whole of the river-flat above the level of its bottom may not have
been deposited by the river in its existing state ; for the channel
and flood-plain may be excavated in the alluvium of an earlier
period, so that the upper surface alone may be of recent origin
(p. 550). If, however, the land were undergoing a very slow subsi-
dence, which should diminish the pitch of the stream, a deposition
of detritus would take place which would raise both its bed and
flood-plain, and the thickness might thus go on increasing as long
as the subsidence continued.
The deposition of detritus which takes place along the course of a river
usually raises the borders of the channel above the general level of the flood-
plain. Along the Lower Mississippi, the pitch of the plain away from the
river amounts, on an average, to seven feet for the first mile. (Humphreys
The earthy alluvium which is formed by a slow deposition of detritus con-
sists of very thin even layers. A vibration or wave-movement in any waters
in which a sediment is falling tends to arrange that sediment in layers, each
layer corresponding to a wave, and showing by a difference of te^xture in its
under and upper portions the progress of the wave. In the ca?e of accumula-
tions from a rapid deposition or pressing forward of material, the lamination is
The pebbles or stones forming beds in the alluvium are brought in by the
upper waters and lateral tributaries during floods. The course of a tributary
across the river-plain is often marked by a wide bed of stones. The sweep of a
freshet over the earthy flood-phiin may carry away the finer earth and leave a
surface of pebbles. The bank of a river struck by a strong current may in a
similar way be made pebbly, while the opposite is muddy or has a sand-bank
forming from the earth carried across.
Still other irregularities result from changes in the river-channel. The trans-
fer of material from one side of a stream to the other ends often in making a
long bend, and finally in cutting off" the bend and turning it into an island, and
ultimately into a part of the mainland by the filling up of the old channel.
The islands in the large rivers are also very unstable. In the Mississippi, as
Humphreys & Abbot observe, they often begin in the lodging of drift-wood on
a sand-bar; this causes the accumulation of detritus; a growth of willow suc-
ceeds ; the height of the alluvium still increases, until finnlly the island reaches
the level of high Avater, or rises even above it, and becomes covered with a
growth of cotton-wood, willow, etc. By a similar process, the island may be
united to the mainland ; or, " by a slight change of direction of the current, the
underlying sand-bar is washed away, the new-made land caves into the rivei',
and the island disappears."
2. Delta formations. ā The larger part of the detritus of a river is car-
ried to the ocean (or lake) into wdiich it empties, and it goes to form
about the mouth of the stream more or less extensive flats. Such
flats, when large and intersected by a network of water-channels,
are called deltas ; they reach a large size only where tlie tides are
quite small or are altogether wanting. They are formed from the
conjoined action of the river and the ocean, and are sometimes
called Jluvio-mariyie formations. Great streams, like the Amazon,
carry their muddy waters hundreds of miles into the ocean ; but-
far the greater part of the detritus, even in the case of the largest
rivers, is beaten back by the waves on soundings and by the shore-
currents, and either falls over the bottom or is thrown upon the
coast near by. In floods, the river-water of the Mississippi is dis-
tinguishable in the Gulf at the distance of only twenty or twenty-
five miles from the bar ; in low water, at the distance of five or ten
miles. (Humphreys & Abbot.)
The eastern North American coast, from Texas to Florida, and
from Florida to New Jersey, is nearly a continuous range of fluvio-
Only a single example ā that of the Mississippi delta ā need here be referred to.
The preceding map (fig. 942) presents its general features. It commences
below the mouth of Red River, where the Atchafalaya '' bayou" begins, ā the
first of the many side-channels that open through the great flats to the Gulf.
The whole area is about 12,300 square miles, and about one-third is a sea-marsh,
only two-thirds lying above the level of the Gulf.
On page G43 the amount of detritus is mentioned which the river annually
furnishes towards the extension of the delta.
According to Humphreys & Abbot, the outer crest of the bar of the Southwest
Pass (the principal one) of the Mississippi advances into the Gulf o38 feet, over
a width of 11,500 feet, annually ; and the erosive power is only ab(mt one-tenth of
its depositing power. The depth of the Gulf where the bar is now formed being
100 feet, the profile and other dimensions of the river, in connection with the
above-mentioned rate of deposit, give for the difTerenee between the cubical con-
tents of yearly deposit and erosion 255,000,000 cubic feet, or a mass one mile
square and nine feet thick : this, therefore, is the volume of earthy matter pushed
into the Gulf each year at the Southwest Pass. The quantities of earthy matter
pushed along by the several passes being in proporti^ā¢n to their volumes of dis-
charge, the whole amount thus carried yearly to the Gulf is 750,000,000 cubic feet,
or a mass cnc mile square and twenty-seven feet thick. As the cubical contents
of the whole mass of the bar of the Southwest Pass are equal to a solid one mile
square and 490 feet thick, it would require fifty-five years to form the bar as it
now exists, or, in other words, to establish the equilibrium between the advancing
rates of erosion and deposit.
The deltas of the Nile, Ganges, Amazon, and other large streams are equally
interesting subjects of study. But it is not necessary to enter into details re-
specting them in this place, as the}"^ illustrate no new principles.
As the forms and stratification of delta deposits depend partly upon wave-
action, this subject comes up again under the head of The Ocean.
B. SUBTERRANEAN WATERS.
It is an obvious fact that a considerable part of the water which
reaches the earth's surface descends into the soil and becomes in a
senee subterranean. But there are also subterranean streams,
which have their rise in hills and mountains, and are fed, like the
surface-rivers, by the rains and snows, and especially those that
fall about elevated regions. The.-e waters become under-ground
streams by following the dip of tilted strata. The layers of sand-
stones and limestones never fit together so closely but that waters
may find their way between them. The subterranean sti'eams
usually flow over limestone or argillaceous strata, and not on
All wells and springs are tappings of these subterranean waters.
The large size of some of these under-ground rivers is proved by
direct observation in caverns, where they have the variety of cas-
cades and quiet waters which characterizes the streams of the sur-
face. The Mammoth Cave of Kentucky, and the Adelsberg, twenty-
two miles northeast of Trieste, are examples. And, again, some-
times, as in the Jura Mountains of Switzerland, they come out of
the hills with sufficient force and volume to turn the wheel of a
The outward flow of the under-ground waters of a continent pre-
vents the in-flow of the salt water on sea-shores. Springs are com-
mon on shores; occasionally their waters rise in large volume in a
harbor, or out at sea some miles distant from a coast.
If subterranean streams have their rise in elevated regions, their
inferior portions beneath the plains of a country must be under
great hydrostatic pressure ; and this should appear, whenever a
boring is made to the waters, by their rising above the surface in a
jet. Borings of this kind have been made in many parts of Europe
and America with this effect. They were first attempted in France,
and are called Artesian wells, from the district of Artois, in France,
where they were early used.
In fig. 943, let ab represent
an argillaceous stratum on
which the water descends,
and b c the boring ; bed is the
jet of water. The rise of tlie
jet falls far short of the height
of the source, because of the
great amount of friction along
the irregular rocky bed of the
stream, and also the resistance
of the air.
It is possible that in some cases subterranean waters may be
under pressure from a stratum of gas over them, which is sufficient
to send them to the surface without other aid.
Sectiou illustrating the origin of Artesian wells.
The Artesian well of Grenelle, near the Hotel cles Invalides, in Paris, is 2000
feet deep. At ISOO feet, water was struck, and it darted out to a height above
the surface of 112 feet and at the rate of nearly one million of gallons a day.
The pressure indicated by the jet was equal to that of a column of water 2612
feet high, or 1160 pounds to the square inch.
Another well, in Westphalia in Germany, is 2385 feet deep.
An Artesian boring at St. Louis has been carried to a depth of 2200 feet; but
the water obtained is not pure. One at Louisville, Kentucky, 2086 feet deep,
supplies an abundance of water, though a little brackish. Several have been
made in New York City connected with manufactories. In California they have
been resorted to successfully for agricultural purposes.
Borings are often succes.'^ful in alluvial regions fifty or one hundred miles
from any high land. A second boring in the same region sometimes seriously
lessens the amount of water afforded by the first, by giving the same subterra-
nean stream a new place of exit. The laj'er from which the boring and jet
rise may be gradually worn through by the flow, and the water, or part of it,
become lost by being thus let off to a lower level.
The mechanical effects of subterranean waters are ā (1) Erosion
and the consequent undermining of strata ; (2) Land-slides:
1. Erosion. ā Running water will wear rocks under ground as well
as above, and may excavate a channel in the same way. Caverns
are made partly by erosion and partly by the dissolving action of
water. A common effect of such excavations is the production of
subsidences of the soil and overlying rocks, and the formation
of sink-holes. Small shakings of the earth may be a consequence
of the fractures of undermined strata.
2. Land-slides. ā Land-slides are of three kinds : ā
(1.) The mass of earth on a side-hill, having over its surface, it
may be, a growth of forest-trees, and, below, beds of gravel and
stones, may become so weighted with the waters of a heavy rain,
and so loosened below by the same means, as to slide down the
slope by gravity.
A slide of this kind occurred during a dark, stormy night in August, 1826,
in the White Mountains, back of the Willey House. It carried rocks, earth, and
trees from the heights to the valley, and left a deluge of stones over the country.
The frightened Willey family fled from the house, and were destroyed: the
house remains, as on an island in the rocky stream.
(2.) A clayey layer overlaid by other horizontal strata some-
times becomes so softened by water from springs or rains that the
sujoerincumbent mass by its weight alone presses it out laterally,
provided its escape is possible, and, sinking down, takes its place.
Near Tivoli, on the Hudson River, a subsidence of this kind took place in
April, 1862. The land sunk down perpendicularly, leaving a straight wall
around the sunken area sixty or eighty feet in height. An equal area of
clay was forced out laterally underneath the shore of the river, forming a
point about an eighth of a mile in cii'cuit, projecting into the cove. , Part
of the surface remained as level as before, with the trees all standing. Three
days afterwards, the slide extended, partially breaking up the surface of the
region which had previously subsided, and making it appear as if an earth-
quake had passed. The whole area measured three or four acres.
(3.) When the rocks are tilted and form the slope of a mountain,
the softening of a clayey or other layer underneath, in the manner
650 DYNAMICAL GEOLOGY.
just explained, may lead to a slide of the suioerincumbent beds
down the declivity.
In 1806, a destructive slide of this kind took place on the Rossberg, near
Goldau, in Switzerland, which covered a region several square miles in area
with masses of conglomerate, and overwhelmed a number of villages. The thick
outer stratum of the mountain moved bodily downward, and tinallj' broke up
and covered the country with ruins, while other portions were buried in the
half-liquid clay that had underlaid it and was the cause of the catastrophe.
Similar subsidences of soil have taken place near Nice, on the Mediterranean.
On one occasion, the village of Roccabruna, with its castle, sunk, or rather slid
down, without disturbing or destroying the buildings upon the surface.
Besides (1) the transfer of rocks and earth, land-slides also cause
(2) a scratching or planing of slopes by the moving strata and
stones; (3) the burial of animal and vegetable life ; (4) the folding
or crumpling of the clayey layer subjected to the pressure, where
the effect does not go so far as to produce its extrusion and destruc-
tion. Such crumpled or folded beds of clay are not very uncom-
mon in alluvial regions (tig. 977).
2. THE OCEAN.
1. OCEANIC FORCES.
The ocean exerts mechanical force by means of its ā
1. General system of currents.
2. Tidal waves and currents.
3. Wind-waves and currents.
The 7'atio between the velocity/ of salt water and its force is the same as
for fresh water (p. 635) ; but in the application of the ratio there is
a ditierence arising from the greater density of the former, ā its
specific gravity being one-thirty-Jifth to oyie-fortieth more than that of
fresh water. Having determined the size of block that any given
velocity would be sufficient to transport, the size for other velo-
cities may be deduced by means of the ratio referred to.
The specific gravitij of sea-water varies for different parts of the ocean. For
the waters of the southern ocean, it is 1.02919; the northern, 1.02757; equator,
1.02777; Mediterranean Sea, 1.0293; Clack Sea, 1.01418 (.Alarect). In most
seas receiving large rivers, and in bays, the density is least. The specific gravity
of the water of East River, off New York City, at high tide, is 1.02038 (Beck).
1. General system of currents.
The system of oceanic currents is briefly exijlained on page 39.
It is part of the organic structure of the globe, irrespective of its
age or condition ; for, whatever the temi:)erature of the poles,
there must always have been a xoarmer tropics under the jyath of
The prominent characteristics of these currents bearing on their
mechanical effects in geological history are the following: ā
1. The rate of movement is slow. ā The maximum velocity of the
Gulf Stream is five miles an hour, and the average less than one
mile and a half.
The Gulf Stream is most rapid off Florida, where the hourly rate is three to
five miles; off Sandy Ilook, it is one mile and a half. The rate of flow of the
polar current is less than one mile an hour. Kane, while shut up in the Arctic,
was carried south by the current, some days, about half a mile an hour. The
great oceaiiic current of the eastern South Pacific varies from three miles an
hour to a fraction of a mile ; and across the middle of the ocean it is barely
appreciable. The current in the Indian Ocean, where most rapid, has the hourly
rate of two miles and a quarter.
In past geological ages the rapidity of these great oceanic cur-
rents must liavo been less than now, if there was any difference,
because of the less difference of temperature then between tlie
equator and the poles.
2. The currents are generally remote from coasts, and are seldom appre-
ciable where the depth is less than one hundred feet, and very feeble -where
less than one hundred fathoms. ā Owing to the great depth of the
oceanic movement, the waters are diverted along the borders of
the oceans by the deep-sea slopes of the continents. In the case
of the Gulf Stream, these approach the coast at Cape Florida, and
somewhat nearly at Cape Ilattcras ; but off New Jersey they are
eiglity to one hundred miles distant; and here runs the western
limit of the stream.
Tlie polar or Labrador current, which is mostlj^ a sub-current,
comes to the surface along the same slope, west of the limit of the
Gulf Stream, and is slightly apparent on the coast plateau, but
rather by its' temperature than by the movement of the waters.
The more western position of the limit of the polar current is ex-
plained on page 41. The fact that it has not more rapid move-
ment on the great shore-plateau is evidence that it belongs to
the deep water. This appears further in the current's underlying
the Gulf Stream, and its banding the stream with colder and
warmer waters, as shown by the Coast Survey under Professor
Bache. The observations of the survey have proved that there
are mountain-ridges apparently parallel Avith the Appalachians
along the course of the stream in its more southern part, and that
above these ridges the surface-waters are cooler, owing to the lifting
652 DYNAMICAL GEOLOGY.
upward of the polar current by the submarine elevations. The fact
that the cold waters jjroduce a temperature of o5Ā° F. at a depth of
six hundred fathoms off Havana (as stated by Bache) is proof of
the great magnitude of the polar current.
Where the current flows close along a coast or submarine bank,
or by an oceanic island, it may produce some effects.
3. As the position of the main flow of the currents is determined partis/ hy
the trend of the continents, their courses may have been different in former
time from what they are 7ww, provided the continents, or large portions of
than, were sufficiently siibmerged. ā Small subsidences would not suffice
to produce a diversion from their present courses, for the reason
just given. Even the barrier of Darien might be removed by sub-
mergence to a de^Dth of five hundred feet, and probably one thou-
sand, without giving passage to much, if any, of the Gulf Stream.
If, however, the straits were so deeply sunk that the Gulf Stream
passed freely into the Pacific (the West India islands being also in
the depths of the ocean, as would be necessary for the result), a
great change would thereby be produced in the temperature both
of the Atlantic and Pacific, ā a loss of heat to the former and a
gain to the latter (see Physiographic Chart). But no facts yet ob-
served prove this supposition to have been a realized fact since the
opening of the Silurian age.
Besides the general system of currents which has been considered, there are
currents between the ocean and some confined seas opening into it, which are
due to the evaporation going on over the surface of those seas. The conse-
quent diminution of water causes a flow from the ocean to supply the loss.
This happens at the Straits of Gibraltar opening into the Mediterranean. In
many seas of this kind the accessions from rivers more than supply the amount
removed by evaporation, and these produce an out-current at the entrance.
2. Tidal -waves and currents.
1. Rise and fall of tides. ā The simplest of tidal actions is the
periodical rising of the waters on a coast. The in-flow acts like a
dam in setting back the waters of springs and rivers. It floods
large areas on flat coasts, which are thereby made salt marshes.
The height of the tide is less in mid-ocean than along the conti-
nents, and is greatly augmented where the two coast-lines con-
verge, as on entering a bay, and especially where there is free
entrance to a channel from two directions. In the middle At-
lantic, at St. Helena, it is two or three feet ; at the Azores, three
feet ; on the Atlantic coast of the United States, from five to twelve
feet ; but in the Bay of Fundy, fifty to seventy feet. In the cen-
tral Pacific, the height is two to four feet ; and at Tahiti, high tide
occurs always at noon.
2. Translation character of the tidal ivaves. ā The tidal waves which
succeed one another around the globe become appreciably trans-
lation or propelling waves on soundings ; and directly upon a
coast, especially along its deeper bays or inlets, they constitute a
force of great energy. The borders of all the continents and
islands feel this power and exhibit its effects.
3. In-jlowing tidal currents. ā The in-coming tide generally strikes
one part of a coast before another, owing to its trend with refer-
ence to the wave, and, consequently, has a progressing movement
along it. This is very marked on the shores of southern New
The tidal current becomes one of great strength where there are
narrow channels to receive and discharge the waters.
The movement may have the violence of a river-torrent when
the entrance to bays is of a kind to temporarily dam uji) the waters
until the far-advanced tide has so accumulated them that they
overcome the resistance and pass on in a body.
In the Bay of Fundy, the waters of the in-coming tide are raised
high above their natural elevation, so that as they advance they
seem to be pouring down a slope, making a turbid waterfall of
majestic extent and power, without foam. The tide at Bristol.
England, has a height of forty feet.
In some cases the whole tide moves in all at once, in a few great
waves. This haj^pens especially at the mouths of rivers where there
is obstruction from sand-bars, and other favoring circumstances
about the entrance. The phenomenon is called an eagre or lore.
The flow of the tides at the Bay of Fundy has something of the
character of an eagre. But the most perfect examples are afforded
at the mouths of the rivers Amazon, Hoogly (one of the mouths of
the Ganges), and Tsien-tang in China. In the case of the last-
mentioned river the wave plunges on like an advancing cataract,
four or five miles in breadth and thirty feet high, and thus passes
up the stream, to a distance of eighty miles, at a rate of twenty-five
miles an hour. The change from ebb to flood tide is almost instan-
taneous. Among the Chusan Islands, Just south of the bay, the
tidal currents run through the funnel-shaped firth with a velocity
of sixteen miles an hour. (Macgowan.)
In the eagre of the Amazon, the whole tide passes up the stream
in five or six waves, following one another in rapid succession, and
each twelve to fifteen feet high.
4. Out-Jlowiny currents. ā The ebbing tide causes an out-flowing
Got DYNAMICAL GEOLOGY.
current, which is directly the courterpart of the in-flowing current.
It is more quiet than the latter in its movement, although often
a rapid and powerful current, because more contracted in width, ā
and especially so in bays, -where the waters of a river add to the
volume of the ebb. Wind-waves may increase greatly the force of
the in-coming tide, but not so with the out-flowing, since waves
always act shoreward.
The piling of the tidal waters to an unusual height in converging
bays, raising them far above their level outside, is another powerful
cause of out-flowing currents. The flow is along the bottom ; and
in a case like that of the Bay of Fundy it must have great power.
3. Ordinary -wind-waves and currents.
1. Waves. ā The winds are almost an incessant wave-making
power. Even in the calmest weather there is some breaking of
wavelets against the rocky headlands or the exposed beach ; and
with ordinary breezes the beaches and rocks are ever under the
beating surge, night and day, from year to year. Most seas, more-
over, have their storms, and in some, as those about Cape Horn,
gales prevail at all seasons. The breakers on the shores of the
Pacific are especially heavy, on account of its extent and depth.
Through a large part of the ocean the winds are constant in
direction, either for the year or half-year.
Stevenson, in his experiments at Skerryvore (w^est of Scotland),
found the average force of the waves for the five summer months
to be 611 pounds per square foot, and for the six winter months,
2086 pounds. He mentions that the Bell Rock Lighthouse, 112
feet high, is sometimes buried in spray from ground-swells when