. This sinter has often
a beautiful semi-opalescent lustre. A recent breccia is also in the act
of forming, composed of obsidian, pumice, and scoriae, cemented by
siliceous sinter.[322]
_Geysers of Iceland._--But the hot springs in various parts of Iceland,
particularly the celebrated geysers, afford the most remarkable example
of the deposition of silex.[323] The circular reservoirs into which the
geysers fall, are lined in the interior with a variety of opal, and
round the edges with sinter. The plants incrusted with the latter
substance have much the same appearance as those incrusted with
calcareous tufa in our own country. They consist of various grasses, the
horse-tail (_Equisetum_), and leaves of the birch-tree, which are the
most common of all, though no trees of this species now exist in the
surrounding country. The petrified stems also of the birch occur in a
state much resembling agatized wood.[324]
By analysis of the water, Mr. Faraday has ascertained that the solution
of the silex is promoted by the presence of the alkali, soda. He
suggests that the deposition of silica in an insoluble state takes place
partly because the water when cooled by exposure to the air is unable
to retain as much silica as when it issues from the earth at a
temperature of 180 degrees or 190 degrees Fahr.; and partly because the
evaporation of the water decomposes the compound of silica and soda
which previously existed. This last change is probably hastened by the
carbonic acid of the atmosphere uniting with the soda. The alkali, when
disunited from the silica, would readily be dissolved in and removed by
running water.[325]
Mineral waters, even when charged with a small proportion of silica, as
those of Ischia, may supply certain species of corals, sponges, and
infusoria, with matter for their siliceous secretions; but there is
little doubt that rivers obtain silex in solution from another and far
more general source, namely, the decomposition of felspar. When this
mineral, which is so abundant an ingredient in the hypogene and trappean
rocks, has disintegrated, it is found that the residue, called porcelain
clay, contains a small proportion only of the silica which existed in
the original felspar, the other part having been dissolved and removed
by water.[326]
_Ferruginous springs._--The waters of almost all springs contain some
iron in solution; and it is a fact familiar to all, that many of them
are so copiously impregnated with this metal, as to stain the rocks or
herbage through which they pass, and to bind together sand and gravel
into solid masses. We may naturally, then, conclude that this iron,
which is constantly conveyed from the interior of the earth into lakes
and seas, and which does not escape again from them into the atmosphere
by evaporation, must act as a coloring and cementing principle in the
subaqueous deposits now in progress. Geologists are aware that many
ancient sandstones and conglomerates are bound together or colored by
iron.
_Brine springs._--So great is the quantity of muriate of soda in some
springs, that they yield one-fourth of their weight in salt. They are
rarely, however, so saturated, and generally contain, intermixed with
salt, carbonate and sulphate of lime, magnesia, and other mineral
ingredients. The brine springs of Cheshire are the richest in our
country; those of Northwich being almost saturated. Those of Barton
also, in Lancashire, and Droitwich in Worcestershire, are extremely
rich.[327] They are known to have flowed for more than 1000 years, and
the quantity of salt which they have carried into the Severn and Mersey
must be enormous. These brine springs rise up through strata of
sandstone and red marl, which contain large beds of rock salt. The
origin of the brine, therefore, may be derived in this and many other
instances from beds of fossil salt; but as muriate of soda is one of the
products of volcanic emanations and of springs in volcanic regions, the
original source of salt may be as deep seated as that of lava.
Many springs in Sicily contain muriate of soda, and the "fiume salso,"
in particular, is impregnated with so large a quantity, that cattle
refuse to drink of it. A hot spring, rising through granite, at Saint
Nectaire, in Auvergne, may be mentioned as one of many, containing a
large proportion of muriate of soda, together with magnesia and other
ingredients.[328]
_Carbonated springs.--Auvergne._--Carbonic acid gas is very plentifully
disengaged from springs in almost all countries, but particularly near
active or extinct volcanoes. This elastic fluid has the property of
decomposing many of the hardest rocks with which it comes in contact,
particularly that numerous class in whose composition felspar is an
ingredient. It renders the oxide of iron soluble in water, and
contributes, as was before stated, to the solution of calcareous matter.
In volcanic districts these gaseous emanations are not confined to
springs, but rise up in the state of pure gas from the soil in various
places. The Grotto del Cane, near Naples, affords an example, and
prodigious quantities are now annually disengaged from every part of the
Limagne d'Auvergne, where it appears to have been developed in equal
quantity from time immemorial. As the acid is invisible, it is not
observed, except an excavation be made, wherein it immediately
accumulates, so that it will extinguish a candle. There are some springs
in this district, where the water is seen bubbling and boiling up with
much noise, in consequence of the abundant disengagement of this gas. In
the environs of Pont-Gibaud, not far from Clermont, a rock belonging to
the gneiss formation, in which lead-mines are worked, has been found to
be quite saturated with carbonic acid gas, which is constantly
disengaged. The carbonates of iron, lime, and manganese are so
dissolved, that the rock is rendered soft, and the quartz alone remains
unattacked.[329] Not far off is the small volcanic cone of Chaluzet,
which once broke up through the gneiss, and sent forth a lava stream.
_Supposed atmosphere of carbonic acid._--Prof. Bischoff in his history
of volcanoes,[330] has shown what enormous quantities of carbonic acid
gas are exhaled in the vicinity of the extinct craters of the Rhine (in
the neighborhood of the Laacher-see, for example, and the Eifel), and
also in the mineral springs of Nassau and other countries, where there
are no immediate traces of volcanic action. It would be easy to
calculate in how short a period the solid carbon, thus emitted from the
interior of the earth in an invisible form, would amount to a quantity
as great as could be obtained from the trees of a large forest, and how
many thousand years would be required to supply the materials of a dense
seam of pure coal from the same source. Geologists who favor the
doctrine of the former existence of an atmosphere highly charged with
carbonic acid, at the period of the ancient coal-plants, have not
sufficiently reflected on the continual disengagement of carbon, which
is taking place in a gaseous form from springs, as also in a free state
from the ground and from volcanic craters into the air. We know that
all plants are now engaged in secreting carbon, and many thousands of
large trees are annually floated down by great rivers, and buried in
their alluvial deposits; but before we can assume that the quantity of
carbon which becomes permanently locked up in the earth by such agency
will bring about an essential change in the chemical composition of the
atmosphere, we must be sure that the trees annually buried contain more
carbon than is given out from the interior of the earth in the same
lapse of time. Every large area covered by a dense mass of peat, bears
ample testimony to the fact, that several million tons of carbon have
been taken from the air, by the powers of vegetable life, and stored up
in the earth's crust, a large quantity of oxygen having been at the same
time set free; but we cannot infer from these circumstances, that the
constitution of the atmosphere has been materially deranged, until we
have data for estimating the rate at which dead animal and vegetable
substances are daily putrefying,--organic remains and various calcareous
rocks decomposing, and volcanic regions emitting fresh volumes of
carbonic acid gas. That the ancient carboniferous period was one of vast
duration all geologists are agreed; instead, therefore, of supposing an
excess of carbonic acid in the air at that epoch, for the support of a
peculiar flora, we may imagine Time to have multiplied the quantity of
carbon given out annually by mineral springs, volcanic craters, and
other sources, until the component elements of any given number of
coal-seams had been evolved from below, without any variation taking
place in the constitution of the atmosphere. It has been too common, in
reasoning on this question, to compute the loss of carbon by the volume
of coal stored up in the ancient strata, and to take no account of the
annual gain, by the restoration of carbonic acid to the atmosphere,
through the machinery above alluded to.[331]
_Disintegrating effects of carbonic acid._--The disintegration of
granite is a striking feature of large districts in Auvergne, especially
in the neighborhood of Clermont. This decay was called by Dolomieu, "la
maladie du granite;" and the rock may with propriety be said to have
_the rot_, for it crumbles to pieces in the hand. The phenomenon may,
without doubt, be ascribed to the continual disengagement of carbonic
acid gas from numerous fissures.
In the plains of the Po, between Verona and Parma, especially at Villa
Franca, south of Mantua, I observed great beds of alluvium, consisting
chiefly of primary pebbles, percolated by spring-water, charged with
carbonate of lime and carbonic acid in great abundance. They are for the
most part incrusted with calc-sinter; and the rounded blocks of gneiss,
which have all the outward appearance of solidity, have been so
disintegrated by the carbonic acid as readily to fall to pieces.
The subtraction of many of the elements of rocks by the solvent power of
carbonic acid, ascending both in a gaseous state and mixed with
spring-water in the crevices of rocks, must be one of the most powerful
sources of those internal changes and rearrangements of particles so
often observed in strata of every age. The calcareous matter, for
example, of shells, is often entirely removed and replaced by carbonate
of iron, pyrites, silex, or some other ingredient, such as mineral
waters usually contain in solution. It rarely happens, except in
limestone rocks, that the carbonic acid can dissolve all the constituent
parts of the mass; and for this reason, probably, calcareous rocks are
almost the only ones in which great caverns and long winding passages
are found.
_Petroleum springs._--Springs of which the waters contain a mixture of
petroleum and the various minerals allied to it, as bitumen, naphtha,
asphaltum, and pitch, are very numerous, and are, in many cases,
undoubtedly connected with subterranean fires, which raise or sublime
the more subtle parts of the bituminous matters contained in rocks. Many
springs in the territory of Modena and Parma, in Italy, produce
petroleum in abundance; but the most powerful, perhaps, yet known, are
those on the Irawadi, in the Burman empire. In one locality there are
said to be 520 wells, which yield annually 400,000 hogsheads of
petroleum.[332]
_Pitch lake of Trinidad._--Fluid bitumen is seen to ooze from the bottom
of the sea, on both sides of the island of Trinidad, and to rise up to
the surface of the water. Near Cape La Braye there is a vortex which, in
stormy weather, according to Captain Mallet, gushes out, raising the
water five or six feet, and covers the surface for a considerable space
with petroleum, or tar; and the same author quotes Gumilla, as stating,
in his "Description of the Orinoco," that about seventy years ago, a
spot of land on the western coast of Trinidad, near half-way between the
capital and an Indian village, sank suddenly, and was immediately
replaced by a small lake of pitch, to the great terror of the
inhabitants.[333]
It is probable that the great pitch lake of Trinidad owes its origin to
a similar cause; and Dr. Nugent has justly remarked, that in that
district all the circumstances are now combined from which deposits of
pitch may have originated. The Orinoco has for ages been rolling down
great quantities of woody and vegetable bodies into the surrounding sea,
where, by the influence of currents and eddies, they may be arrested and
accumulated in particular places. The frequent occurrence of earthquakes
and other indications of volcanic action in those parts lend countenance
to the opinion, that these vegetable substances may have undergone, by
the agency of subterranean fire, those transformations and chemical
changes which produce petroleum; and this may, by the same causes, be
forced up to the surface, where, by exposure to the air, it becomes
inspissated, and forms the different varieties of pure and earthy pitch,
or asphaltum, so abundant in the island.[334]
It may be stated generally, that a large portion of the finer particles
and the more crystalline substances, found in sedimentary rocks of
different ages, are composed of the same elements as are now held in
solution by springs, while the coarser materials bear an equally strong
resemblance to the pebbles and sedimentary matter carried down by
torrents and rivers. It should also be remembered, that it is not only
during inundations, when the muddy sediment is apparent, that rivers are
busy in conveying solid matter to the sea, but that even when their
waters are perfectly transparent, they are annually bearing along vast
masses of carbon, lime, and silica to the ocean.