The processes which develop coastal inde_Geology, Vol. 1 [of 3]

antecedent subaërial and the subsequent wave gradation, account for

most of the islands which affect indented coasts. Some of them are

high and some low for reasons which will be readily understood. The

long narrow belts of land constituting irregularities parallel to the

general trend of the coast (Figs. 319 and 320) are usually the result

of deposition in shallow water. They are usually sand or coral reefs,

built up above water-level by waves. The deposits at the debouchures

of streams give rise to projecting deltas. Most small irregularities

of angular form, especially if high (Pl. XX), indicate wave-erosion,

and their details of form are determined by the structure of the rock

along shore, while most irregularities of curved outline involve

something of shore-deposition, if not due wholly to it. Glaciation, or

glaciation and subsidence, may also give rise to peninsulas, capes, and

islands of curved outlines (Pl. XXIV, coast of Maine). Curving outlines

may, however, be developed by erosion alone in weak rock structures.

This is illustrated by the weak rock structures (clay, sand, etc.) of

most of the Atlantic coastal plain. Thus inspection of the horizontal

configuration of coasts will often indicate the processes which

have been dominant there in recent times. On the other hand, the

interpretations of many coastal irregularities, such as Hudson Bay,

Puget Sound, the Gulf of California, the Baltic Sea, etc., are not to

be read from the map. In such cases, diastrophism and gradation have

usually coöperated, but the relative importance of the two processes

can only be determined by detailed study in the field. When it is

remembered that the tendency of shore-erosion is to reduce great

irregularities of horizontal configuration, though not to obliterate

small ones if the coast be heterogeneous in composition (p. 353), and

that the tendency of shore-deposition is also to regularity, it is

clear that the great irregularities of coast-lines are due neither to

shore-erosion nor to shore-deposition, though minor ones may be due to

either.

THE WORK OF OCEAN-CURRENTS.

As agents of erosion, ocean-currents are not, in general, of great

importance. Currents which reach the bottom are comparable, in

their effects, to rivers of the same velocity and volume; but most

ocean-currents do not touch bottom, and, therefore, do not erode it.

Where the current agitates the bottom sensibly, as it often does in

shallow water, the bottom is abraded, and in the lee of such places

it is doubtless aggraded. Since ocean-currents do not, for the most

part, flow in shallow water, their erosive work is, on the whole,

relatively slight; but where they are forced through narrow and shallow

passageways, their abrasive work may be considerable. Thus the Gulf

Stream, where it issues from the Gulf, has a velocity of four or five

miles per hour, and its shallow and narrow channel is current-swept.

A rough test of the abrasive work of an ocean-current is found in the

nature of the bottom beneath it. If this be hard, it indicates that the

loose sediment on the floor of the ocean has been swept away, while the

presence of fine detritus indicates that the current is not wearing.

Thus the abrasive power of the Gulf Stream is known to continue

somewhat beyond its narrow channel, for on the Blake plateau (between

the Bahamas and Cape Hatteras), where the water is 600 fathoms and less

in depth, “the bottom of the Gulf Stream ... is swept clean of lime and

ooze and is nearly barren of animal life.”[168] Other illustrations

of the erosive power of currents have been noted near Gibraltar in

water 500 fathoms deep, and between the Canary Islands at depths of

1000 fathoms.[169] In spite of these examples, and of many others

which probably exist in similar situations, it yet remains true that

ocean-currents are on the whole but feeble agents of erosion.

As agents of transportation, ocean-currents are scarcely more important

than as agents of corrasion, for they transport only what they erode,

if the life which inhabits them be left out of consideration. This

phase of their work has probably been exaggerated through a confusion

of transporting energy and actual transportation. Ocean-currents which

do not touch bottom roll no sediment and carry only what may be held

in suspension. A river’s power of transporting sediment in suspension

is due largely to the cross-currents occasioned by the unevenness of

its resistant bottom (p. 117). If a particle of mud in suspension in

a river drops to the bottom, as it frequently does, it may be picked

up again and carried forward. If, on the other hand, a particle in

suspension in an ocean-current once escapes the moving water by

settling through it, the current which does not drag bottom has no

chance to pick it up again. Very fine sediment may be carried by an

ocean-current far beyond the point where it was acquired, but currents

which do not touch bottom are rarely strong enough to hold any but the

finest material for any considerable length of time. As transporters

of sediment, therefore, ocean-currents are at a great disadvantage as

compared with rivers.

How readily particles of extreme fineness may be kept in suspension,

and how little agitation is necessary to keep them from sinking, is

shown by the experiments of Sorby, who showed that while a sand grain

¹⁄₁₀₀ of an inch in diameter will settle one foot per second in still

water, fine particles of clay require days to sink through the same

distance. The Challenger found fine sediment derived from the land 400

miles from the coast of Africa, and that not opposite the debouchure of

any large river. Sediment settles more readily in salt water than in

fresh, despite the fact that the former is heavier. This is presumably

because the salt diminishes the cohesion of the water.

Deposition by ocean-currents is limited by their transportation. Only

where they erode their bottoms do they gather coarse materials, and

only in the lee of such places are their deposits coarse. Since the

material which they carry is generally fine, it is widely distributed

before deposition.

Ocean-currents have little influence on the configuration of

coast-lines.

DEPOSITS ON THE OCEAN-BED.

Something has already been said concerning the sediments which

accumulate in the shallow waters along shores; but the area of marine

sedimentation is as extensive as the ocean itself, and the deposits

must now be reviewed from another point of view.

Oceanic deposits may be conveniently divided into two chief groups,

dependent on the depth of the water in which they are made.[170] These

groups are (1) _shallow-water deposits_, made in water less than some

such depth as 100 fathoms, and (2) _deep-sea deposits_, laid down in

water of greater depth. The selection of the 100-fathom line as the

dividing depth is less arbitrary than it seems, for passing outward

from the shore, it is at about this depth that the bottom ceases to be

commonly disturbed by the action of currents and waves; that sunlight

and vegetable life cease to be important at the bottom; and that the

coarser sediments which predominate along shore give place, as a rule,

to muds and oozes. Furthermore, the 100-fathom line (or some line very

near it) is an important one in the physical relief of the globe, for

it appears to mark, approximately, the junction of continental plateaus

and ocean-basins. Only because the latter are a little over-full does

the water run over their rims, covering about 10,000,000 square miles

of the borders of the continents, converting them from land into

epicontinental seas.

Aside from the deposits made by organisms, shallow-water deposits are

divisible into two groups—(_a_) those immediately along the shore, the

_littoral deposits_, and (_b_) those made between the littoral zone

and the 100-fathom line. Both are terrigenous. The deep-sea deposits

likewise are divisible into two groups, (_a_) _terrigenous deposits_

formed close to land, and made up chiefly of materials derived

immediately from the disintegration of land formations; and (_b_) the

_pelagic deposits_, made up chiefly of the remains of pelagic organisms

and the ultimate products arising from the decomposition of rocks and

minerals. The former predominate in the less deep waters relatively

near shore; the latter in the deeper water far from land. The shallow-

and deep-water deposits grade into each other in a belt along the

100-fathom line.

_Shallow-water Deposits._

=Littoral deposits.=—The littoral zone is the zone between high- and

low-water marks. It is the zone in which bowlders, gravels, sands, and

all coarser materials accumulate, though muds are occasionally met

with in sheltered estuaries. Generally speaking, the nature of these

deposits is determined by the character of the adjoining lands and the

nature of the local organisms. “The heavier materials brought by rivers

from high terrestrial regions, or thrown up by the tides and waves

of the sea, are here arranged with great diversity of stratification

through the alternate play of the winds and waves. Twice in the

twenty-four hours the littoral zone is covered by water and exposed

to the direct rays of the sun or the cooling effects of the night.

There is a great range of temperature; mechanical agencies produce

their maximum effects,”[171] and physical conditions in general are

most varied. Still greater diversity is introduced by the fact that

the zone is inhabited by both marine and terrestrial organisms, while

the evaporation of the sea-water which flows over tidal marshes and

lagoons leads to the formation of saline deposits. If the length of

the coast-lines of the world be taken at 125,000 miles (about 200,000

kilometers), and the average width of this zone at half a mile, these

deposits are now forming over an area of 62,500 square miles (about

160,000 square kilometers) of the earth’s surface.

=Non-littoral, mechanical deposits in shallow water.=—These deposits

are laid down in the zone of the ocean between low-water mark and the

100-fathom line. They cover about 10,000,000 square miles.[172] Their

composition is much the same as that of the littoral deposits, with

which they are continuous, though on the whole they are finer. At

their lower limit they pass insensibly into the fine deposits of the

deep sea. Coarse material, such as gravel and sand, prevails, though

in special situations, such as depressions and inclosed basins, muddy

deposits are found. While some of the deposits are wholly composed

of inorganic débris, organic remains are freely mingled with others.

The mechanical effects of tides, currents, and waves are everywhere

present, but become less and less well marked as the 100-fathom line

is approached. The forms of vegetable and animal life are numerous,

though the former decrease as depths which exclude the sunlight are

approached.

Both littoral deposits and deposits in shallow water outside the

littoral zone have already been referred to in connection with the work

of waves and currents (pp. 355–66). A few additional points only need

here be added.

[Illustration: +Figs.+ 321 and 322.—Diagrams showing how shallow-water

deposits may attain considerable depth by the shifting of the zone of

deposition seaward.]

[Illustration: +Fig.+ 323.—Diagram showing the interwedging of gravel-,

sand-, and mud-beds.]

In general the coarser sediments are lodged near shore and those

farther from the land become progressively finer. Even the coarser

part of the material carried in suspension by the undertow is partly

left in the shallow water. On the other hand, waves of exceptional

strength may carry coarse material into water of some depth. Thus

coarse shingle (gravel) and even bowlders have been found at depths of

10 fathoms.[173] Coarse deposits may extend far out from land if the

waves are strong, and especially if the water is shallow, and since

the zone of shallow water may be extended seaward by the aggradation

of the bottom, shallow-water deposits may cover extensive areas. They

may become deep at the same time, for as the outer border of the

shallow-water zone is shifted seaward by aggradation, the vertical

space to be filled becomes greater (compare Figs. 321 and 322).

Again, if the coast be sinking, new deposits of coarse material may

be made on older ones. In this way also great thicknesses of sediment

may be accumulated, all parts of which were deposited in shallow water.

The great thickness of some of the conglomerate beds of the past shows

how far this process may go.

[Illustration: +Fig.+ 324.—Ripple-marks.]

[Illustration: +Fig.+ 325.—Rill-marks resembling impressions of

seaweeds. Beach at Noyes Point, R. I. (Walcott, U. S. Geol. Surv.)]

[Illustration: +Fig.+ 326.—Rill-marks. Same locality as 325. (Walcott.)]

As a rule, no definite line marks the seaward terminus of the coarse

detritus, since coarse material is carried farther out when the waves

run high (and the undertow is strong) than when they are feeble. In

calm weather, therefore, fine sediment may be deposited where coarse

had been laid down in the preceding storm, only to be covered in turn

by other deposits of a different character. Thus gravel grades off into

sand, with more or less overlapping or interwedging, and sand grades

off into silt in the same way. This is diagrammatically illustrated by

Fig. 323.

=Characteristics of shallow-water deposits.=—Clastic sediments laid

down in shallow water have several distinctive characteristics. While

they are, in the aggregate, coarse, they are characterized by frequent

variations in coarseness. The surfaces of successive beds are likely

to be ripple- and rill-marked (Figs. 324, 325, 326), and cross-bedding

(Fig. 327) is of common occurrence. Clayey sediments accumulated

between high and low water are often sun-cracked (Fig. 328), and the

tracks of land animals are sometimes preserved on their surfaces.

Shallow-water deposits often contain fossils of organisms which live

in waters of slight depth. These characteristics are sufficient to

differentiate sedimentary formations made in shallow water from those

made in deep water, even after they have been converted into solid

rock and after the rock has emerged from the sea. Many of these

characteristics are, however, shared by deposits made by streams on the

land. Subaërial and lacustrine sediments are usually distinguishable

from those made in the sea by their fossils, and sometimes by their

distribution.

[Illustration: +Fig.+ 327.—Cross-bedding. (Gilbert.)]

[Illustration: +Fig.+ 328.—Sun-cracks. These cracks were on the

mud-flats of the Missouri a few miles above Kansas City, but

the sun-cracks on shore-deposits are not essentially different.

(Calvin.)]

=Topography of shallow-water deposits.=—The shallow-water deposits

have, on the whole, a rather plane surface, though there are some

notable departures from flatness. The steep slopes of the delta fronts

and of wave-built terraces have already been spoken of. Barriers often

shut in depressions, and the disposition of the material deposited is

sometimes uneven, owing to shore and tidal currents. The result is that

the surface of the shallow-water deposits is often affected by low

elevations and by shallow depressions. The elevations and depressions

may be elongate, circular, or irregular in form. These general facts

are shown in Figs. 319, 320, and 329. This topography is sometimes

preserved on newly emerged lands, as at various points on the Coastal

Plain of the United States.

[Illustration: +Fig.+ 329.—Irregularities of topography of

shallow-water deposits. The depths of the water are shown in fathoms.

(Chart of C. and G. Surv.)]

=Chemical and organic deposits.=—There is no sharp line of distinction

between the deposits usually classed as chemical and those regarded as

organic. The latter are chemical in the broader sense of the term, but

as they are immediately associated with life and are dependent upon it,

it is a matter of practical convenience to separate them. Aside from

the organic deposits, the chemical deposits made in shallow sea-water

embrace (1) those due to reactions between constituents so brought

together that new and insoluble compounds are formed and precipitated,

and (2) those due to evaporation. The points of saturation for the

various substances dissolved in sea-water are reached at different

stages, and hence they are deposited more or less in succession.

The chemical deposits made in the shallow water of the sea, or in

shallow bodies of water isolated from the sea, are chiefly simple

precipitates resulting from evaporation; but new combinations are

sometimes made in the process of concentration and precipitation.

All substances in solution are necessarily precipitated on complete

evaporation, but since the sea-water is in general far from saturation,

so far as all its leading salts are concerned, only a few are

thrown down in quantity sufficient to have geological importance

where evaporation is incomplete. The leading deposits are lime

carbonate (CaCO₃), lime sulphate (gypsum, CaSO₄,2H₂O), common salt

(rock-salt, NaCl), and the magnesium salts, usually the chlorides and

sulphates, which are later changed to carbonates. In investigations

on Mediterranean water[174] which had an initial density of 1.02, no

deposit took place until concentration by evaporation had brought the

water to a specific gravity of 1.05. Between this density and that

of 1.13, lime carbonate and some iron oxide were deposited. Between

1.13 and 1.22, lime sulphate was the most abundant precipitate, while

between 1.22 and 1.31, 95% of the deposit was common salt. With still

further concentration, the remaining substances in solution, especially

the magnesium salts, were thrown down.

While there is somewhat more than ten times as much lime sulphate as

lime carbonate in the ocean (p. 324), the deposits of the carbonate

(including the organic) have been very much greater than those of the

sulphate. This is due partly to the fact that the sulphate is much more

soluble in natural waters than the carbonate. Rivers bring much more

carbonate than sulphate to the sea, so that the point of saturation for

the sulphate would normally be reached much later than that of the

carbonate. The more important fact, however, is that marine plants and

animals use lime carbonate freely for skeletal and housing purposes.

It is held by some that they get their lime from the sulphate, but if

so they convert it into carbonate before it takes the form of shells,

coral, etc., the sulphuric acid set free in the process reproducing,

directly or indirectly, more sulphate. The secretion of lime carbonate

by organisms is not dependent on the saturation of the water, but may

be carried on when the amount in solution is very small.

There can be little doubt that the chief deposits of lime carbonate

have been and are being made through the agency of plants and animals

in the form of shells, coral, bones, teeth, and other devices for

supporting, stiffening, housing, protecting, and arming themselves;

but while it is agreed that the larger part of the lime carbonate

deposited in the open sea is of organic origin, it is equally clear

that in closed seas subject to concentration from evaporation, simple

precipitation takes place freely. There is some difference of opinion

as to the importance of these two classes of deposits, past and

present. The debated point is whether simple precipitation takes place

in any appreciable degree under the usual oceanic conditions. There is

much more evidence of solution by sea-water than of precipitation from

it. The ocean appears to be under-saturated with lime carbonate on the

whole, though it is still possible that deposition may take place in

favorable situations, as, for example, where the very calcareous waters

of rivers are spread out in thin sheets on the surface of the heavier

salt water, and thus exposed to exceptional evaporation, or where there

is very exceptional agitation and aëration.[175]

Gypsum appears to be deposited in quantity only in the closed basins of

arid regions where concentration reaches an advanced state.

Since normal sea-water is far from saturation with common salt,

the latter is precipitated only in lagoons, closed seas, or other

situations favorable to great concentration. This is usually achieved

only in notably arid regions, and in basins that receive little or no

drainage from the land.

Deposits of salt usually, therefore, signify highly arid conditions,

and where they occur over wide ranges in latitude and longitude, as

in certain periods of the past, unusual aridity is inferred. Where

confined to limited areas, their climatic significance is less, for

topographic conditions may determine local aridity. The total area

where salt is now being precipitated is small, though on the whole

the present is probably to be regarded as a rather arid period of the

earth’s history. On the other hand, ancient deposits of salt preserved

in the sedimentary strata show that the area of salt deposition has

been much more considerable than now at one time and another in the

earth’s history. The salt and gypsum deposits of the past seem,

therefore, to tell an interesting tale of the climates of the past.

The magnesium salts are among the last to be thrown down as the

sea-water is evaporated, and they most commonly take the form of

sulphates and chlorides. They often form double salts with potassium,

a relatively small and soluble constituent of sea-water. In the

artificial evaporation of salt water to obtain common salt, the process

is usually stopped before the saturation-point for the magnesium salts

is reached, and the residue, the “mother-liquor,” or “bittern,” is

drawn off to prevent these “bitter” salts from mixing with the common

salt. The magnesium salts are among the last to be precipitated, not

only because they are readily soluble, but because their quantity is

small; yet in the original rock from which all the sea-salts came,

there is at least as much magnesium as sodium, while in the sea there

is about five times as much sodium as magnesium. Just what becomes of

the remaining magnesium is not yet well understood. It has a notable

disposition to form double salts with some other constituent, as noted

above. In the earlier marine strata, dolomite, that is, limestone

composed partly or wholly of the double carbonate of lime and magnesia,

(CaMg)CO₃, abounds. This appears to have been formed by a gradual

substitution of molecules of magnesium for those of calcium, but

just how and when and why it was done has not been fully worked out.

It appears to be a case where the saline matter of the sea made its

contribution to the sedimentary deposits by chemical reaction upon

them, rather than by precipitation because of saturation.

The relatively small amount of potash in the sea-water is probably due

to its disposition to remain united with the clays and earths of the

mantle rock and of the shaley deposits.

To some extent the salts in solution act directly on the earthy matter

brought down into the sea by rivers, but where sedimentation is rapid,

as it often is in shallow water, this action is limited and obscure.

In the main, the ocean-waters protect the sediments from weathering

and similar changes, except as organic matter buried with them induces

change.

While the lime deposits are by far the greatest of the chemical and

organic deposits of the sea, plants and animals also secrete notable

quantities of silica. Silica deposits of organic origin are relatively

much more important in the deep sea than in shallow water, and will be

mentioned in that connection.

=Limestone.=—Something concerning the origin of limestone has already

been given in the preceding paragraphs, but because of the importance

of this formation, it may be added by way of summary that shallow seas

free, or nearly free, from terrigenous sediment, and abounding in

lime-secreting life, furnish the conditions for nearly pure deposits

of limestone, and that most of the limestone within the areas of the

present continents appears to have originated under such conditions.

The common notion that limestone is normally a deep-water formation is

a serious error. Although limestones are formed in deep as well as in

shallow waters, by far the more important classes of lime-secreting

organisms are photobathic, i.e. are limited to the depths to which

light penetrates. In the shallow waters, these plants and animals are

in part free and in part attached. Within the areas of deep water they

are free and at the surface, and their remains drop to the bottom,

if not sooner dissolved. But few forms live on the deep, dark, cold

bottoms of abysmal depths. Clear waters, free from abundant terrigenous

sediments and abounding in lime-secreting life, rather than deep

waters, are, therefore, the most favorable conditions for the origin of

limestone.

The purely chemical deposits of limestone are probably all of

shallow-water origin. Once made, they are subject to solution,

redeposition, and other mutations like other deposits. As a result,

they often lose many of their original characteristics, but enough

usually remain to tell the story of their origin.

_Deep-sea Deposits._

=Contrasted with shallow-water deposits.=—The deep-sea deposits cover

the ocean-bottom below the 100-fathom line. Their area is considerably

more than half the earth’s surface. The characteristic deposits are

muds, organic oozes, and clays, which in their physical characteristics

are remarkably uniform. In regions of floating ice, greater diversity

is introduced from the varied nature of the materials which the ice

transports, but gravels and sands, comparable to those of shallow

water, are rarely found. “Tides, currents, and waves produce some

mechanical effects at the upper limits of the deep-sea region, but

on the whole there is an absence of the phenomena of erosion, and

mechanical action would appear to be absent except in the case of

submarine eruptions. The depth is too great for sunlight to penetrate,

and vegetable life is limited to the upper zone. Animal life is present

in the same zone and on the bottom, but absent or nearly so in the

middle depths. The temperature (at the bottom) is below 40° Fahr.

throughout the larger part of the area, and if subject to variation

with latitude or change of season, these changes affect only the depths

immediately beyond the 100-fathom line. Throughout the whole region

there is a very uniform set of conditions. In the shallow-water and

littoral zones, owing to the rapid accumulation and the mechanical

effects of transportation and erosion, the effects of chemical

modification are not very apparent in the deposits; but in deep-sea

deposits, in consequence of the less rapid rate of accumulation,

absence of transport, the nature and small size of the particles, many

evident chemical reactions have taken place, resulting in the formation

_in situ_ of glauconite, phosphatic and manganese nodules, zeolites,

and other secondary products.”[1] With increasing depth and distance

from the shore, the character of the deposits undergoes a change. There

is less and less material derived directly from the land, and more

“amorphous matter arising from the ultimate decomposition of minerals

and rocks, and accompanied, in all moderate depths, by an increase

[relative] of the remains of pelagic organisms. We thus pass insensibly

from those deep-sea deposits of a terrestrial origin, which we call

‘terrigenous,’ to those deep-sea deposits denominated ‘pelagic,’ in

which the remains of calcareous and siliceous organisms, clays and

other substances of secondary origin play the principal rôle.”[176]

The following table[177] shows the relations of the various groups of

marine deposits.

1. Deep-sea deposits beyond { Red clay } I. Pelagic deposits

100 fathoms { Radiolarian ooze } formed in deep

{ Diatom ooze } water removed

{ Globigerina ooze } from land.

{ Pteropod ooze }

{

{ Blue mud }

{ Red mud }

{ Green mud }

{ Volcanic mud } II. Terrigenous

{ Coral mud } deposits formed

} in deep and

2. Shallow-water deposits } Sands, gravels, } shallow water,

between low-water mark } muds, etc. } mostly close

and 100 fathoms } } to land.

}

3. Littoral deposits between } Sands, gravels, }

high- and low-water marks } muds, etc. }

=Sources.=—The pelagic deposits are made up in part of materials of

organic origin, and in part of materials of inorganic origin. The

inorganic materials may be of mechanical or chemical origin. Mechanical

pelagic deposits originate in various ways. They may come (1) from

the land by the ordinary processes of gradation, (2) from volcanic

vents, or (3) from extra-terrestrial sources. Chemical deposits may

be formed (1) _in situ_ by the chemical interaction of substances in

the sea-water on materials of organic and inorganic origin, and (2) by

direct precipitation from the sea-water.

=Mechanical inorganic deposits.=—The terrigenous materials which reach

the deep sea are, as a rule, only the finest products of land decay,

and are carried out by movements of water or by the winds. They are

not commonly recognized in the dredgings more than 200 miles from

the shore, but opposite the mouths of great rivers they extend much

farther,—1000 miles in the case of the Amazon. They are especially

abundant on the slopes of the continental shelves. Here occur the

_blue_, _green_, and _red muds_, with which are associated volcanic

and coral muds. The color of these various muds is dependent in part

on the changes which they have undergone since their deposition.

The green muds usually contain enough glauconite to give them their

color, and are most commonly found off bold coasts where sedimentation

is not rapid. The blue muds indicate lack of oxidation, or perhaps

deoxidation. Red muds are not common, though they have been found in

some situations. In general, these deposits are analogous to certain

shales, marls, etc., found within the continents.

Though coarse materials derived from the land are occasionally found in

the deep-sea deposits, their presence must be looked upon as in some

sense accidental. Occasional pebbles, or even bowlders, are carried out

into the ocean entangled in the roots of floating trees. Within limits,

too, icebergs have carried out land débris, though it is probable that

transportation by this means has been exaggerated. The amount which

icebergs might carry, if fully loaded, is far greater than the amount

which they do carry.

Of the identifiable inorganic materials in the deep sea, the most

abundant are of volcanic origin, and among these the most common is

pumice, which is frequently so light that it floats readily until it

becomes water-logged. Pieces of pumice brought up by the Challenger and

thoroughly dried were found to float for months in sea-water before

settling even through the depth of water contained in the vessel

in which the experiment was performed.[178] The next most abundant

substance of volcanic origin in pelagic deposits is _volcanic glass_.

This ranges from pieces of the size of a walnut down to the smallest

fragments, which often serve as centers for concretions. _Lapilli_

(cinders) and _volcanic ash_ also are abundant in parts of the deep

sea. The distribution of these volcanic products is essentially

universal, though by no means uniform. Some of them are probably from

submarine volcanoes.

The study of the deep sea deposits has revealed the presence of many

nodules and grains which are believed to be of extra-terrestrial

origin. Many of them are magnetic.[179] The dust of countless meteors

which enter the atmosphere daily settles on land and sea alike, and

enters into the sediment of the bottom of the latter. It is probably

no more abundant in deep water than in shallow, but it is relatively

more important, since other sedimentation is more meager. The number of

meteorites which enter the atmosphere daily has been estimated at from

15,000,000 to 20,000,000.[180] If on the average the meteorites weigh

ten grains each, probably a rather high estimate, the total amount of

extra-terrestrial matter reaching the earth yearly would be 5,000 to

7,000 tons, and something like three-fourths of this must, on the

average, fall into the sea. But even at this rate it would take some

fifty billion years to cover the sea-bottom with a layer one foot in

thickness.

=Organic constituents of pelagic deposits.=—With increasing distance

from shores, and especially with increasing depth of water, terrigenous

deposits become less and less abundant, and sediments derived from

pelagic life increase in relative importance. Beyond the upper part of

the outer slopes of the continental shelves, the pelagic deposits are

largely made up of shells and skeletons of marine organisms which live

in the surface-waters. Pelagic molluscs, foraminifera, and algæ secrete

shells of lime carbonate, while diatoms and radiolarians secrete shells

of silica. When the organisms die, they sink to the bottom with their

shells, and these mineral matters of organic origin are mingled with

the volcanic products which are universal over the sea-floor. Pelagic

deposits of organic origin are named according to their characteristic

constituents. Thus there are pteropod oozes, globigerina oozes, diatom

oozes, radiolarian oozes, etc.[181] It is not to be understood that

these oozes are made up exclusively of the shells which give them their

names. Diatom ooze is an ooze in which diatom shells are abundant, not

an ooze made up wholly of diatom shells; and globigerina ooze is an

ooze in which globigerina shells are abundant, though in many cases

they do not make up even the bulk of the matter. While samples of these

various oozes might be selected which are thoroughly distinct from one

another, there are all gradations between them, since pelagic life does

not recognize boundary-lines.

It is a significant fact that with increasing depth the proportion

of lime carbonate in the ooze decreases. Thus in tropical regions

remote from land where the depths are less than 600 fathoms, the

carbonate of lime of the shells of pelagic organisms may constitute

80% or 90% of the deposit. With the same surface conditions, but with

increasing depth, the percentage of lime carbonate decreases, until at

2000 fathoms it is less than 60%; at 2400 fathoms, 30%, and at 2600

fathoms, 10%. Beyond this depth there are usually no more than traces

of carbonate of lime. The data at hand show that the percentage of lime

carbonate falls off below 2200 fathoms more rapidly than at lesser

depths.

When the percentage of lime carbonate becomes very low, the calcareous

oozes grade off into the red clay with which the sea-floor below 2400

to 2600 fathoms is covered.

=Chemical deposits.=—The chemical deposits of the deep sea are chiefly

the alteration products of sediments which reach the sea-bottom

by mechanical means. All sediment deposited in the sea undergoes

more or less chemical change, but it is only when the change is

very considerable that the product is referred to this class. Where

sedimentation is rapid and the sediment coarse, the chemical change is

relatively slight; but where the sedimentation is slow and the sediment

fine, the chemical change is relatively great; for the longer exposure

to the sea-water and the greater proportion of surface exposed to

attack, both favor change. Both the area and the mass of sea-bottom

sediment radically changed in this way are large, but most of the

deposit does not correspond to any formation known on the land.

The red clay already referred to belongs to this class of deposits.

Its origin has been the subject of much discussion. It contains much

volcanic débris, various concretions, bones of mammals, zeolitic

crystals, and extra-terrestrial spherules, and doubtless the insoluble

products of the shells of pelagic life; but it is still a mooted

question how far the clay itself is the product of decomposed shells,

and how far the altered product of pulverized pumice, volcanic ash,

dust, etc. Pelagic life does not seem to be less abundant at the

surface where the water is deep than where it is shallow, and it would

appear that the shells must sink in such situations as elsewhere. If

the lime carbonate of globigerina ooze be removed by dilute acid, the

inorganic residue is similar to the red clay in the ocean-bottom. This

suggests that owing to the more complete solution in the very deep

water, the lime carbonate of the shells has been dissolved, leaving

the red clay as a residuum. The more complete solution at the bottom

might be the result either of the greater pressure, or of a greater

percentage of CO₂ in the water due to emanations from the sea-floor, or

to both; but the suddenness of the transition from oozes to red clay,

with increasing depth, does not seem to be fully explained by these

assumptions. The study of the dredgings has inclined the students of

these materials to the conclusion that volcanic materials, rather than

shells, are the principal source of the red clay.[182] The volcanic

materials are thought to have accumulated slowly and to have been long

exposed to the action of sea-water. The various nodules and crystals in

the clay are believed to be secondary products, the materials for which

were derived from the decomposition of the same materials. Eolian dust

may be a notable constituent of the red clay.

Various specific products of chemical change may be briefly referred

to. The decomposition of certain mineral particles, such as feldspar,

gives rise to _kaolin_, and kaolin is a very considerable constituent

of most of the clayey deposits of the ocean-bottom. The kaolinization

of feldspar may take place both on land and in the sea. _Manganiferous_

deposits are widespread in the ocean-bottom, occurring both as coatings

on grains of mechanical sediments, shells, etc., and as concretions

ranging in sizes from minute particles to nodules an inch or more in

diameter. The concretions are sometimes approximately spheroidal, but

often botryoidal. These manganiferous nodules are believed to have

arisen from the decay of fragments of volcanic rocks. In their decay,

the manganese and iron are believed to have been first changed to

carbonates, and subsequently to oxides. After manganese oxide, iron

oxide and silica are by far the most abundant constituents, but many

other substances enter into their composition in minor quantities.

Another substance somewhat widely distributed in the sea-bed, though

by no means universal, is _glauconite_, a complex silicate of alumina,

iron, potassium, etc. Glauconite is, on the whole, most abundant

along the edges of the continental shelves, though it is by no means

universal in this position. It is not commonly found in deep water,

nor very near the shore, but approximately at the “mud-line.” The

glauconite grains begin to form, as a rule, in tiny shells, chiefly

the shells of foraminifera. After filling the shell, the shell itself

may disappear, while the glauconite goes on accumulating around the

core already formed, until the grain attains considerable size.

Glauconite is believed to be an alteration product of certain sorts

of mechanical sediment, the change being effected under the influence

of the decaying organic matter in the shells.[183] It does not occur

where sedimentation is rapid, and its formation appears to be favored

by considerable changes of temperature. Glauconite deposits occur on

the land and are commonly known as _green sand marl_. Glauconite also

occurs sparingly in many other sedimentary rocks.

[Illustration: +Fig.+ 330.—Distribution of various sorts of deep-sea

deposits. (Murray. Challenger Reports.)]

Another substance which is somewhat widespread in the ocean-bottom is

_phosphate of lime_, which occurs in various sorts of oozes, in the

manganiferous nodules, in glauconite, and in independent nodules. Like

the grains of glauconite, the grains of phosphate of lime appear to

have started as concretions in shells, and to be the result of the

reaction of organic matter on the contents of sea-water. The immediate

source of the lime phosphate in the water appears to have been the

shells or bones of the numerous animals living in the sea.

Secondary minerals made from the constituents of volcanic matter which

has been decomposed occur not uncommonly in the bottom of the sea.

These minerals belong to the general class of _zeolites_, phillipsite

being the most abundant. Their distribution is somewhat wide, but their

quantity is slight.

Unfortunately, knowledge of the deep-sea deposits is limited to their

superficial layers. Soundings do not usually penetrate more than a few

inches, or at most a foot or two.

Unlike shallow-water deposits, those of the really deep sea seem to

find no correlatives in the known rock formations of the land.

LAKES.

Most of the phenomena of the ocean are repeated on a smaller scale in

lakes. The waves of lakes and their attendant undertows and littoral

currents are governed by the same laws and do the same sort of work

as the corresponding movements of the ocean. Tides are absent, or

insignificant, but slight changes of level, known as _seiches_,[184]

have been observed in many lakes. They are probably caused by sudden

changes in atmospheric pressure. While they are generally very slight,

they frequently amount to as much as a foot, and occasionally to

several feet. The seiches are oscillatory movements, and their period

is influenced by the length and depth of the lake. They have been

studied most carefully in Switzerland. Currents corresponding to those

of the ocean are slight or wanting in lakes, but since most lakes have

inlets and outlets, their waters are in constant movement toward the

latter. In most cases this movement is too slow to be readily noted, or

to do effective work either in corrasion or transportation. The work of

the ice, on the other hand, is relatively more important in lakes than

in the sea.

=Changes taking place in lakes.=—The processes in operation in lakes

are easily observed and readily understood. (1) The waves wear the

shores, and the material thus derived is transported, assorted, and

deposited as in the sea, and all the topographic forms resulting from

erosion or deposition along the seacoast are reproduced on their

appropriate scale in lakes. (2) Streams bear their burden of gravel,

sand, and mud into lakes and leave it there. (3) The winds blow dust

and sand into the lakes, and in some places pile the sand up into dunes

along the shores. (4) Animals of various sorts live in the lakes, and

their shells and bones give rise to deposits comparable to the animal

deposits in the sea. (5) Abundant plants grow in the shallow water

about the borders of many ponds and lakes, and as they die, their

substance accumulates on the bottom. (6) At the outlet the water is

constantly lowering its channel. The lowering of the outlet is often

slow, especially if the rock be coherent, for the outflowing water

is usually clear, and therefore inefficient in corrasive work. These

six processes are essentially universal, and all conspire against the

perpetuity of the lakes. (7) In lakes where the temperature is low

enough for ice to be formed, it crowds on the shores and develops

phenomena peculiar to itself. The ice of the sea may work in similar

ways, but its work is restricted to high latitudes. (8) In lakes in

arid regions, deposits are often made by precipitation from solution.

The first five and the last of these processes are filling the basins

of the lakes. As the sediment is deposited, a corresponding volume

of water is displaced, and, if there be outlets, forced out of the

basins; the sixth process is equally antagonistic to the lakes, while

the seventh has little influence on their permanence. Given time

enough, these processes must bring the history of any lake to an end.

The lowering of the outlet will alone accomplish this result if the

bottom of the basin is above base-level. Many lakes have already become

extinct, either through the filling or draining of their basins, or

through both combined. The antagonism of rivers and lakes long ago

led to the epigram “Rivers are the mortal enemies of lakes.” True as

this statement is, it does not follow that lakes will ever cease to

exist, for the causes which produce new lakes may be in operation

contemporaneously with those which bring lakes now in existence to an

end.

=Lacustrine deposits.=—The beds of sediment deposited in lakes are

similar in kind, in structure, and in disposition to beds of sediment

laid down in the sea, but river-borne sediment is more commonly

concentrated into deltas, since waves and shore-currents are less

effective. Even the limestone of the sea has its correlative in some

lakes. Some of it was made of the shells of fresh-water animals which

throve where the inwash of terrigenous sediment was slight, some of

it from the calcareous secretions of plants,[185] and some of it was

precipitated from solution.[186] Salt and iron-ore[187] deposits are

also sometimes made in lakes.

=Extinct lakes.=—The former existence of lakes where none now exist

may be known in various ways. If the lake basin was filled, its former

area is a flat, the beds of which bear evidence, in their composition,

their structure, and often in their fossil contents, of their origin

in standing water. Such a flat is commonly so situated topographically

that the basin would be reproduced if the lacustrine deposits were

removed. To this general rule there might be exceptions, as where a

glacier formed one side of the basin when it was filled. If the lake

was destroyed by the reduction of its outlet, or by the removal of some

other barrier, such as glacier ice, or by desiccation, shore phenomena,

such as beaches, spits, etc., may be found. In time such evidences are

destroyed by subaërial erosion, so that they are most distinct soon

after the lake becomes extinct.

Many lakes, some of them large[188] and many of them small, are known

to have become extinct, while many others are now in their last stages,

namely, marshes. Many others have been greatly reduced in size. Such

reductions are often obvious where deltas are built into lakes. Thus

the delta built by the Rhone into Lake Geneva is several miles in

length, and has been lengthened nearly two miles since the time of

the Roman occupation. The end of Seneca (N. Y.) lake has been crowded

northward some two miles by deposition at its head. Similar changes

have taken place and are now in progress in many other lakes.

=Lake ice.=[189]—Since fresh water is densest at 39° Fahr., ice does

not commonly form on the surface until the temperature from top to

bottom is reduced to this point. Cooled below this temperature, the

surface-water fails to sink, and with sufficient reduction freezes. If

the lake be small, and especially if it be shallow, it is likely to

freeze over completely in any region where the temperature is notably

below the freezing-point for fresh water for any considerable period

of time. It is under these circumstances that the ice becomes most

effective.

[Illustration: +Fig.+ 331.—Ice crowding upon low shore. Clear Lake, Ia.

(Calvin.)]

Suppose a lake in temperate latitudes, where the range of temperature

is considerable, to be frozen over when the temperature is 20° Fahr.

If now the temperature be suddenly lowered to -10°, and such change of

temperature is not uncommon in the northern part of the United States,

the ice contracts notably. In contracting, it either pulls away from

the shores or cracks. If the former, the water from which the ice is

withdrawn quickly freezes; if the latter, water rises in the cracks

and freezes there. In either case, the ice-cover of the lake is again

complete. If the temperature now rises to 20° the ice expands. The

cover is now too large for the lake, and it must either crowd up on the

shores (Fig. 331) or arch up (wrinkle) elsewhere. It follows the one

course or the other, or both, according to the resistance offered by

the shore.

If the water near the shore is very shallow, the ice freezes to the

sand, gravel, and bowlders at the bottom. If the adjacent land is low,

the ice in expanding may shove up over it, carrying the débris frozen

in its bottom. It may even push up loose gravel and sand in front of

its edge if they be present on the shore. Where bowlders are frozen

to the bottom of the ice, the shoreward thrust in expanding has the

effect of shifting them in the same direction, and even of lifting

them a little above the normal water-level. This constant process of

concentrating bowlders at the shore-line gives rise to the “walled”

lakes, which are not uncommon in the northern part of the United

States. The “wall” does not commonly extend entirely around a lake,

though it exists at various points on the shores of many lakes. In

making the walls, the ice shoved up by winds, especially in the spring

when the ice is breaking up, coöperates.

[Illustration: +Fig.+ 333.—Calcareous tufa domes. Pyramid Lake, Nev.

(Russell.)]

If the lake be bordered by a low marsh, the ice and frozen earth of the

latter are really continuous with the ice of the lake, and the push of

the latter sometimes arches up the former into distinct anticlines,

the frozen part only being involved in the deformation. A succession

of colder and less cold periods may give rise to a succession of such

anticlines.[190] If the shore be steep and of non-resistant material,

the crowding of the ice produces different but not less striking

results. Where the thrust of the ice is against a low cliff of yielding

material, such as clay, it disturbs all above the shore-line. Where the

cliff is sufficiently resistant, it withstands the push of the ice, and

the ice itself is warped and broken.

=Saline lakes.=—A few lakes, especially in arid or semi-arid regions,

are salt, and others are “bitter.” Beside sodium chloride, salt

lakes usually contain magnesium chloride, and magnesium and calcium

sulphates. “Bitter” lakes usually contain much sodium carbonate, as

well as some sodium chloride and sulphate, and sometimes borax. The

degrees of saltness and bitterness vary from freshness on the one

hand to saturation on the other. The water of the Caspian Sea (lake)

contains, on the average, less salt than that of the sea; that of Great

Salt Lake contains about 18%; that of the Dead Sea, about 24%; and that

of Lake Van (eastern Turkestan), the densest body of water known, about

33%. See accompanying table.

Many salt lakes, such as the Dead Sea and Great Salt Lake, are

descended from fresh-water ancestors, while others, like the Caspian

and Aral Seas, are probably isolated portions of the ocean. Lakes of

the former class have usually become salt through a decrease in the

humidity of the region where they occur. The water begins to be salt

when the aridity is such that evaporation from the lake exceeds its

inflow. In this case the inflowing waters bring in small amounts of

saline and alkaline matter, which is concentrated as evaporation takes

place. The concentration may go on until the point of saturation is

reached, or until chemical reactions cause precipitation. In general

the least soluble minerals are precipitated first. Thus gypsum begins

to be deposited from sea-water when 37% of it has been evaporated; but

the saturation-point for salt is not reached until 93% of the water has

been evaporated (see p. 375). The relations in lakes are similar, and

gypsum deposits often underlie those of salt. Deposits of salt and

other mineral matters once in solution are making in some salt lakes

at the present time, and considerable formations of the same sort have

been so made in the past. Buried beneath sediments of other sorts, beds

of common salt or of other precipitates are preserved for ages. Lime

carbonate has been precipitated in quantity from some extinct lakes

(Fig. 333).

The lakes which originate by the isolation of portions of the sea

are salt at the outset. If inflow exceeds evaporation, they become

fresher and may ultimately become fresh; otherwise they remain salt.

If evaporation exceeds inflow they diminish in size and their waters

become more and more salt or bitter.

=Indirect effects of lakes.=—Lakes tend to modify the climate of

the region where they occur, both by increasing its humidity and

by decreasing its range of temperature. They act as reservoirs

for surface-waters, and so tend to restrain floods and to promote

regularity of stream flow. They purify the waters which enter them by

allowing their sediments to settle, and so influence the work and the

life of the waters below.

=Composition of lake-waters.=—The accompanying table[191] shows the

composition of various inclosed lake-waters, and gives some idea of

the wide range, both in kind and quantity, of the mineral matter

held in solution by them. It is to be noted that the table shows the

composition of the waters of exceptional, rather than common, lakes.

The waters of fresh lakes do not depart widely from those of rivers (p.

107).

TABLE—ANALYSES OF THE WATERS OF INCLOSED LAKES.

[+Reduced to Parts per 1000 by Dr. H. J. Van Housen.+]

+----------------+----------------+-----------------+---------------+--------------+

| | | | W. S. W. | |

|Specific gravity| 1023.17 | | | |

| | | | | |

|Date | May 3, 1883 | | | |

| | | | | |

| | | | | |

| | | 284 | I, p. 89 | Vol. I, p. 89|

+----------------+----------------+-----------------+---------------+--------------+

|Sodium, Na | 2.838 | 74.700 | 1.4440 | .3081 |

| | | | | |

|Potassium, K | 10.880 | 1.041 | .0398 | |

| | | | | |

|Rubidium, Rb | | ...... | ...... | ...... |

| | | | | |

|Calcium, Ca | ...... | 3.647 | .1854 | .1238 |

| | | | | |

|Magnesium, Mg | .002 | 13.777 | .4095 | .0728 |

| | | | | |

| | | | | |

|Iron, Fe | ...... | ...... | ...... | ...... |

| | | | | |

|Chlorine, Cl | 8.410 | 163.344 | 2.7376 | .4576 |

| | | | | |

|Bromine, Br | ...... | .043 | Trace | ...... |

| | | | | |

| gas, CO₂ | 4.653 | ...... | .1382 | .3746 |

| | | | | |

| H₂SO₄ | .509 | .198 | 1.3372 | .3109 |

| | | | | |

| HPO₄ | | | | |

| | | | | |

| NO₃ | | | | |

| | | | | |

| H₃BO₃ | ...... | | | |

| | | | | |

|Silica, SiO₂ | .064 | | | |

| | | | | |

| Al₂O₃ | ...... | | ...... | ...... |

| | | | | |

| bicarbonates, H| | | .0023 | .0062 |

| | | | | |

|Ammonium, NH₄ | | ...... | | |

| | | | | |

|Organic matter | ...... | ...... | ...... | ...... |

| +----------------+-----------------+---------------+--------------+

| | 27.357 | 256.750 | 6.2940 | 1.6540 |

+----------------+----------------+-----------------+---------------+--------------+

+-----------------+-----------------+-----------------+-----------------+--------------+--------------+

| | | | | | |

| 1.0216 | 1.1647 | 1.2225 | 1.2300 | | |

| | | | | | |

| Mar. 20, 1864 | Apr. 7, 1864 | Mar. 15, 1804 | Mar. 15, 1864 | April | August |

| | | | | | |

| | | | | | |

| | | | | I, p. 403–405| I, p. 403–405|

+-----------------+-----------------+-----------------+-----------------+--------------+--------------+

| .885 | 22.400 | 25.071 | 25.107 | 51.590 | 29.300 |

| | | | | | |

| .474 | 3.547 | 3.990 | 4.503 | 1.162 | |

| | | | | | |

| ...... | ...... | ...... | ...... | | ...... |

| | | | | | |

| 2.150 | 9.094 | 3.704 | 4.218 | ...... | .106 |

| | | | | | |

| 4.197 | 25.529 | 41.306 | 42.006 | 29.971 | 45.598 |

| | | | | | |

| ...... | ...... | ...... | ...... | | |

| | | | | | |

| | | | | | |

| 17.628 | 126.521 | 166.340 | 170.425 | 159.498 | 166.890 |

| | | | | | |

| .167 | 4.568 | 4.870 | 4.385 | .059 | ...... |

| | | | | | |

| | | | | | |

| .202 | .494 | .451 | .459 | 13.320 | 17.734 |

| | | | | | |

| ...... | ...... | ...... | ...... | | |

| | | | | | |

| | | | | | |

| | | | | | |

| ...... | ...... | ...... | ...... | | |

| | | | | | |

| | | | | | |

+-----------------+-----------------+-----------------+-----------------+--------------+--------------+

| 25.709 | 192.153 | 245.732 | 251.103 | 255.600 | 264.980 |

+-----------------+-----------------+-----------------+-----------------+--------------+--------------+

+-----------------+-----------------+-----------------+-----------------+-----------------+--------------+

| | | | | | |

| 1.27288 | 1.170 | 2.4 | 1.102 | 1.007 | ...... |

| | | | | | |

| October | 1850 | 1869 | Aug., 1873 | ...... | ....... |

| | | | | | |

| | | | | | |

| I, p. 403–405 | Lake, p. 410 | p. 435 | | | p. 284 |

+-----------------+-----------------+-----------------+-----------------+-----------------+--------------+

| 15.060 | 85.330 | 49.690 | 38.3 | .27842 | 94.050 |

| | | | | | |

| 1.204 | | 2.407 | 9.9 | .06083 | .529 |

| | | | | | |

| | ...... | ...... | ...... | ...... | ...... |

| | | | | | |

| ...... | Trace[196] | .255 | .6 | .01257 | .123 |

| | | | | | |

| 60.540 | .636 | 3.780 | 3.0 | .01648 | 5.076 |

| | | | | | |

| | | | | | |

| ...... | ...... | ...... | ...... | ...... | ...... |

| | | | | | |

| 171.936 | 124.454 | 83.946 | 73.6 | .29545 | 158.687 |

| | | | | | |

| | | Trace | | ...... | |

| | | | | | |

| ...... | ...... | ...... | ...... | .20126 | ...... |

| | | | | | |

| 42.560 | 12.400 | 9.858 | 8.8 | .03040 | 3.065 |

| | | | | | |

| | | | | .00069 | |

| | | | | | |

| | | ...... | | ...... | |

| | | | | | |

| | | | | | |

| | | ...... | | .03250 | |

| | | | | | |

| ...... | | | | | |

| | | | | | |

| Trace | ...... | ...... | ...... | ...... | ..... |

+-----------------+-----------------+-----------------+-----------------+-----------------+--------------+

| 291.300 | 222.820 | 149.936 | 134.2 | .92800 | 261.530 |

+-----------------+-----------------+-----------------+-----------------+-----------------+--------------+

+-----------------+-----------------+------------------+-----------------+-----------------+

| | | | | |

| 1.101 | 1.101 | 1.048 | 1.155 | 1.051 |

| | | | | |

| ...... | ...... | July 16, 1883 | ...... | ...... |

| | | | | |

| | | | | |

| | | | | p. 190 |

+-----------------+-----------------+------------------+-----------------+-----------------+

| 41.632 | 40.206 | 18.100 | 74.890 | 21.650 |

| | | | | |

| 2.290 | 2.425 | 1.111 | | 2.751 |

| | | | | |

| | | ...... | ...... | ...... |

| | | | | |

| ...... | ...... | .278 | .529 | Trace |

| | | | | |

| .245 | .245 | .125 | 2.914 | Trace |

| | | | | |

| | ...... | | | Trace |

| | | | | |

| ...... | ...... | ...... | ...... | ...... |

| | | | | |

| 41.496 | 40.206 | 11.610 | 119.496 | 13.440 |

| | | | | |

| ...... | ...... | ...... | | ...... |

| | | | | |

| 15.650[7] | 18.058[7] | 14.465[7] | ...... | 13.140 |

| | | | | |

| 11.771 | 11.943 | 6.520 | 7.671 | 9.362 |

| | | | | |

| | ...... | | | Trace |

| | | | | |

| ...... | ...... | ...... | | Trace |

| | | | | |

| .285 | .287 | .153 | | Trace |

| | | | | |

| .275 | .281 | .268 | | .164 |

| | | | | |

| ...... | | | | Trace |

| | | | | |

| | | | | ...... |

| | | | | |

| | | | | ...... |

| | | | | |

| ...... | ...... | ...... | ...... | Trace |

+-----------------+-----------------+------------------+-----------------+-----------------+

| 113.644 | 113.651 | 49.630 | 205.500 | 60.507 |

+-----------------+-----------------+------------------+-----------------+-----------------+

+----------------+-----------------+-----------------+-----------------+------------------+-----------------+

| | | | | | |

| Nev. | Utah | Nev. | Lake, Nev. | | |

| | | | | | |

| ...... | ...... | 1.003 | 1.001 | | |

| | | | | | |

| Aug. 1882 | 1872 | Sept., 1882 | Aug., 1882 | | |

| | | | | | |

| | | | | | |

| | p. 144 | | | p. 94 | |

+----------------+-----------------+-----------------+-----------------+------------------+-----------------+

| 1.1796 | 28.840 | .85535 | 1.2970 | 8.502[4] | 2.4512 |

| | | | | | |

| .0733 | | Trace | .0686 | .246 | .0584 |

| | | | | | |

| ...... | ...... | ...... | ...... | | .0022 |

| | | | | | |

| .0089 | .118 | .02215 | .0196 | ...... | .4581 |

| | | | | | |

| .0797 | 2.000 | .03830 | .0173 | .157[4] | .5965 |

| | | | | | |

| | | | | ...... | ..... |

| | | | | | |

| ...... | ...... | ...... | ..... | Trace[197] | .0008 |

| | | | | | |

| 1.4300 | 45.500 | .58375 | 1.6934 | 5.693 | 3.8386 |

| | | | | | |

| ...... | | ...... | ...... | ...... | .0029 |

| | | | | | |

| .4900[7] | ...... | .47445[7] | .3458[198] | 5.267[195] | .0918 |

| | | | | | |

| .1822 | 9.345 | .52000 | .1333 | 2.555 | 3.3368 |

| | | | | | |

| | | ...... | | | .0011 |

| | | | | | |

| | | | | | |

| ...... | | ...... | | ...... | ...... |

| | | | | | |

| .0334 | | .00750 | .0275 | .180 | .0032 |

| | | | | | |

| | | | ....... | ...... | ...... |

| | | | | | |

| | | | | | ...... |

| | | | | | |

| | | | | | |

+----------------+-----------------+-----------------+-----------------+------------------+-----------------+

| 3.4861 | 86.403 | 2.50150 | 3.6025 | 22.600 | 10.8416 |

+----------------+-----------------+-----------------+-----------------+------------------+-----------------+

CHAPTER VII.

THE ORIGIN AND DESCENT OF ROCKS.

It has been the current opinion that the earth was once in a molten

state, and thence cooled to a solid condition, and hence that all the

primitive rocks were igneous. Even those who think that the earth may

never have passed through a molten state agree that the oldest known

rocks are either true igneous rocks, or rocks of very similar nature. A

molten magma may, therefore, be taken as the mother state of the rocks.

Starting with this conception, the natural order of events suggests the

inquiries (1) how rocks are formed from molten magmas, (2) what natures

they assume, (3) how other rocks are derived from them, (4) how still

other rocks are derived from these derivatives, and so on. To answer

these inquiries is to trace out the generations of rocks and learn the

general history of rock-formation.

(1) The process by which igneous rocks are formed from lavas is

actually taking place in existing volcanoes. As these are widely

scattered over the face of the earth, the material poured out by

them represents different parts of the interior and varies in nature

accordingly. This affords the means of studying the differences that

arise from differences of material. This is a radical consideration,

for variations in composition give rise to the most fundamental

distinctions between rocks, though by no means the only ones. Rocks

which have the same composition often differ greatly in texture or

structure, owing to the varying conditions under which they were

formed. In the solidification of rocks from the molten state, the

rate of cooling causes many differences. A means of studying this is

afforded by the various lava flows that are now being poured out on

the surface under different conditions; but a more important means is

afforded by extinct volcanoes, especially by those which have been

deeply cut open by erosion. In certain very ancient volcanoes, not

only have the solidified lava streams of the surface been cut across

by erosion, but the lava that remained in the crater, or in the neck

that led up from below, is laid bare for inspection. Exposures of even

more profound nature have been made by the great disruptions which

the outer part of the crust has suffered. In certain tracts there have

been profound fractures, and the formations on one side of these have

settled down and on the other side have been pushed up (faulted), so as

to expose parts that were once much below the surface. Sometimes also

the crust has been folded and crumpled, and the wrinkles thus formed

have afterwards been worn away or cut open by deep valleys, and rocks

that were once deeply buried have been laid bare. By the revelations

made in these and other ways, it has been learned that at various times

in the history of the earth molten matter has been thrust into fissures

or intruded between layers of the crust and cooled there, without

coming to the surface. Sometimes the lava appears to have forced its

way into the rocks, and sometimes to have lifted the upper beds and

formed great subterranean layers or tumor-like aggregates, called

bathyliths and laccoliths (Fig. 334). Such intruded bodies of molten

rock, solidifying under the varying conditions of such subterranean

situations, are a fruitful source of instruction respecting the

influence of varying rates and modes of cooling, as well as of other

attendant conditions.

[Illustration: +Fig.+ 334.—Diagram of a laccolith. (Gilbert.)]

It will thus be readily seen that the rate of cooling of the various

molten rocks must have differed very greatly. In the portions

poured out upon the surface there were sometimes narrow streams and

thin sheets, giving large exposure in proportion to the mass (Fig.

335), and sometimes thick flows and deep pondings in basins and

choked valleys, giving massive bodies with relatively small surface

exposure. There were explosions of the lava into minute particles

with almost instantaneous cooling, and there were eruptions beneath

the sea the peculiar effects of which are rather matters of inference

than of positive knowledge. In the portions underground there were

insinuations into thin fissures, on the one hand, and in-thrustings of

thick bodies, on the other. Some intrusions entered the upper part of

the crust where the rocks were cold and wet, and some were thrust into

the deeper portions where the rocks were warmer and less penetrated

by water. Sometimes the lava rose rapidly and was little cooled in

passage, sometimes slowly with more cooling en route, and sometimes

there were long halts between eruptions, with much opportunity to cool.

An almost infinite variety of conditions is thus presented, and with it

a rich field for the study of the modes of solidification.

[Illustration: +Fig.+ 335.—Fresh lava flow, with large surface

exposure. Holemaumau, Hawaii. (Libbey.)]

In the underground intrusions the additional factor of high pressure

was also present, and this is the third important condition in

determining the nature of igneous rocks.

The three factors, _composition_, _rate of cooling_, and _degree of

pressure_, require special consideration.

_Composition of Igneous Rocks._

All or nearly all the chemical elements known on the earth are found

in greater or less amounts in igneous rocks, and in a broad sense are

constituents of them. If there are any exceptions, they are most likely

to be found in the rarer elements in the atmosphere. Oxygen, nitrogen,

hydrogen, aqueous vapor, and carbonic acid, which make up the mass of

the present atmosphere, are all found in lavas and in their cooled

products. Probably all the rarer elements also occur in igneous rocks.

Helium is known to be given forth by springs.

=Leading elements.=—But although nearly or quite all the known chemical

elements enter into the igneous rocks, only a few of them are abundant.

These are regarded as normal or essential constituents, while the rarer

substances are regarded as incidental. By combining a large number of

the most trustworthy analyses of rocks of all sorts, F. W. Clarke[199]

has estimated the relative amounts of the more abundant elements in the

crust of the earth with the following result:

Percent. in

Element. Symbol. the Solid Crust.

Oxygen (O) 47.02

Silicon (Si) 28.06

Aluminum (Al) 8.16

Iron (Fe) 4.64

Calcium (Ca) 3.50

Magnesium (Mg) 2.62

Sodium (Na) 2.63

Potassium (K) 2.32

Titanium (Ti) .41

Hydrogen (H) .17

Carbon (C) .12

Phosphorus (P) .09

Manganese (Mn) .07

Sulphur (S) .07

Barium (Ba) .05

Strontium (Sr) .02

Chromium (Cr) .01

Nickel (Ni) .01

Lithium (Li) .01

Chlorine (Cl) .01

Fluorine (Fl) .01

------

100.00

It will be seen that only eight of the elements hold a high rank

in quantity. Many that are of the utmost importance in the history

of the earth and the affairs of men are low in the list, or do not

even appear in it at all, because their quantity is too small to be

estimated in percentages. The precious metals, and even some of the

more common metals, as lead, zinc, and copper, are too scarce to form

an appreciable percentage.

=Union of elements.=—In a general study of the igneous rocks we may

for the present neglect all but the first eight of these elements.

Out of these elements spring various chemical combinations, and out

of these combinations spring the various minerals, and out of the

combinations of minerals come the various rocks. The union of oxygen

with the other seven elements may be taken as a fundamental step in

this series of combinations. The result is the following oxides: Silica

(SiO₂), alumina (Al₂O₃), ferrous, ferric, and magnetic oxide (FeO,

Fe₂O₃, and Fe₃O₄), magnesia (MgO), calcium oxide or lime (CaO), soda

(Na₂O), and potash (K₂O). The oxygen sometimes unites in proportions

different from those here given, but such exceptions may be neglected

in a general study. We thus have nine leading oxides. Of these, silica

acts as an acid, or more strictly according to the newer chemical view,

as an acid anhydride. All the rest, except the magnetic oxide of iron,

and sometimes the oxide of aluminum, act as basic oxides.

In the older chemical philosophy these oxides were supposed to combine

by the simple union of an acid oxide with a basic oxide, and to remain

as oxide joined to oxide; thus silica (SiO₂) and lime (CaO) formed

silicate of lime (CaO,Si₂). The symbols express the idea better than

the words. This method is used in the older geological works and in

some of the later. But in the newer chemical doctrine, the oxides are

not believed to remain so distinct after their union, and the symbols

are written CaSiO₃, and the compound is named calcium silicate.

According to the modern doctrine of solution, some of the calcium,

silicon, and oxygen may exist as free ions in molten rock. The precise

way in which the elements are related to each other in these compounds

can scarcely be said to be known. For the general purposes of geology

it is most convenient to think of these oxides as uniting in the

simple fashion first named, and this involves no apparent geological

error in general studies, since they are oxides when they enter the

compound, and if the compound is decomposed they usually come forth

again as oxides; but in closer studies more complex unions, attended by

dissociations (ionization), must be recognized.

=Formation of minerals.=—As but one of the leading oxides that abound

in an average magma plays the part of an acid, the silica, a very

simple conception of the general nature of igneous rocks may be

reached by noting that they are mostly silicates of the seven leading

basic oxides—alumina, potash, soda, lime, magnesia, and the iron

oxides. This general idea is a very useful one and represents a most

important truth; but in its use we must not forget that there are many

exceptions. Sulphur, phosphorus, chlorine, and other elements unite

with the bases to form sulphates, sulphides, phosphates, phosphides,

chlorides, etc. So also there are many minor bases that form silicates;

and these minor bases unite with the minor acids to form many more or

less rare minerals. Again, there are native metals in some igneous

rocks. But altogether these hardly reach more than one or two percent.

of the whole.

There are, however, two exceptions of more importance. In the molten

magma the acid and basic elements are not always evenly matched. When

there is an excess of silica, a portion remains free and takes the

form of quartz (SiO₂). If there is an excess of the basic oxides, the

weakest one is usually left out of the combination. This is commonly

the iron oxide, which then usually takes the form of magnetite (Fe₃O₄).

It is a singular fact that quartz often forms when there is no excess

of silica, and magnetite when there is no excess of base. Quartz

(free acid anhydride) and magnetite (free basic oxide) sometimes

occur in the same rock. The explanation for this is yet to be found.

These form rather important exceptions to the generalization that the

igneous rocks are mostly made up of silicates, but, thus qualified,

it expresses the essential truth, and has the merit of embodying the

central chemical fact relative to these rocks.

=Sources of complexity.=—But here simplicity ends. As we pass on to

the specific silicates that are formed, we encounter several sources

of complexity. In the first place, the silica unites with the bases in

different ratios and thus gives rise to unisilicates or orthosilicates

(ratio of oxygen of bases to oxygen of silica, 1:1), subsilicates

(ratio more than 1), bisilicates (ratio 1:2), trisilicates or

polysilicates (ratio 1:3 or higher), and combinations of these. All the

bases are not known to combine in all these ways, but many do in more

than one of them. Still, if the silica were content to unite with each

of the bases by itself alone, the results would remain comparatively

simple; but instead of this it unites with two or more at the same

time; and, more than that, it unites with them in varying amounts. The

case would still remain measurably simple if these chemical compounds

always crystallized out by themselves, each compound forming one

mineral, and but one; but the different silicates have the confusing

habit of crystallizing together in the same mineral. A crystal

may thus sometimes be seen, under the microscope, to be made up of

alternating layers of different silicates; e.g., a microscopic layer

of an aluminum-calcium silicate may be overlain by a microscopic layer

of an aluminum-sodium silicate, and the alternation may be repeated

throughout the crystal, giving it a banded structure. There is reason

to believe that this is true in many cases where the microscope fails

to detect it, and that less symmetrical comminglings of silicates may

take place. As such alternations or mixtures are not governed by any

known mathematical law, as is the case in chemical compounds, there is

no determinate limit to the number of combinations that may arise. As

a matter of fact, new ones are still being discovered in the progress

of research, and the total number that may ultimately be found can

scarcely be prophesied.

As a result of all this fertility of combination, the total number of

silicious minerals in igneous rocks is large. It is the function of the

mineralogist to treat of these minerals as such. The geologist deals

with them as constituents of the earth and as factors in its history.

Only a few of them are so abundant as to require special individual

notice in a general study of the earth. It may be remarked also that

only a few of them can be identified by simple inspection as they

occur in the rocks, partly because of the delicacy of the distinctions

between many of them, and partly because of their minuteness and

intricate intermixture. The resources of the polarizing microscope

are necessary for safe determination in most cases. The student need

not feel embarrassment or discouragement if he is often unable to

recognize the constituents of the intimately crystalline rocks. Their

determination has grown to be a profession by itself.

=The leading minerals of igneous rocks.=—Fortunately for the simplicity

of geological study, a few minerals make up the great mass of the

igneous rocks. These few are _quartz_, the _feldspathic minerals_,

the _ferromagnesian minerals_, and the _iron oxides_. Quartz (silica,

SiO₂) is the free acid already mentioned. The feldspathic and

ferromagnesian minerals are the leading silicates of the earth’s crust,

and vastly surpass all others in abundance. The feldspathic group

embraces minerals formed by silica in union with alumina, together

with either potash, soda, or lime, or two or more of these together.

The ferromagnesian group embraces minerals formed by the union of

silica with iron, magnesia, and lime, together with more or less of the

other basic oxides. These statements are only true in a very general

sense. Admixtures, replacements, and impurities are so frequent as to

break down all sharp, simple definitions. The feldspathic minerals

are normally light in color, ranging from white to red or gray. The

ferromagnesian minerals are normally dark (commonly greenish) from the

presence of iron, the great coloring element of rocks. But these color

distinctions do not hold good in detail and cannot be much trusted as a

means of identification.

=The feldspathic minerals= (p. 462) embrace the potash feldspars,

_orthoclase_ and _microcline_; the soda feldspar, _albite_; the lime

feldspar, _anorthite_; and the mixed feldspars intermediate between

albite and anorthite, viz., the soda-lime feldspar, _oligoclase_, the

lime-soda feldspar, _andesine_, in which lime and soda are nearly

equal, and the lime-soda feldspar, _labradorite_, in which the lime

predominates; together with _leucite_, a potash silicate higher in

alkali than orthoclase, and _nephelite_, a soda silicate higher in

soda than albite. Leucite and nephelite are usually classified as

_feldspathoids_, not as feldspars. It is to be understood that alumina

is normally present in all these. Additional details respecting these

minerals may be found in the reference list, p. 460.

Among the =ferromagnesian minerals= the most important are the

pyroxenes, the amphiboles, and the biotite type of mica. Olivine is

of subordinate importance. The pyroxenes (p. 465) and amphiboles

(p. 460) have nearly the same chemical composition, but differ in

crystallization and physical properties. _Hornblende_ (an amphibole)

has been melted, and on cooling under proper conditions found to take

on the form of _augite_ (a pyroxene). Pyroxene is sometimes altered

into _uralite_, one of the amphiboles. The pyroxenes and amphiboles

are the most abundant of the dark minerals in crystalline rocks.

The leading members of the pyroxene group are _augite_, _diallage_,

_hypersthene_, _enstatite_, and _soda pyroxene_. The chief members of

the amphibole group are _hornblende_ and the _soda amphiboles_. All are

essentially silicates of magnesia and iron oxide, with or without the

addition of lime, soda, and alumina. Details respecting these may be

found in the reference list.

The two leading =micas= are the iron-magnesia mica, _biotite_, and the

potash mica, _muscovite_, the familiar “isinglass” of the stove-door.

Chemically, muscovite should go with the potash feldspars, but it

is distinguished from them by its crystalline habit and physical

properties. The biotite should go chemically with the pyroxenes and

amphiboles, which it closely resembles except in its crystalline

properties. Details respecting the micas may be found in the reference

list, p. 464.

Two =iron oxides=, magnetite (Fe₃O₄) and hematite (Fe₂O₃) are widely

disseminated in igneous rocks. They constitute the free bases already

mentioned.

=Summary of salient facts.=—The salient facts are, therefore, (1) that

out of the seventy-odd chemical elements in the earth, eight form the

chief part of it; (2) that one of these elements uniting with the rest

forms nine leading oxides; (3) that one of these oxides acts as an

acid and the rest as bases; (4) that by their combination they form a

series of silicates of which a few are easily chief; (5) that these

silicates crystallize into a multitude of minerals of which again a few

are chief; and (6) that these minerals are aggregated in various ways

to form rocks. Possessed of these leading ideas, we are prepared to

turn to the consideration of some of the conditions under which these

combinations take place in the formation of rocks from molten magmas.

THE NATURE OF MOLTEN MAGMAS.

We easily fall into the habit of thinking of molten rock as we think

of a molten metal, merely as a substance which has passed from the

solid to the liquid condition because of high temperature. With the

return of low temperature a molten metal returns to the solid state

usually in the same molecular condition which it possessed before.

The point of fusion and the point of solidification are the same and

are rigidly fixed. If this were true of the constituents of a rock,

a definite order for the solidification of the several minerals

might be anticipated. As a matter of fact, the order is not the

same under all conditions, and, what is especially significant, the

order is far from being that in which the constituents would fuse

or would solidify separately. For instance, in a granite composed

chiefly of quartz, feldspar, and mica, the quartz is often the last

to take form, although it is more infusible than the feldspar or the

mica. This and other phenomena show that a molten magma is not to be

viewed simply as a fused substance, but rather as a _solution_ of

one silicate in another, or as a solution of several silicates in

one another mutually. The high temperature is to be regarded merely

as a condition prerequisite to solution, or as the condition of

fusion of some one constituent which then dissolves the others. If

crystals of snow, sugar, and salt be mixed at a low temperature and

compacted, the mass may be regarded as an artificial rock. On raising

the temperature, all will pass into solution while the temperature is

still somewhat below the melting-point of the snow, the most fusible,

and while it is much below that of either the sugar or the salt. This

particular case is instructive because the ice is not simply fused by

temperature; the affinity of the salt plays a part. If the temperature

were again lowered, the sugar and salt would not crystallize out at

their fusing-points, but would remain in solution down to and even

below the normal freezing-point of water; in other words, they would

remain in solution until the water crystallized out and forced them

to take the solid state. This holds good when the amounts of the

sugar and salt are small relative to the water. If, on the contrary,

their quantity is large relatively, crystallization will take place

at higher temperatures and before the water crystallizes to ice. From

this it appears that the salt and sugar might crystallize either before

the water or after it, according to the degree of concentration. The

behavior of mixtures of minerals in passing into and out of the molten

condition appears to be quite analogous to this, and hence a great

variety of results attend the process, dependent upon the number, the

nature, and the relative quantities of the ingredients. The approved

conception of the genesis of a rock from a molten magma (when ample

time is given) is that one compound after another crystallizes out

as the temperature falls and its point of _saturation_ for each is

reached, until the whole has been solidified. The modes of combination

of the elements in the molten magma are not necessarily the same as

those in the derivative crystals; indeed, the combinations doubtless

change as the process proceeds; certain constituents being taken out,

the remaining ones probably rearrange themselves.

=Time required in crystallization.=—The liquid magma of igneous rocks

is essentially a fluid glass or slag. It is analogous to common glass,

which is a silicate of potash, soda, or other base, except that

usually common glass is relatively free from iron and other coloring

substances, while these abound in the natural magmas and render them

dark and more or less opaque; but the fundamental nature is the

same, except that the natural lavas are usually mixtures of several

silicates, while the artificial glasses consist of only one, or at most

a few. Furnace slag is essentially an artificial lava.

When a lava is cooled quickly, the commingled silicates solidify in the

diffused condition essentially as they were in the liquid; for there

is no time for the silicate molecules of a like kind to come together,

particle by particle, in regular systematic order, as required in

crystallization. The essential feature of crystallization is this

systematic arrangement of the molecules according to a definite plan,

giving a specific crystal form, as a cube, a hexagonal prism, etc.

There are six (sometimes made seven) fundamental systems of

crystallization, and a multitude of variations of special form in each

system. The treatment of these forms belongs to mineralogy.

In a thick viscid liquid, this systematic arrangement of molecules

into definite crystal forms takes place slowly, for the crystalline

force in the silicates is far less energetic than that in water,

which crystallizes into ice with much rapidity and with great force.

Because of this slowness, the solidification of the lava may catch

the process of crystallization at any stage. If the lava is cooled

quickly, the result is a glass; if less quickly, part glass and part

crystals; if slowly enough, all becomes crystalline. In general the

slower the growth the larger the crystals. The solidification product

may, therefore, range from a glass to a mass of crystals; i.e., it may

be (1) wholly glass, (2) a glassy matrix with a few small crystals

scattered through it, (3) a less abundant glassy matrix with more and

larger crystals, (4) a mere remnant of glass in a mass of crystals, or

(5) a mass of crystals with no glass.

=Successive stages of crystallization.=—Since eruptions take place

intermittently, it is obvious that cooling of the lava may be in

progress in its hidden reservoir during the quiescent intervals between

eruptions. After a certain stage of partial crystallization has been

reached during such time of quiet, a renewal of eruption may take place

and the whole mass of lava be shifted into quite new conditions, and

a second phase of solidification may be superposed on the one already

started. The rock will then show two phases of crystallization: (1)

large crystals of the kind or kinds most prone to develop in the given

lava may have grown during the first long stage of slow subterranean

cooling, while the greater part of the lava still remained liquid; and

(2) small crystals or glass may have developed when the more rapid

cooling under the new conditions took place. The result would be large

crystals set in a matrix of small crystals or of glass, a combination

styled _porphyritic_. In such cases the lava, in its later stages,

carries the large crystals floating throughout its mass, and is not a

simple liquid.

THE FRAGMENTAL PRODUCTS OF SUDDEN COOLING.

=Pyroclastic rocks.=—The extreme example of sudden cooling is presented

when lavas are violently exploded into the air and solidify almost

instantly. The resulting glassy particles or filaments, if small,

constitute _volcanic ash_. The explosion appears to be due to steam and

other gases which are held in the deeper lava under great pressure, but

which, as they rise toward the surface of the lava where the pressure

is relieved, expand with explosive violence. It is probably also due in

part to progressive crystallization, which forces the gases out from

the part that crystallizes and overcharges the rest. Sometimes the

projected particles draw after themselves long _filaments_ like the

threads of spun glass, and sometimes while in the air they divide and

draw apart, spinning a filament of viscid lava between them. A variety

of this kind at the volcano of Kilauea in Hawaii is known as “Pele’s

hair.” These light filaments drift with the wind and lodge on the lee

side of the volcano, covering the surface “like mown grass” (Dana).

[Illustration: +Fig.+ 336.—Volcanic bomb. About half natural size.

(Photo. by Church.)]

When the exploded fragments are coarser they fall about the volcanic

vent and form the _tuffs_ (_tufa_) of which most steep volcanic cones

are chiefly built. In these larger fragments, crystals are not

infrequently found, and the same is even true of the volcanic ash.

These crystals are undoubtedly such as had already been formed in the

lava before it exploded, and their formation, as suggested above, may

have contributed to the explosion.

Fragments too large to be borne far away by the air, but still small,

are known as _lapilli_, especially if they are somewhat rounded and

gravel-like. A finer variety, of the nature of sand, much used in

making Portland cement, is locally known as _puzzolana_.

[Illustration: +Fig.+ 337.—Volcanic bomb of unusual form, 13 foot long.

Cinder Buttes, Idaho. (Russell, U. S. Geol. Surv.)]

The rougher, irregular fragments of a clinker-like nature ejected by

volcanoes are known as _scoriæ_ or _cinders_. They are more or less

distended by gas-bubbles and are hence light and pumiceous.

The larger masses of lava ejected into the air are often caused to

rotate by the unequal force of the projection, or by the unequal

friction of the air, and to assume spheroidal forms, the internal gases

at the same time often expanding and rendering the mass vesicular.

These rounded projectiles are known as _volcanic bombs_ (Figs. 336 and

337). Balls of lava that have originated in rolling movements of the

seething mass, or in other ways, are also styled bombs. Usage is not

altogether harmonious or consistent in the application of the term.

The larger masses that are projected into the air are more or less

vesicular from the expansion of included gases, as already noted, and

so the fragmental products of volcanic action grade into the vesicular.

The type of this class is _pumice_, in which the gas cavities make up

by far the larger part of the volume of the whole mass, and the whole

is reduced to the condition of a solidified froth or foam. So thin are

the dividing films of glassy material in some cases that the whole is

pure white, though the same material in solid mass would be dark. This

solidified glassy froth is often lighter than water and floats freely

on the sea until it becomes “water-logged” and sinks. Dredgings of

the deep sea show that much pumice has accumulated there, and being

far from the land has escaped burial by the sediments borne in by the

rivers.

All of these fragmental rocks produced by volcanic action are known

as _pyroclastic_ (fire-fragmented) rocks, a general term of much

convenience in distinguishing them from lava-flows, on the one hand,

and from the fragmental rocks produced by air and water (ordinary

clastics), on the other.

THE GLASSY ROCKS.

=The solid glasses.=—The quick cooling of lava-flows into solid glasses

is chiefly dependent on their exposure at the surface. Hence it is

often the case that the exterior of a lava-flow is glassy in greater

or lesser degree, while the interior is more or less crystalline.

Quick cooling is sometimes also due to the intrusion of the lava in

thin sheets into fissures in cold rocks. When massive bodies of lavas

penetrate solid rocks, the lava does not usually cool so fast as to

prevent some degree of crystallization, and the crystallization may

even become complete; but if the intruded lava sheet be very thin,

the lava is liable to be cooled to a nearly perfect glass. The glassy

condition is, therefore, subject to indefinite gradations. As a rule,

the acid lavas are stiffer at the same temperature than the basic ones,

and crystallize more slowly, so that acid glasses are more common than

basic ones. The basic rocks usually crystallize pretty thoroughly,

except on the immediate surface of the flows.

=The first stages of crystallization.=—The microscopic study of the

volcanic glasses reveals great numbers of minute forms known as

_crystallites_, _microlites_, _globulites_, etc., that appear to

be first steps in crystallization, though many of them do not take

definite geometrical shapes and some do not show the optical characters

of crystals. There are minute globules (globulites), needles, and

hair-like bodies (trichites) of more or less indeterminate nature,

together with other forms that can be seen to be certainly the initial

forms of well-known minerals.

[Illustration: +Fig.+ 338.—Flow structure in rhyolite. Nearly natural

size. (Photo. by Church.)]

=The obsidians.=—Of the compact glassy rocks, _obsidian_ is the best

type. It is essentially a natural glass, formed usually of acid

silicates. It has the close texture, conchoidal fracture, and other

qualities of glass. It is usually black, but sometimes red, brown,

purple, bluish, or gray. While chiefly of glass, it usually contains

more or less of the incipient crystals above described, showing

that even here the first step in the gradation to the next or the

crystalline stage has been taken. These incipient crystals sometimes

become so abundant as to change the texture from the vitreous to the

stony order. In some cases, the stony texture seems to have been

developed in the obsidian after it was formed, the change being a part

of a subsequent process of devitrification, but in other cases the

crystals seem to be original. Besides these, there are often small

globular bodies known as spherulites.

Varieties of glassy rock in which the embryo crystals are more numerous

and the glassy texture less perfect, are known as _pitchstones_. The

fresh surfaces of these have rather the aspect of pitch or resin

than that of true glass; hence their name. Like the obsidians, they

are usually dark, but they take on greenish, brownish, yellowish,

and light-colored hues as well. Sometimes glassy rock fractures in

small spheroidal forms like pearls, and is known as _perlite_. Basic

glasses are relatively rare, and while usually included under the term

obsidian, are sometimes given special names.

[Illustration: +Fig.+ 339.—Flow structure in volcanic glass. About half

natural size. (Photo. by Church.)]

[Illustration: +Fig.+ 340.—Flow structure in porphyry, shown by the

position of the large crystals. About two-thirds natural size.

(Photo. by Church.)]

[Illustration: +Fig.+ 341.—Scoriaceous texture. About four-fifths

natural size. (Photo. by Church.)]

SPECIAL STRUCTURES.

=Flow structure.=—Lavas that cool into glassy rocks frequently contain

gas cavities, colored spots and variations of texture which, together

with the hair-like embryo crystals, are drawn out into lines, streaks,

and parallel belts by the flow of the viscous mass, giving rise to

_rhyolitic_ or flow structure (Figs. 338 and 339). Rocks in which this

is the most pronounced feature were formerly known as rhyolites, though

the term has drifted away from this original meaning and has been

applied to a class of acidic rocks. The obsidians and pitchstones may

be more or less rhyolitic under the microscope, though to the naked

eye they may appear only as a glassy or resinous mass. The rhyolites

generally have but an imperfect glassy texture, since the crystals

and the cavities sometimes make up a notable part of the mass, the

glassy portion being scarcely more than a matrix in which the crystals,

spherulites, and cavities are carried. By an increase of the crystals

in number and size, the rock passes by gradations into porphyry or

phanerite.

[Illustration: +Fig.+ 342.—Porphyritic texture. Two-thirds natural

size. (Photo. by Church.)]

=Amygdaloids.=—In lava-flows the included steam often collects in

bubbles near the surface as the lava cools and forms a vesicular

portion with a scoriaceous texture (Fig. 341). In its upper part,

the vapor bubbles may be numerous, while below they become more and

more scattered until they disappear. Similar bubbles are also often

found near the bottom of a sheet of lava. This is perhaps due to the

rolling under of the frontal surface of the lava-stream as it flows.

Later, these cavities often become filled with minerals deposited from

solution and the rock then becomes an amygdaloid, but this filling is a

secondary action.

[Illustration: +Fig.+ 343.—Porphyritic texture. Natural size. (Photo.

by Church.)]

THE PORPHYRITIC ROCKS.

When the conditions are such that after a part of the magma has formed

distinct crystals floating in the remaining liquid lava, there is

a change which causes the rest to solidify as a glass or as a mass

of small crystals, the structure is known as _porphyritic_, and the

rocks possessing it are called _porphyries_. This differentiation into

distinct crystals set in a ground-mass of minute crystals or of glass

often gives a mottled or variegated aspect to the rock, especially

if the matrix of glass or minute crystals differs in color from the

distinct crystals. This structure is much oftener developed in acidic

rocks than in basic ones, because the latter crystallize more readily.

The most common porphyritic crystals are feldspar and quartz, though

they are by no means the only ones. The matrix is also usually felsitic

or quartzose, but not necessarily so. The character is a structural

one, and is not dependent upon any special chemical or mineralogical

constitution. The distinct crystals are known as phenocrysts, and the

varieties of porphyries are named from the characteristic phenocryst,

e.g., quartzophyre (quartz-porphyry) if the conspicuous crystals

are quartz, orthophyre if orthoclase, augitophyre if augite, etc. A

convenient classification has recently been proposed[200] into (1)

_leucophyre_ (white porphyries), which have a light-colored ground-mass

set with phenocrysts of any kind, and (2) _melaphyres_ (black

porphyries), which have a dark-colored ground-mass, with phenocrysts

of any kind. While it is to be hoped this usage will prevail, it is to

be noted that these terms, especially the latter, have been used in a

different sense. (See reference list of rocks, p. 445.)

In many cases the ground-mass itself becomes minutely crystalline

and the porphyritic aspect is due simply to large distinct crystals

set in a mass of minute obscure ones. The rock is then really

_holocrystalline_, but the term porphyry is applied to it. In other

rocks the crystals of the ground-mass become more and more distinct,

the porphyritic aspect gradually disappears, and there is a graduation

into the next class.

THE PHANEROCRYSTALLINE ROCKS.

=The phanerites.=—When time enough is given for the cooling process the

molten magma becomes completely crystalline. The holocrystalline rocks

hence include a large series, ranging from the most acid to the most

basic. In this class the differentiation of the rock material and the

formation of distinct minerals reach a high stage, and as a natural

result the varieties of rock are numerous. Taken as a group they are

phanerites. If they are to be more particularly characterized, it is

usually done on the basis of the minerals of which they are composed.

The following are the leading types, beginning with those which are

rich in silica and poor in basic oxides, and ending with those which

are rich in basic oxides and poor in silica.

[Illustration: +Fig.+ 344.—Granitic texture. About half natural size.

(Photo. by Church.)]

=The granites.=—The term granite was originally used to designate

a granular, i.e., a distinctly crystalline, rock, and it is still

popularly and properly so used. In scientific treatises it has usually

been confined to a special aggregate of crystals of quartz, feldspar,

and mica. It has recently been proposed to give it again a more general

application, though not quite its original one, by including under it

all holocrystalline rocks composed of dominant quartz and feldspar

of any kind, with mica, hornblende, or other minerals in subordinate

amount. In scientific literature as it now stands, granite consists of

quartz, feldspar, and mica, the feldspar being of the alkali-potash

or soda variety (orthoclase, microcline, or albite), and the mica,

either muscovite or biotite. In the type form the crystals are distinct

and sometimes large (Fig. 344). They are intimately mingled with one

another, and in growing, interfered more or less with each other and

so became interlocked. The granites are among the most common and

easily recognized of the holocrystalline rocks. Their color is mainly

dependent upon the feldspar, the red and pink varieties of the mineral

giving rise to red granite, and the white varieties to gray granite.

[Illustration: +Fig.+ 345.—Graphic granitic (or pegmatitic) texture.

Nearly natural size. (Photo. by Church.)]

Very few granites conform strictly to the type. They vary by the

addition and substitution of other minerals, and these sometimes

become as prominent as the type minerals. The soda-lime feldspars

sometimes take the place of the orthoclase, or accompany it; hornblende

and other minerals take the place of the biotite, or occur with it;

and so on. Whenever one of these replacing or accessory minerals is

notable in quantity, its name is often prefixed, as hornblende-granite,

oligoclase-granite, zircon-granite, etc. In this way the rock grades

almost insensibly into the syenites, diorites, etc. Variations also

arise from the absence of one of the three leading minerals. If mica

is absent, the rock is termed an _aplite_ (quartz and feldspar). If

the feldspar is absent, it is called a _greisen_ (quartz and mica).

If quartz is absent, it is termed a _minette_ (feldspar and mica).

These varietal terms are neither universally nor always consistently

used, and it is to be hoped they will be replaced by the systematic

nomenclature recently proposed and outlined later (p. 451).

The granites were formed from a magma rich in silica, alumina, potash,

and soda, but generally poor in lime, iron, and magnesia. Incidentally

other substances were present. The alumina, potash, and other bases

united with so much of the silica as was required to form the feldspars

and micas, and the remaining silica crystallized into quartz.

Granite is normally a massive rock without foliation or banding. If

it takes on these characters, it becomes a _gneiss_, and passes into

the foliated or schistose class of rocks, to be discussed later. The

texture of graphic granite (see pegmatite) is notably peculiar, due to

the simultaneous crystallization of the quartz and feldspar (Fig. 345).

=The syenites.=—When the mica of a granite is replaced by hornblende,

the rock is now commonly known as a _hornblende-granite_, but it was

formerly called _syenite_, because found at Syene on the Nile. The term

syenite is now applied to a rock consisting essentially of feldspar

and hornblende or mica, but there is a complete gradation from the

granites to the syenites. The magma of the syenites was richer in

iron and magnesium than the typical granitic magma. The syenites also

grade into other classes, as do the granites, and are named by similar

prefixes, as augite-syenite, etc., and some of these varieties have

special names. The syenites are red or gray, according to the color of

the feldspar, and are usually darker than the granites. The texture of

syenite is like that of granite. In the scheme of field names recently

proposed, syenite is made to include all holocrystalline rocks composed

mainly of feldspar of any kind, with subordinate amounts of mica,

hornblende, pyroxene, and other minerals, but without a noticeable

amount of quartz.

=The diorites.=—These embrace rocks which were crystallized from a

magma still poorer in silica and the alkalies, and richer in the earthy

bases. In composition they closely approach the ideal average rock,

but usually fall a little below it in silica and the alkalies, and

rise a little above it in the earthy bases. In current usage, diorite

is defined as an intimate mixture of crystals of hornblende and a

plagioclase feldspar. It differs from the syenite in having plagioclase

feldspar instead of orthoclase. By substitutions and the addition of

accessory minerals, the diorites graduate toward the granites and

syenites on the one hand, as already noted, and into gabbros on the

other.

In the scheme recently proposed, all holocrystalline rocks in which

hornblende is dominant and feldspar subordinate are classed as diorites.

=The gabbros.=—The name gabbro was formerly applied to a coarse-grained

basic rock consisting of labradorite and diallage, but the name has

been gradually extended until it embraces a large group of rocks that

have essentially the same composition as the dolerites mentioned below,

but are coarser in crystallization, and the crystals do not embrace one

another (i.e., are not ophitic). The principal minerals are plagioclase

(normally labradorite) and pyroxene (normally diallage) with magnetite

or ilmenite. They are usually dark, heavy rocks. The pearly luster of

the cleavage faces of the diallage, when present, gives a peculiar

sheen to a fresh surface of the rock. In the recently proposed field

names, gabbro is made to include all phanerocrystalline rocks in which

pyroxene predominates, attended by feldspar of any kind in subordinate

quantity, with or without hornblende or mica.

=The peridotites.=—These stand at the basic end of the series, having

been formed from a magma in which the silica was low (39–45 per cent.),

as were also the alumina, lime, and alkalies, but in which the magnesia

was relatively very high, ranging from 35 to 48 per cent. The rock

consists very largely of olivine associated with pyroxene, magnetite,

and other very basic minerals. Little or no feldspar is present. The

peridotites are much less abundant than the preceding classes and

represent a very distinctive phase of the magma in which the magnesia

was greatly concentrated.

Closely allied to the peridotites are rocks which are made up largely

of a single basic mineral, as _augitite_, _pyroxenite_, _hornblendite_,

rocks essentially formed of the minerals augite, pyroxene, and

hornblende respectively. It will be noted that in these rocks the magma

became quite simple in nature, just as at the acid end of the series

certain rocks become comparatively simple from the concentration of the

acid element, as in certain acidic granites, felsites, etc. (See pp.

523–524.)

=The basalts.=—The term basalt is used in a somewhat comprehensive

way to embrace dark, compact, igneous rocks that appear to be nearly

homogeneous, owing to the minuteness of the crystals, which are

usually so small as to be identifiable only under the microscope.

In some cases the crystals are scattered throughout a ground-mass

after the porphyritic fashion. In some of these cases there is a true

glassy base, and in such cases the rock does not strictly belong in

the holocrystalline group. In the more typical cases the constituent

minerals are very minutely crystallized and intimately intermixed. The

leading minerals are plagioclase (usually labradorite or anorthite)

and pyroxene (usually augite), with olivine and magnetite or ilmenite

usually present. There is a considerable range in chemical nature,

but the basalts are relatively poor in silica, usually also low in

potash and soda, but rich in lime, magnesia, and the iron oxides. They

are classed as basic and are sometimes highly so. The magmas of the

basalts are especially fluid, and when poured forth upon the surface

easily spread out in thin sheets. In cooling they are prone to take on

a columnar or basaltic structure, the columns standing at right angles

to the surfaces exposed to cooling. The columns are sometimes curved,

owing to the peculiar attitude of the cooling surface. The columns of

Giant’s Causeway and Fingal’s Cave are familiar examples.

=The dolerites.=—The basalts graduate insensibly into the dolerites;

indeed the dolerites may be regarded simply as basalts of coarser

crystallization. The minerals are evident to the eye and range up

to medium size. The more abundant minerals are plagioclase feldspar

(labradorite or anorthite), with one or more of the ferromagnesian

minerals (augite, olivine, or biotite), and magnetite or ilmenite. In

the growth of the minerals one crystal frequently embraces others,

giving an ophitic structure. The dolerites have many varieties, due

either to accessory minerals or to the development of some of the

constituents more amply than the rest. The type may be said to consist

of plagioclase and augite, the other minerals being regarded as

accessories. Magnetite or ilmenite is almost universally present. The

varieties are usually designated by prefixes, as olivine-dolerite,

enstatite-dolerite, etc., but special names are also used for some of

these.

[Illustration: +Fig.+ 346.—Conglomerate, Carboniferous series. Bancroft

Place, Newport, R. I. (Walcott, U. S. Geol. Surv.)]

The ancient dolerites have usually undergone internal changes and

such rocks are often called _diabases_. While the use of the term has

not been uniform, it accords with the better practice to regard the

diabases simply as partially altered dolerites and basalts. In general,

therefore, the diabases are but ancient dolerites.

_General names._

The difficulty of distinguishing many of the foregoing rocks from each

other by any means available in the field, owing to the minuteness

of the crystals, and to the gradation of one type of rock into

another, makes it desirable to employ certain general names which will

correctly express the leading character of the rock without implying a

knowledge of the precise mineral composition. A convenient term of this

kind is _greenstone_, which merely indicates that the ferromagnesian

minerals are prominent and usually give a greenish or dark cast to the

rock. The greenstones embrace the diorites, dolerites, some of the

gabbros and the basalts, and may even extend to the peridotites and

some of the more hornblendic of the granitoid rocks. Another convenient

name is _trap_, which may be used for any dark, heavy igneous rock. The

name (from _trappe_, stairs) refers to the step-like arrangement which

the edges of the superimposed sheets of lava often take, especially

when the lava is of the free-flowing, basaltic kind.

[Illustration: +Fig.+ 347.—Brecciated limestone, Calciferous formation.

One mile south of Highgate Falls, Vt.]

The term basalt is sometimes used to embrace any of the very

fine-grained dark igneous rocks. In such cases, it covers the very

fine-grained dolerites, diorites, peridotites, etc. The term granite

was used originally for any coarse-grained crystalline rock, and there

is a tendency to revive this early use. In general descriptions, some

of our best petrographers call any coarsely crystalline rock (e.g.,

coarse-grained syenites, diorites, gabbros, etc.) granite. The term

_granitoids_ may be used with strict propriety to cover all rocks of

this class.

DERIVATION OF SECONDARY ROCKS.

Rocks, though commonly made the symbol of the abiding, are subject

to constant slow changes. Through these changes newer rocks have

been derived from older ones, and still others in turn from these

derivatives, and so on in an endless chain. All derived rocks are

conveniently termed secondary, though they may be several generations

removed from the primitive rocks, and even the primitive rocks, as we

now understand them, may be themselves derived. The ordinary changes

of rock are most active at or near the surface, and the processes

of such change have already been discussed in part under the titles

“Weathering” (pp. 54 and 110), “Erosion” (pp. 119–123 and 342–349),

“Transportation” (pp. 115–119 and 354–355), and “Deposition” (pp.

177–204 and 355–363).

[Illustration: +Fig.+ 348.—Quartzitic breccia. About one-third natural

size. (Photo. by Church.)]

[Illustration: +Fig.+ 349.—Section of limestone showing abundant

fossils imbedded in a matrix made up of comminuted shell matter.

About two-thirds natural size. (Photo. by Church.)]

[Illustration: +Fig.+ 350.—Limestone composed chiefly of shells. About

three-fourths natural size. (Photo. by Church.)]

=Regolith.=—The first great product of the surface changes is

_mantle-rock_ (_regolith_), which comprehends all the loose matter

that springs from rock decay, wear, fracture, and other forms of

disintegration. It lies in an unconsolidated sheet on the face of the

land, whether as soil, sand, clay, earth, gravel, or loose rock.

=Disrupted products: arkose and wacke.=—In dry regions, in cold

regions, on mountain heights and precipitous slopes, and under other

conditions where sudden changes of temperature and frost action work

efficiently, rocks are broken down into fine fragments without much

chemical decomposition. Such _disaggregated_ rather than decomposed

matter, if derived from granitic and similar crystalline rocks, is

termed _arkose_, or arkose sand, and consists of fragments of quartz,

feldspar, mica, etc. Common sand consists essentially of quartz grains.

If the fine fragments are derived from the darker igneous rocks, and

consist mainly of grains of plagioclase feldspar, and ferromagnesian

minerals, it is sometimes called _wacke_. This term is not widely used

in just this sense, but there seems to be an important place for it,

and it will be so employed in this work. These disaggregated sands are

but special phases of the mantle-rock.

=Disintegrated products.=—When the surface-rock is chemically

decomposed, the residual material is confined mainly to the insoluble

portions, i.e., the silicious and clayey parts; while the lime,

magnesia, soda, potash, and similar substances are largely dissolved

and borne to the ocean. The potash is somewhat more disposed to remain

with the clays than the soda, lime, or magnesia; but residues of all

are usually present.

_Classes of Sedimentary Rocks._

=Shales, sandstones, and conglomerates.=—As already shown in the

discussion of the atmosphere and surface-waters, the mantle-rock is

constantly being borne away and redeposited in lodgment spots on the

land or in the basins of the sea, while it is constantly being renewed

below. It is an evanescent but ever-renewed derivative mantle. In this

process of renewal, removal, and redeposition, the mantle material is

usually assorted into mud, sand, and gravel; and these several classes

of material are laid down more or less separately, and usually take

the stratified form, because their deposition depends on different

degrees of motion of the transporting waters or wind. When these

several classes of material become cemented or otherwise hardened,

they give rise to _shales_ (cemented muds), _sandstones_ (cemented

sands), and _conglomerates_ (or pudding-stones, cemented gravel, Fig.

346). If the coarse material remains angular, they form _breccia_

instead of conglomerates (Figs. 347 and 348). For the most part, the

deposits of mud, sand, and gravel are made under the sea or in lakes

and estuaries, but they are also formed on the land in lodgment basins,

in low-gradient valleys, and on base-plains. The deposits of sediment

on land have received less recognition than they deserve. When formed

under the sea or in other life-sustaining waters, shells and other

organic material are liable to be entrapped and to form a part of the

rock. These organic remains, or fossils, greatly aid in interpreting

the deposits in which they occur. Fossils are less liable to be

preserved in sedimentary deposits formed on land. There is, therefore,

some ground to suspect that great series of sandstones and shales which

do not contain marine or fresh-water fossils were formed in lodgment

basins on land, though the absence of fossils cannot be regarded as

proof of such origin.

[Illustration: +Fig.+ 351.—Globigerina ooze. Magnified 20 times.

(Murray and Renard.)]

[Illustration: +Fig.+ 352.—Pteropod ooze. Magnified 4 times. (Murray

and Renard.)]

=Limestones and dolomites.=—Of the lime, magnesia, soda, and potash

leached out of the surface-rocks and carried to the ocean in solution,

the lime is largely extracted to form the shells, skeletons, teeth,

armor, and other hard parts of sea-animals and sea-plants. These limy

parts are at length left on the floor of the ocean and become more or

less disintegrated and help to form beds of lime-mud and lime-sand

which in time are cemented into _limestone_ (Figs. 349, 350, 351, and

352). A larger proportion of the magnesia remains in solution in the

sea-water, but in ways not yet well understood, the magnesia sometimes

unites with the lime to form _dolomite_, a double carbonate of lime

and magnesia (Ca,Mg)CO₃. This change is sometimes local, and sometimes

affects great series of beds, more commonly the ancient ones than the

modern. Sometimes the dolomization appears to have taken place long

after the original limestone was formed and probably sometimes after

it was lifted out of the sea, while in other cases it seems to have

taken place while the sediment was accumulating, or at least before the

next overlying beds were laid down. The potash in solution is to some

large extent taken up by the land- and sea-plants or is retained in the

clays, and through them becomes again incorporated in the sediments.

The soda largely remains in solution in the sea-water.

=Precipitates.=—When a portion of the ocean-water is isolated in a

region where evaporation from the surface of the water is greater than

the rainfall on it, and the inflow from the tributary basin, the lime,

magnesia, soda, potash, and other dissolved substances (solutes) are

concentrated until the water becomes saturated. The solutes are then

precipitated in the order in which they reach the point of saturation.

This order, when taken in strict and full detail, gives a very complex

series, but the leading deposits are calcium carbonate (_limestone_),

calcium sulphate (_gypsum_), and sodium chloride (_halite_ or _rock

salt_) (see p. 375). Isolated lakes in arid regions may give rise

to similar deposits. It has sometimes been thought that the ancient

limestones were produced largely by precipitation from concentrated

sea-water. While this is probably the case in some instances and to

some degree, it has not been demonstrated that the great limestone

formations were made to any large extent in this way. The more accepted

view is that the limestones in the main were made from organic remains.

The lime in solution in the ocean is chiefly in the form of the

sulphate.

[Illustration: +Fig.+ 353.—Diatom ooze. Magnified 150 times. (Murray.)]

=Iron Ore-beds.=—In a somewhat different way iron ore-deposits are

formed by the precipitation of iron oxide or iron carbonate from

solutions of ferrous compounds. The ferrous compounds in solution

were leached from iron-bearing rocks by percolating waters. The most

familiar case is that of iron-bearing springs. On exposure to the air,

the iron compounds in solution undergo change, and ferric oxide is

thrown down, usually forming _limonite_ (Fe₂O₃,3H₂O), but sometimes

_hematite_ (Fe₂O₃). This change is common in marshes and gives origin

to “bog-ore.” Similar deposits take place in certain shallow lakes, and

hence are known as “lake ore.” Iron ore sometimes also forms at the

bottom of a peaty bed or in muddy soil. In connection with the great

coal formations, beds of iron carbonate (_siderite_) occur. Organic

matter seems to play a great part both in the original solution and

the later deposition of these ores. From certain soils and clay-beds

on which the ancient coal-producing forests grew, the iron has been

almost completely removed, either by the action of the roots, or

more probably by organic acids arising from their decay and from the

decaying vegetation on the surface. On flowing into shallow bodies

of water or into marshes, the waters containing such dissolved iron

compounds usually throw down their iron content either as a carbonate

(siderite), or as a hydrous ferric oxide (limonite). The siderite is

formed where decaying vegetation is present to furnish abundant carbon

dioxide and to partially protect the iron solution from oxidation, and

the limonite where free oxidation takes place. Sand, silt, clay, or

calcium carbonate often accumulates with the iron precipitate, and the

result is an impure deposit which becomes an _ironstone_. Such deposits

often become segregated into nodules, as will be explained later. It

is thought that diatoms sometimes aid in the deposit of iron ore in

shallow waters.

=Silicious deposits.=—In the decomposition of igneous rocks, a certain

portion of the silica, as well as of the bases, is dissolved and

carried away in solution. Certain organisms extract this from solution

for their skeletons, just as others extract calcium carbonate. The

accumulation of these silicious skeletons often forms silicious

rocks. The _diatom_, _radiolarian_, and other oozes (Fig. 353) of

the deep sea are the great examples. Sometimes layers of infusorial

earth, _tripolite_, arise from the shells of diatoms and other aquatic

organisms secreting silica. The waters in which such earths accumulate

are rather shallow, and either fresh or salt. The most familiar

examples of indurated rocks formed in this general method are the

_flints_ and _cherts_ (impure flints) that occur in limestone and

chalk, chiefly as nodules, but sometimes in distinct beds.

The processes which develop coastal indentations, together with the