=Depth of source.=—Attempts have been made to ascertain the depth from

which lavas rise, by means of the earthquake tremors that accompany

eruptions. The estimates have ranged from seven or eight to thirty

miles. The mode of estimate is that discussed under earthquakes, and is

subject to the corrections there indicated. If these could be perfectly

applied, the estimates might probably all fall within ten miles, and

not improbably all within six miles of the surface. But in any case the

method really tells very little as to the true point of origin of the

lava. At most it probably only tells where the ascending lava begins

to _rupture_ the rock through which it passes, and rupture may not be

possible below the zone of fracture, which is probably not more than

six miles deep. In the zone of flowage below, where the pressure is too

great to permit fracture, the lava not improbably makes its way by some

boring or fluxing process, which might not, because of its nature, be

capable of giving rise to seismic tremors. The behavior of the tremors

perhaps forces us to locate the origin of lava movement _at least as

low as the bottom of the fracture zone_, but it probably offers no

sufficient ground for limiting the lava’s origin to this or any other

specific depth.

VOLCANIC GASES.

The most distinctive feature of volcanoes is the explosive action

arising from the gases and vapors pent up in the lava. There is not a

little explosive action of a secondary character arising from the mere

outer contact of surface-waters with lavas or with the hot rocks of the

crater walls, or with the hot ashes and rocks thrown out; but these are

incidental, not essential, features.

The precise nature of the occlusion or absorption of gases and vapors

has not yet been determined. It is thought that lava spontaneously

absorbs such gases when at high temperatures, and especially when the

gases are under great pressure, and that as the pressure is relieved

and the lava is cooled and solidified, the larger part of the gases

escapes. In those cases in which the eruption is quiet, the escape

of the gases is but partial while the lava is in the crater, and

much gas remains to be given out from the molten material after it

has been extruded and is about to congeal. The gases are then given

off with relative slowness and quietness. If, however, the lavas are

surcharged with gases, and if these are restrained from free escape

by the viscosity of the lavas, the gases gather in large vesicles in

the lava in the throat of the volcano, and on coming to the surface

explode, hurling the enveloping lava upwards and outwards, often to

great distances. The violence of projection reduces a portion of the

lava to a finely divided state constituting the “ash” and “smoke” of

the volcano. Other portions less divided are inflated by the gases

disseminated through them, and form “pumice” and “scoria,” according

to the degree of inflation, while masses of lava that have already

solidified into more or less rounded masses in the crater are hurled

forth as “bombs”; not infrequently portions of the walls of the crater

or of the duct below are also disrupted and shot forth.

=Differences in gas action.=—The causes of the differences of gas

action in different volcanoes are undetermined, but the following

suggestions may point to a part of the truth: (1) Doubtless some lavas

contain more gases than others, and hence are predisposed to be more

explosive; (2) some are more viscous than others and hence hold the

gases more tenaciously until they accumulate and acquire explosive

force, while the more liquid lavas allow their gases to escape more

freely and easily; (3) some are hotter than others, and hence hold

their gases until after they have escaped from the crater, when they

give them off from their expanded surfaces in the open air, where there

is no restraint to develop explosiveness; (4) some flows are so massive

that they cool to the chief gas-discharging point only after they are

spread out on the surface, when quiet escape is possible; (5) probably

a main occasion of the very violent explosions lies in the fact that

the lavas have begun to crystallize while yet in the duct of the

volcano. The crystals, in forming in the magma, exclude the gases from

themselves, and this excluded portion overcharges the remaining portion

of the lava. This process continues as the lava rises and grows cooler

until the gases acquire great volume and explosive force. This view is

sustained by the fact that the pumice and ash of such extraordinarily

explosive eruptions as those of Krakatoa and Pelée contain many small

crystals which had certainly formed before the explosive inflation took

place. Incipient crystallization does not, however, appear to be a

universal accompaniment of explosive action.

=Spasmodic action.=—The discharge of the gases is spasmodic, and

usually consists of a succession of distinct explosions. Sometimes

these succeed one another at rather constant and frequent intervals,

as in Stromboli, where the explosions follow one another at intervals

of three to ten or more minutes. In many others the outbursts are

rhythmic, while in others the spasms are distant and irregular.

=Kinds of gases.=—Steam is the chief volcanic gas. Its constituents,

hydrogen and oxygen, are also present in the free state, and are

perhaps the result of the dissociation of the steam at the very

high temperatures of the lavas. Carbon dioxide is probably next in

abundance. No positive statement as to the relative amounts of the

subordinate gases can be made because of the obvious difficulties

of obtaining anything like a representative analysis of the gases

concerned in the great volcanic eruptions. The materials for the

analyses which have been made were derived chiefly from little

secondary or “parasitic” vents, or from side-wall crevices, through

which the volcanic gases rise. Such vents probably derive their gases

from the very border of the main mass, where it is most subject to

the influence of waters and gases from the adjacent walls, and it is

uncertain how far they are truly representative of the gases in the

interior of the lava itself. The data now at command seem to indicate

that carbon dioxide increases greatly in relative abundance as volcanic

action dies away. Great quantities of this gas are often given forth

long after all signs of active vulcanism have disappeared. Such gases

have been attributed to the action of the lavas on buried beds of

limestone or other carbonates, but in many cases the geology of the

region offers no special support to this hypothesis. It does not seem

inherently probable that the heat of the lava would be sufficient to

decompose limestone at a period very long after the active eruption. An

alternative suggestion is that the stronger volcanic acids mentioned

below are gradually conveyed into the adjacent rocks and there act on

limestones or on partially carbonated crystalline rocks, setting free

carbon dioxide. Whatever may be true with regard to secondary gases of

this kind, it is quite certain that the lavas themselves contain large

quantities of carbon dioxide, and also of carbon monoxide, doubtless

reduced from the dioxide. Sulphur gases are very common accompaniments

of volcanic eruptions. They take the forms of sulphuretted hydrogen

and sulphurous acid and perhaps of sublimated sulphur, all of which

are liable to pass by oxidation and hydration into sulphuric acid.

Chlorine and hydrochloric gases are also common, particularly at high

temperatures. Fluorine and other gases are occasionally present.

Certain gases, such as hydrogen and chlorine, are especially associated

with high temperatures and energetic action, and are probably dependent

on them. Hydrochloric acid and the sulphurous gases are also mainly

associated with high temperatures, while sulphuretted hydrogen is

commoner at lower temperatures. Oxygen, nitrogen, and probably

carbon dioxide or carbon monoxide are present throughout all ranges

of temperature. Nitrogen is a rather frequent but not very abundant

constituent of the volcanic gases. How far it results from admixture

of the atmosphere and how far it is original, is not determined. It

is, however, one of the gases found in volcanic rocks after they have

cooled, and is presumably original in part. A large series of secondary

vapors naturally arise from the volatilization of substances contained

in the lavas, such as the oxides, chlorides, and sulphides of the

metals, etc.

=Residual gases in volcanic rocks.=—Some light upon the vital question

of the original, as distinguished from the secondary gases of lavas may

be found in the analyses of the gases that remain in the lavas after

they are solidified. When the lavas lodged underground without free

communication with the surface, there is reason to think that they

retained a larger percentage of their original gases in solidification

than in cases of free exposure at the surface; at any rate, such

rocks contain notable quantities of gases occluded in some way within

themselves. Recent surface-lavas also contain gases of similar kinds,

but not in equal degree, so far as available analyses show. The gases

are in part held in numerous small cavities within the constituent

minerals, especially in the quartz. This is perhaps due to the fact

that quartz usually crystallizes late in the process of solidification,

and its mother-material becomes crowded with gases excluded by the

previous crystallization of other minerals. Analyses of twenty-five

crystalline rocks of various kinds from many typical localities by

Tilden,[285] gave an average volume of gas, under ordinary atmospheric

pressure, four and a half times that of the containing rock. This shows

the condensed condition in which the gases are held. Of these gases,

the chief is hydrogen, which much exceeds all the rest. Next in order

of abundance is carbon dioxide, followed by carbon monoxide, marsh

gas (CH₄), and nitrogen. Water is frequently present and free oxygen

almost universally absent. The average ratio of hydrogen to carbon

dioxide by volume in these analyses is about 70 : 30. Five complete

analyses gave the following averages: H₂, 52.134; CO₂, 34.104; CO,

8.422; CH₄, 3.224; N₂, 2.072. It will be seen that the gases contained

in these rocks are in proportions radically different from those of

the atmosphere, and it is doubtful whether they can be reasonably

assigned to any other source than the lavas from which the rocks

were formed. It is to be noted, however, that some sedimentary and

meta-sedimentary rocks, such as quartzite and quartz-schist, contain

similar gases, but this may be because the granules of the original

rock retain them, notwithstanding the secondary processes through

which they have passed. Analyses of meteorites show essentially the

same gases in much the same proportions. If evidence of this kind can

be trusted, the standard original gases of lavas are the elements or

compounds of hydrogen, carbon, and nitrogen, in the order named, while

the chlorine and sulphur gases are to be regarded as accessory. Because

of their intensely energetic and noxious character, these latter

gases make themselves disproportionately manifest in the vicinity

of active volcanoes. That they are really not preponderant seems to

be implied by the fact that the volcanic rains, which are extremely

copious, are usually fresh, and only in rare cases is the presence of

the hydrochloric or sulphurous elements sufficient to produce noxious

effects. Volcanic and meteoric data seem to indicate that steam is held

less tenaciously than the other gases in the magmas as they solidify

into rocks.

=The source of the gases.=—As already noted, it is one of the

outstanding problems of geology to determine (_a_) how far the gases of

lavas were possessed by them from their origin, whatever that may be,

and (_b_) how far they have been acquired in the lava’s ascent to the

surface. It is recognized that lavas have the power of absorbing gases,

and one of the views entertained is that surface-waters, percolating

through the rocks and coming in contact with the ascending column of

lava, are converted into steam, which is absorbed into the lava and

rises with it to the surface. There are two phases of this view. (1)

The more conservative one supposes that the water merely penetrates

the fracture zone of the surface of the earth through the ordinary

means of passage of underground-waters, and so makes a comparatively

short circuit. Under this view, the steam and other gases given forth

are not a contribution to the atmosphere and hydrosphere, but merely

a restoration to them of water and dissolved gases previously carried

down from the surface. An even narrower view is sometimes entertained

which supposes that the larger part of the water descends through the

volcanic cone itself, or immediately about its base. The presence of

chlorine gases in the volcanic emanations and the nearness of most

existing volcanoes to the sea have been the basis for the idea that

sea-water, penetrating to the lava, is a chief source of the volcanic

gases. (2) The broader phase of the view assumes that the waters

penetrate not only the outer fracture zone of the earth, which is

probably limited to five or six miles in depth, but that they diffuse

themselves through the continuous unfractured zone down to depths

where temperatures of fusion prevail, and that they there enter into

combination with the lavas or with hot rock to form lavas. It is well

known that aqueous vapor facilitates the fusion, or more accurately,

the mutual solution of the minerals. This view is a part of one of the

hypotheses concerning the origin of the lavas themselves.

The opposing view supposes that the gases were in the main original.

Of this view there are two phases: (1) One supposes that the lavas

are remnants of an original molten globe which absorbed gases from

the primitive atmosphere and retained them till the time of their

eruption. The possible absorption of steam and air into the supposed

molten globe has been much neglected in current conceptions of

early conditions. If the lavas of the supposed molten globe absorbed

proportionately as much water-vapor as the volcanic lavas often

contain, it would probably take forty or fifty times the present ocean

and atmosphere to supply them. Any remnants of these original lavas

might well be supposed to hold gases. Even rocks derived from them by

deep-seated solidification might retain much gas. (2) The other phase

of the view assumes that the gases were entrapped when the globe was

built up of meteoroidal or planetesimal matter, as assumed in the

accretion hypothesis.

Under either of the last two views the gases may be said to be primary,

and genetically connected with the origin of the lavas themselves. Such

gases would be a contribution to the atmosphere and the hydrosphere.

This view does not exclude the idea that as the lava rises through the

surface-rocks, other gases are formed by contact, and that they may be

absorbed into the rising column. On the contrary, the view recognizes

the possibility that a tongue of lava rising into the upper formations

may encounter bodies of water, or masses of thoroughly water-soaked

rock, from which great quantities of steam may be generated, and that

this accessory steam may be a large factor in the initial explosion

which often accompanies the development of a new volcano, or the new

eruption of an old one after a long period of quiescence.

A decision on the vital question whether the volcanic gases are largely

primary, or are essentially secondary, has not yet been reached;

but it will doubtless be reached when a sufficient number of really

representative analyses of volcanic gases have been made, and when the

phenomena of the gases occluded in igneous rocks have been thoroughly

investigated.

The peculiar proportions of the rock-gases, in which hydrogen and

carbon dioxide so greatly preponderate, seem to imply that they are

not derived from the atmosphere; at least if they were so derived,

there must have been a selective absorption of a most remarkable kind,

because hydrogen is present in the atmosphere in exceedingly small

quantities, while carbon dioxide is a very minor constituent. At the

same time, as already remarked, no free oxygen is usually found in

these absorbed gases.

The question as to whether the larger part of the volcanic gases is

original or is merely a special form of convective circulation, has an

important bearing on the supply of the atmosphere, which is constantly

being depleted by the oxidation of the rocks and by the formation of

carbonates and carbonaceous deposits. This vital phase of the subject

will receive further consideration. While recognizing the lack of

decisive proof, it would seem that the preponderance of evidence lies

in favor of the view that a notable portion, at least, of gaseous

volcanic emanations is derived from the interior of the earth, and

is really a contribution to the atmosphere and the hydrosphere. The

hydrogen, on coming in contact with the atmosphere, ignites and adds

itself to the hydrosphere. The carbon dioxide is in part decomposed by

plants, and adds to the supply of atmospheric oxygen. The nitrogen,

being comparatively inert, doubtless gradually accumulates in the air

and has thus come to be its preponderant constituent.

THE CAUSE OF VULCANISM.

The extraordinary facts involved in volcanic phenomena cannot well be

discussed fully until the origin of the earth is considered, and the

great agencies, as well as the peculiar conditions, which the earth

inherited from its birth, are duly weighed, for these were, with little

doubt, the true causal antecedents of vulcanism. We shall return to

the subject after a sketch of the early conditions of the earth, but

the views that have been entertained may be reviewed here while the

phenomena are fresh in mind.

The explanation of vulcanism involves two essential elements. These

are (1) the origin of the lavas, which involves a consideration of the

necessary temperatures, pressures, and other conditions, and (2) the

forces by which the lavas are expelled.

Nearly all current explanations of vulcanism are founded upon

conditions supposed to be derived from a molten globe, and fall under

two general classes: (I) those which assume that the lavas are residual

portions of the original molten mass, and (II) those which assign the

lavas to the local melting of rock.

I. _On the Assumption that the Lavas are Original._

In this case it is not necessary to assume any special accession of

heat, but merely to account for extrusion. There are two phases of

this view, (1) the one postulating a general molten interior, (2) the

other limiting the molten matter to local reservoirs.

=Hypothesis I. Lava outflows from a molten interior.=—In the early

days of geology, when the earth was supposed to have a thin crust and

a molten interior, it was very naturally assumed that volcanoes were

but pipes leading down to the molten mass within. This view has been

essentially abandoned. The independence of adjacent vents is in itself

almost a fatal objection, when it is recalled that the height of recent

volcanic craters ranges from nearly 20,000 feet above the sea, to

10,000 to 20,000 feet below. The view would involve the conception of

lava-columns connected with a common reservoir varying possibly 30,000

to 40,000 feet in altitude, and certainly more than half that much,

simultaneously. The lower outlets should as certainly be selected for

the outflow of the great interior sea of fluid rock, as the lowest

sag in the rim of a lake for its outflow, for no great differences in

specific gravity are presumable under this hypothesis. An equally grave

objection arises from tidal strain. If the earth were liquid within and

merely crusted over by a shell of rock of moderate thickness, it would

yield appreciably to tidal stresses, and this yielding would change the

capacity of the interior so that with every distortion of the spheroid

a portion of its fluid interior would be forced to the outside, and

with every return to the more spheroidal form there would either be

a re-flow to the interior or a shrinking of the crust. In any case a

very demonstrative response to tidal influence would tell the story of

interior fluidity. No such effects are observed. The tidal strains may

perhaps have a slight effect in hastening a given eruption when the

forces are approaching a delicate balance and an eruption is imminent,

but the very triviality of this influence implies not only the absence

of a general liquid interior, but also of extensive reservoirs.

=Hypothesis 2. Lavas assigned to molten reservoirs.=—A modification of

the preceding view has been made to escape the difficulties involved

in the hypothesis as stated above. It is supposed that while nearly

all the subcrust solidified, numerous liquid spots were left scattered

through it. This honeycombed substratum is supposed to connect the

continuous outer crust with a central solid body, solidified because

of pressure in spite of its high temperature. This hypothesis escapes

only a portion of the objections. For instance, the lavas in Mauna Loa

and Kilauea in Hawaii differ nearly 10,000 feet in height, and hence

cannot well be supposed to connect with the same reservoir, but they

are both on the same vast cone, which implies at least an equally large

molten reservoir as its source. If there were two distinct reservoirs

of the required magnitude, they must be singularly placed to supply

vents so near and yet so independent. The difficulty grows greater when

the whole Hawaiian chain is considered, for the points of eruption seem

to have migrated from the northwesterly islands, where the volcanoes

are old, to the southeastern end, where volcanic activity is now in

progress.

It would be natural under this view to suppose that these residuary

lakelets of liquid rock should be gradually exhausted as time goes

on, and that vulcanism should be a declining phenomenon. It is not

clear that this is the case. The great number of existing volcanoes in

regions where great extrusions took place in earlier ages does not seem

to be in harmony with the hypothesis.

II. _On the Assumption that the Lavas are Secondary._

The serious difficulties that arise in interpreting volcanic lavas

as remnants of an original molten mass, and the strong arguments of

recent years for a very solid earth, have turned inquiry chiefly

toward the second class of hypotheses, which refer the origin of

lavas to the local melting of deep-seated rock. These differ widely

among themselves. One group seeks for a cause of the melting in

the penetration of surface air and water; another, in the relief

of pressure; a third, in crushing and shearing; a fourth, in the

depression of sediments into the heated interior zone; and a fifth, in

the outward flow of deep-seated heat.

=Hypothesis 3. Lavas assigned to the reaction of water and air

penetrating to hot rocks.=—As steam is one of the great factors in the

explosions of volcanoes, and as water reduces the melting-point of

rocks, it is a natural and simple view that water penetrating through

the fissures and pores of the outer crust and coming into contact with

the heated rocks below, is absorbed into them and renders them liquid,

and that then, being rendered swollen and lighter by the process, they

ascend and discharge quietly or explosively according to the special

conditions of the case. Naturally the suggestion arises that the waters

would be converted into steam long before they could reach rock hot

enough to be melted, and that this steam would be forced back along

its own track, as the line of least resistance, rather than force

itself into the rock material and rise in the lava-column; but to

this it is answered that an experiment of Daubree’s has shown that

water will penetrate the capillaries of sandstone against high steam

pressure and add itself to the steam within. The fact is also cited

that certain substances, when highly heated, absorb gases which they

give out when they cool. The absorption of hydrogen by platinum, and

of oxygen by molten silver, are illustrations. It is certain that the

lavas do contain large quantities of absorbed gases, and that these

are partly, and in most cases largely, given out in cooling, when the

cooling takes place at the surface. The presumption is that the lavas

would take the gases up again on remelting under similar conditions.

If the lavas of actual volcanoes had the temperatures of aqueo-igneous

fusion (700°-1000° Fahr.) only, it would strengthen this view; but as

temperatures of lavas often exceed 2000° Fahr., and probably sometimes

reach 2500° Fahr., and perhaps 3000° Fahr., it is not easy to account

for such temperatures under this hypothesis, because they would only

be reached at levels far below those at which aqueo-igneous fusion

might be presumed to take place. Perhaps this could be met by invoking

pressure which might prevent even aqueo-igneous fusion from taking

place until these temperatures were reached, but pressure brings in a

grave difficulty in another line, as we shall presently see.

There is a phase of the water-penetration hypothesis which seeks to

account for an accession of heat. It is affirmed that the outer rocks

are oxidized, while the inner ones were not originally, or at least

not completely oxidized, and that air and water from the surface,

reaching the unoxidized zone, enter into combination and generate the

necessary heat. This view was pardonable before the development of

modern thermo-chemistry, but is now quite untenable, as may be shown by

working out the reactions thermally.

All views which assign the penetration of surface air or water as

a cause meet with a grave, if not insuperable, difficulty in the

condition of the lower part of the earth’s crust (see p. 218). The

fractured condition of the crust, which permits a ready penetration of

water, is a very superficial phenomenon. Below the first few thousand

feet the crevices and porosities of the rock are rapidly closed by the

pressure of overlying rock, and all appreciable crevices and pores

probably disappear at a depth of five or six miles. The effective

function of fissures is, therefore, limited to the upper few miles of

the crust, and even here to certain portions only. The great pressures

in gas- and oil-wells show that in many quite superficial beds, even

when arched, there are no fissures or pores capable of letting even

gas escape effectively. The depths at which the temperatures of lavas

are reached are usually estimated, from the downward increase of

temperature, at 20 to 30 miles. This leaves from 14 to 24 miles of the

compressed zone between the lowest assignable limit of the fissured

zone, and the highest assignable zone for the origin of lavas. This

thick zone of dense rock must be reckoned with in all hypotheses that

involve the penetration of air and water from without, and, as well,

the extrusion of lavas from within. In addition to the difficulties

of the penetration of ground-water, the limitations of its heat, at

penetrable depths, also bear adversely (see p. 219), on the descent of

air and water.

=Hypothesis 4. Lavas assigned to relief of pressure.=—It seems to

be demonstrated that pressure raises the melting-point of average

rock, and hence at twenty or thirty miles’ depth there may be rocks

hot enough to melt at the surface, but still solid because of high

pressure. If this pressure were in some way relieved they would become

liquid. Pressure may be locally relieved somewhat (1) by denudation,

(2) by certain phases of faulting, (3) by anticlinal arches, and (4) by

continental deformation.

(1) In most cases of denudation, cooling below probably keeps pace

with loss above. At any rate, many volcanoes rise from the bottoms

of the oceans where no denudation takes place, and this phase of the

hypothesis is not workable there.

(2) The theory of relief by faulting finds encouragement in the fact

that many volcanoes occur on fault-lines. There is no evidence,

however, that this is a universal or necessary relation. Computation as

to the amount of lowering of the melting-point that might arise from

the faulting associated with volcanoes indicates that it is necessary

to suppose that the rocks were already very close to the melting-point

when the faulting took place, to make the doctrine applicable. It is

to be observed that in faulting the relief of pressure on one side

of the fault-line is likely to be balanced by increased pressure on

the other side, and that this difference in pressure may be lost by

distribution at a depth of 20 or 30 miles, where, at the nearest, this

delicate relation between solidity and liquidity, on which the theory

is dependent, may perhaps be reached.

(3) Immediately under an anticlinal arch there may doubtless be some

relief of pressure within the limits of strength of the arch, which is

not great (p. 582). The pressure under the arch _as a whole_ is greater

than before it was bowed up by lateral thrust, and in depth this excess

becomes distributed so as to obliterate the local relief under the

center of the arch, and so _adds_ the effects of folding to the average

pressure of the crust. Besides, as a matter of fact, volcanoes do not

appear to be especially associated with mountain folds where arching

reaches its best expression.

(4) The same general considerations bear on the assignment of

liquefaction to relief of pressure in connection with the more general

deformation of the earth’s body. Besides, while relief of pressure

might account for liquefaction, it leaves the extrusion without an

obvious cause; indeed, it would seem to furnish a condition opposed to

extrusion, and if pressure were subsequently added to force the liquid

out, it would tend to restore the solid condition.

=Hypothesis 5. Lavas assigned to melting by crushing.=—Mallet[286]

and others have attributed melting to the crushing of rock. Crushing,

in the ordinary sense of the term, can only take place in the zone of

fracture, and that is apparently too shallow to meet the requirements

of the case. Below this zone, the pressure on all sides is too great to

permit any separation of fragments, and a solid mass can only change

its form by what is called “solid flowage.” The rock under these

conditions may be compressed, and this compression must give rise to

heat, but at the same time the melting-point is raised, according to

all experiments. It seems improbable that melting can be produced in

this way. If great pressure could be brought to bear upon a tract of

rock so as to heat it by compression, and if then the pressure were

relaxed before the heat generated could be distributed by conduction,

and if re-expansion did not follow, possibly melting could be effected,

but this makes the process complicated and apparently inapplicable. It

is scarcely possible that such a sequence of events can have affected

all the tracts that are now volcanic, much less all those that have

been such throughout geologic history. As noted in the preceding

case, relaxation would seem to be unfavorable to expulsion. Besides,

volcanoes do not seem to be confined to tracts that show signs of great

crumpling and crushing, as the Alps, the Appalachians, and the closely

folded ranges generally. Extrusions seem rather more common with

faulted ranges where crushing is less notable and where surface tension

replaces compression.

=Hypothesis 6. Lavas assigned to melting by depression.=—It is observed

that in certain regions great thicknesses of sediments have accumulated

by the slow settling of the crust below, and as these sediments

obstruct the outward flow of heat while the lower beds settle nearer

to the interior source of heat, it is conceived that they become

heated below and, being saturated with water, take on aqueo-igneous

fusion and rise as lavas, well supplied with internal gas and steam

from the water and volatile constituents that were entrapped and

carried down with them. The question obviously arises whether such

depression is sufficient to give the temperatures the lavas show,

and whether volcanic action is confined to such areas of depression

and deep sedimentation. At the highest credible estimates—which are

none the less to be taken with reserve—the post-Archean sediments

rarely reach five or six miles in thickness at any given point, and

probably never exceed ten or twelve, while twenty or thirty miles is

the computed depth required for the acquisition of the temperatures

the lavas actually possess. If the Archean terranes be included among

the sedimentaries, the thickness may be adequate, but what then of the

Archean vulcanism, which much surpasses that of later times, and the

other early extrusions before the sediments were thick; and what of the

moon, where there are probably no sediments at all?

Besides, it is not at all clear that the distribution of vulcanism is

specially related to that of thick sediments, as it should be if this

hypothesis were the true one. There are many volcanoes in the heart of

the great oceans where sedimentation is now inappreciable, and probably

has been in all past periods.

=Hypothesis 7. Vulcanism assigned to the outflow of deep-seated

heat.=—If the earth grew up by slow accessions of meteoroidal or

planetesimal matter, in a manner to be more fully set forth in the

discussion of the origin of the earth, and if its interior heat be due

chiefly to compression by its own gravity, the internal temperature

would be originally distributed according to the degree of compression,

and this would depend on the intensity of the internal pressure. This

can be approximately computed, and is shown in the diagram on page 563,

where this subject has been treated. On not improbable assumptions

regarding the thermometric conductivity, the flow of heat from the

deep interior to the middle zone would be greater than the loss of

this zone to the superficial zone. This middle zone should, under

this view, experience a rising temperature. By hypothesis, this zone

is composed of various kinds of matter mixed as they happened to fall

in. Hence as the temperature rises, the fusion-points of some of these

constituents will be reached before those of others. More strictly,

the temperatures at which some of these constituents will mutually

dissolve one another will be reached, while other constituents remain

undissolved, and thus a partial and distributed liquefaction will

arise. The gases and volatile constituents in the mixed material would

naturally enter largely into the liquefied portion. It is assumed

that with a continued rise of temperature, the partial liquefactions

would increase until the liquefied parts found means of uniting, and

the lighter portions, embracing the gaseous contingent, were able to

work their way toward the surface. As they rose by fusing or fluxing

their way, the pressure upon them became less and less, and hence the

temperature necessary for liquefaction gradually fell, leaving them a

constantly renewed margin of temperature available for melting their

way through the upper horizons. Thus it is conceived that these fusible

and fluxing selections from the middle zone might thread their ways up

to the zone of fracture and thence, taking advantage of fissures and

fractures, reach the surface. It is conceived that such liquefaction

and extrusion would carry out from the middle zone the excess of

temperature received from the deeper interior, and thus regulate its

temperature and forestall general liquefaction, the zone as a whole

remaining always solid. The independence of volcanoes is assigned

to the independence of the liquid threads that worked their way to

the surface. Nothing like a reservoir or molten lake enters into the

conception. The prolonged action of volcanoes is attributed to the

slow feeding of the liquid threads from the locally fused middle zone.

The frequent pauses in action are assigned to temporary deficiencies

of supply; the renewals to the gathering of new supplies after a

sufficient period of accumulation. The distribution of volcanoes in

essentially all latitudes and longitudes is assigned to the general

nature of the cause. The special surface distributions are assumed to

be influenced, though not altogether controlled, by the favorable or

unfavorable conditions for escape presented by the crustal segments of

the earth. The persistence of volcanic action in time is attributed

to the magnitude of the interior source, to its deep-seated location,

and to the slowness of conduction of heat in the earth’s interior. The

force of expulsion is found in the stress-differences in the interior,

particularly the periodic tidal and other astronomic stresses (see p.

580), and in the slow pressure brought to bear on the slender threads

of liquid by the creep of the adjacent rock. The violent expulsions are

due to the included gases, of which steam is chief. Little efficiency

is assigned to surface-waters, and that little is regarded as wholly

secondary and incidental. The true volcanic gases are regarded as

coming from the deep interior and as being true accessions to the

atmosphere and hydrosphere. The standing of the lavas in volcanic ducts

for hundreds and even thousands of years with only small outflows,

as in some of the best-known volcanoes, is regarded as an exhibition

of an approximate equilibrium between the hydrostatic pressure of

the deep-penetrating column of lava, and the flowage-tendency of the

rock-walls, the outflow being, of course, also conditioned on the

slow rate of supply below, and the periodic stress-differences of the

interior.

For the present these hypotheses must be left to work out their own

destiny, serving in the mean time as stimulants of research. All but

the last have been for some time under the consideration of geologists,

and are set forth in the literature of the subject (p. 636).

A few special phases of the problem need further discussion, though

they have been incidentally touched upon.

_Modes of Reaching the Surface._

All of the views that locate the origin of the lavas deep in the earth

must face the difficulties of the passage through the dense portion of

the sphere below the fracture zone. Near the surface, the lavas usually

take advantage of fissures or bedding-planes already existing or made

by themselves. There is little evidence that they bore their way by

melting, though they round out their ducts into pipes as they use them,

much as streamlets on glaciers falling into crevices round out moulins.

But this use of fissures and bedding-planes for passage is probably

merely a matter of least resistance where the lavas are relatively

cool, and their capacity for melting is low or perhaps even gone.

Daly has recently urged that lavas work out reservoirs and enlarge

passageways for themselves by detaching masses of rock from the roofs

and sides of the spaces already occupied by them, these masses either

melting and mingling with the lava, or else sinking to lower positions

in the column. This process he designates _stoping_.[287]

In the denser and warmer zone below, the alternatives seem to be (1)

melting or fluxing, or (2) mechanical penetration without fracture. As

rocks “flow” in this zone by differential pressure without rupture,

an included liquid mass may be forced to flow through the zone by

sufficient differential pressure. If local differential pressures at

the surface be neglected as probably incompetent, there only remain the

stress-differences of the interior and the differences of hydrostatic

pressure between the lava-column and the surrounding solid columns.

The latter would not be great until a column of liquid of much depth

was formed, and the former would probably not be concentrated on the

liquid in such a way as to force it bodily through the solid rock.

Probably fusing or fluxing its way with the aid of stress-differences

is the chief resource of the lava in the initial stages. In this it

may be supposed to be assisted by its gases, by its selective fusible

and fluxing nature, by its very high temperature if it comes from

very great depths, as held in the seventh hypothesis, and by the

stress-differences which prevail in the deep interior, as shown in the

last chapter. In ascending from lower to higher horizons, the lava

would be constantly invading regions of lower melting-point because of

less pressure. It would thus always have an excess of heat above the

local melting temperature until it invaded the external, cool zone,

where the regional temperature is below the melting-point of surface

pressure. From that point on it must constantly lose portions of its

excess of temperature by contact with cooler rocks, and probably in

the process of fluxing its way in the compact zone. If this excess is

insufficient to enable it to reach the zone of fracture, the ascending

column is arrested and becomes merely a plutonic pipe or mass. If

it suffices to reach the zone of fracture, advantage may be taken

thereafter of fissures and of rupturing, and the problem of further

ascent probably becomes chiefly one of hydrostatic pressure, in which

the ascent of the lava-column is favored by its high temperature

and its included gases. The hydrostatic contest is here between

the lava-column, _measured to its extreme base_, and the adjacent

rock-columns, _measured to the same extreme depth_. The result is,

therefore, not necessarily dependent on the flowage of the outer

rocks, but may be essentially or wholly dependent on the deep-seated

flowage of the rock of the lower horizons. The ascending column may

reach hydrostatic equilibrium before it reaches the surface, and may

then form underground intrusions of various sorts without superficial

eruption, or it may only find equilibrium by coming to the surface and

pouring out a portion of its substance and discharging its gases.

_Additional Considerations Relative to the Gases._

The question whether the volcanic gases are a contribution to the

atmosphere and hydrosphere is so important in its bearings on the whole

history of the atmosphere as to merit additional consideration here. As

already noted, if the volcanic gases arise from water and absorbed air

that have previously passed down through the strata, there is no real

contribution to the hydrosphere and atmosphere, but merely circulation.

If the gases are chiefly derived from the deep interior, they are an

important accession to the atmosphere and hydrosphere.

Most views are more or less intermediate, assigning a part of the

gases to the interior and a part to the exterior. No one will question

that some part at least of the steam is due to the contact between the

ground-waters and the hot lava, and probably no one will question that

some gas comes from the interior if the lavas originate there. The

vital question is, whence comes the major portion? Are the constant

ebullitions of some volcanoes and the terrific explosions of others due

mainly to surface-waters, or to interior gases?

It seems to be certain that in most cases the gases are diffused

through the substance of the lava, and are not simply in contact with

the walls of the column or with its summit. Without doubt steam is

generated around the lava-column by external contact, and perhaps some

explosions are due to the entrance of the rising lava upon a crevice

or cavern filled with water, or to the invasion of a lake gathered in

an old crater; but it still remains a question whether the importance

of such explosions has not been exaggerated. Such action does not

seem competent to produce _inflated_ lavas, but merely shattered

ones. Water thus “suddenly flashed into steam” could scarcely diffuse

itself intimately through the lava, for the process of diffusion is

exceedingly slow. But inflated lavas, pumice, scoriæ, and cinders are

the typical products of explosive vulcanism. Not only in the ordinary

Vesuvian type, but in the extraordinary Krakatoan type, inflated lavas

are the dominant product. Prodigious quantities of this covered

the sea about Krakatoa after its tremendous explosion in 1883. Judd

estimates that the volume of included steam involved in the inflation

of the pumice examined by him, was from three to five times that of the

rock, and that the amount involved in exploding the lava into the fine

dust that floated in the upper atmosphere for months, was presumably

much greater.

If the sudden flashing into steam of bodies of water in external

contact with hot lava be rejected as only an incidental source of

explosion, it remains to be considered whether waters permeating the

rock and becoming converted into steam may not be absorbed into the

rising lava, become diffused through it, and ascending with it, explode

at the surface. So far as access through fissures and cavities large

enough to be entered by lava are concerned, it may safely be concluded

that since the hydrostatic pressure of the lava must be greater than

that of the water in the fissures, or else it could not rise, the lava

will enter them, forcing back the water or the steam generated from

it, and, having penetrated as far as accessible, will solidify as a

dike, and plug up the avenue of contact between the ground-water and

the portion of the lava still remaining molten. The numerous dikes

that attend volcanic necks testify to the prevalence of this action.

The capillary pores of the wall-rock, which cannot be thus bodily

occupied by the lava, must doubtless become filled with steam, and

this, following the principles of Daubree’s experiment, will force

itself into contact with the lava and be absorbed by it, but whether

this will be in sufficient quantity, and will become sufficiently

diffused through the body of the lava-column to produce the observed

effects, is an open question. The increasing testimony of deep mining

is that relatively little water flows through the deeper horizons. It

is urged that the water remaining in solidified lavas is very unequal

in distribution, as though due to unequal access and partial diffusion.

The argument seems strong, but to make it thoroughly good, it must be

shown that this inequality is not due to _irregularity of discharge_

of the gases during and after eruption, rather than to irregularity of

original _accession_. There is, perhaps, as much ground for assigning

differences in the degree of parting with the included gases, as in

acquiring them. Doubtless those lavas that boiled and seethed for a

long period in the caldron were more fully deprived of their gases than

those that were more promptly disgorged and cooled with less convection

and surface exposure.

=Thermal considerations.=—Probably the most important consideration

relates to the heat effects. If underground-waters enter the

lava-column and come forth as steam, great quantities of heat are

consumed in the process. Has the lava a sufficient excess of heat to

stand this? Can ebullition be maintained for the observed periods if

the steam comes from ground-waters?

Many lavas probably do not carry a very large excess of heat above

that necessary for liquefaction, for not a few of them contain

crystals already forming, which shows that they are within the range

of the temperatures of solidification of their constituents. The same

conclusion is indicated by the limited fusing effects shown by the

walls of dikes and sills. On the other hand, as already remarked,

dikes and sills often show the effects of a rather rapid cooling from

the walls. The method of flow often implies the same condition for

the acidic lavas, since they usually behave as stiff, pasty masses

of limited liquidity. On the other hand, the basic lavas, whose

fusion-point is much lower, often flow freely and reach great distances

before solidifying. The facts taken altogether imply that the average

temperature of the lavas is not much above the fusing-point of the

acidic lavas, while it may probably be very considerably above the

fusing-point of basalt. For a rough estimate, with no pretensions to

accuracy, it may be assumed that in an average case there are 500°

Fahr. excess, but probably not 1000° Fahr. A computation based on even

so rough an estimate as this may, by showing the order of magnitude

of the thermal considerations, indicate their radical bearing. The

average temperature of the ground-water of the upper two or three miles

of the crust—the only portion through which water probably penetrates

with sufficient freedom to be effective in this case—is probably less

than 200° Fahr. The specific heat of rock appears to average somewhat

less than 0.2. The temperature of the lava may be taken at 3000° Fahr.

as a sufficiently high average. From these data it follows that if

an amount of ground-water equal to five percent. of the volume of

the lava entered the lava and was brought up to its temperature and

then discharged, the temperature of the whole mass would be lowered

550° Fahr. It is therefore evident that only a small percentage of

surface-water can pass through the lava consistently with its continued

fluidity.

M. Fouqué estimated that the discharge of steam from a merely

_parasitic cone_ of Etna during 100 days was equal to 2,100,000 cubic

meters of water. If this were ground-water, and the lava from which

it issued had an excess of 500° Fahr. above the fusion-point, the

formation of this steam would congeal a column 400 feet in diameter

and 3000 feet deep in the time given. If this case is typical, and

if Fouqué’s estimate is not greatly exaggerated or very exceptional,

the view that any large portion of the steam from volcanoes comes

from surface-waters seems to be incompatible with the persistence of

ebullition and explosion which many of them exhibit. Stromboli has been

in constant eruption as far back as the history of the region runs. It

is now exploding every three to ten minutes, and yet the mass of lava

seems to be small and its outflow inconsiderable. Is it possible that

a current of steam, given out with this activity for so long a period,

was derived from adjacent ground-waters, and has not yet solidified the

lava?

The problem takes on a very different aspect if the steam, or at least

some large part of it, rises from great depths and brings thence an

excess of heat. It then becomes an agency for the maintenance of

the liquidity of the lava, for giving it convective motion, and for

promoting explosive action, so long as it continues to rise.

For these and other reasons the balance of present evidence seems to us

to favor the view that most of the steam and other gases come with the

lava from its original source deep in the earth.

_References on vulcanism._—G. P. Scrope, Volcanoes, London,

1872. R. Mallet, on Volcanic Energy, Phil. Trans., 1873. C.

Darwin, Geological Observations on Volcanic Islands, London,

1876. E. Reyer, Beitrag zur Physik der Eruptionen, Vienna, 1877;

Theoretische Geologie, 1888. Fouqué, Santorin et ses Éruptions,

Paris, 1879, Sartoris von Waltershausen and A. von Lasaulx, Der

Aetna, Leipzig, 1880. C. E. Dutton, Geology of the High Plateaus of

Utah, U. S. Geog. and Geol. Surv., 1880; The Hawaiian Volcanoes,

Fourth Ann. Rept. U. S. Geol. Surv., 1883. Judd, Volcanoes, 1881;

The Eruption of Krakatoa (Com. of the Roy. Soc.), 1888. J. D.

Dana, Characteristics of Volcanoes, 1890. H. J. Johnston-Lavis,

The South Italian Volcanoes, Naples, 1891. E. Hull, Volcanoes,

Past and Present, 1892. Milne and Burton, The Volcanoes of Japan,

1892. J. P. Iddings, The Origin of Igneous Rocks, Bull. Phil. Soc.,

Washington, Vol. XII, 1892. A. C. Lane, Geologic Activity of the

Earth’s Originally Absorbed Gases, Bull. Geol. Soc. Am., Vol. V,

1894. A. Geikie, Ancient Volcanoes of Great Britain, London, 1897.

I. C. Russell, Volcanoes of North America, 1897. T. G. Bonney,

Volcanoes, Their Structure and Significance, New York (and London),

1899. F. Miron, Étude des Phénomènes Volcaniques, Paris, 1903.

G. C. Curtis, Secondary Phenomena of the West Indian Volcanic

Eruptions of 1902, Jour. Geol., Vol. XI, No. 2, 1903. A. Heilprin,

Mont Pelée and the Tragedy of Martinique, Philadelphia (and

London), 1903. Robert T. Hill, Report on the Volcanic Disturbances

in the West Indies, Nat’l Geog. Mag., Vol. XIII, No. 7, 1902. I.

C. Russell, The Recent Volcanic Eruptions in the West Indies,

Nat’l Geog. Mag., Vol. XIII, No. 7, 1902; Volcanic Eruptions on

Martinique and St. Vincent, Nat’l Geog. Mag., Vol. XIII, No. 12,

1902. J. S. Diller, Volcanic Rocks of Martinique and St. Vincent,

Nat’l Geog. Mag., Vol. XIII, No. 7, 1902. W. F. Hillebrand,

Chemical Discussion of Analyses of Volcanic Ejecta from Martinique

and St. Vincent, Nat’l Geog. Mag., Vol. XIII, No. 7, 1902. E. O.

Hovey, The Eruptions of La Soufrière, St. Vincent, in May, 1902,

Nat’l Geog. Mag., Vol. XIII, No. 12, 1902.

CHAPTER XI.

THE GEOLOGIC FUNCTIONS OF LIFE.

I. THE DISTINCTIVE FEATURES OF ORGANIC PROCESSES.

There is no reason to suppose that life processes, as we know them,

were in operation in the earliest stages of the earth’s history. They

were introduced and developed gradually during its progress. With

life there came into the processes of the earth’s development three

distinctive factors:

A. Certain chemical actions giving rise to compounds that are not known

to occur independently of life.

B. Certain modes of aggregation of material, and certain kinds of

bodily movements, not known except in association with life.

C. The mental element, under the direction of which certain new

processes were inaugurated, and certain previous processes were

modified and controlled.

+A. The Chemical Work of Life.+

The peculiar chemical phenomena connected with life chiefly concern

the carbon compounds. In the inorganic world the carbon compounds are

few and simple. In the organic world they become extremely numerous

and complicated. These compounds are very unstable, for the greater

part, and their partial decomposition gives rise to many additional

compounds. Some of the true organic compounds and some of their

decomposition products have the power of combining with inorganic

substances, and so produce an additional series of semi-organic

combinations. The total number of the compounds thus directly and

indirectly connected with life greatly exceeds that of all inorganic

compounds. Their mass, however, is very greatly inferior.

=Life material chiefly atmospheric.=—In the building up of the organic

compounds, a necessary step is the decomposition of certain inorganic

compounds. The chief of these is the carbon dioxide of the atmosphere

and hydrosphere, the decomposition of which furnishes the carbon

needed for the organic compounds. On this account carbon dioxide may

be regarded as in some sense the basal material or the fundamental

food of the organic kingdom, and hence it plays a radical rôle in the

life-history of the earth.

Water, and the constituents of water, oxygen and hydrogen, play a

larger part quantitatively, but a less distinctive part.

Nitrogen is also an essential element, and usually stands next to

carbon, oxygen, and hydrogen in quantity.

These, it will be noted, are all atmospheric constituents, and the

material of life is, therefore, dominantly atmospheric. This is

even true of aquatic life, for it lives largely on the atmospheric

constituents dissolved in the water. The function of life, considered

from the material point of view, is not only fundamentally concerned

with the atmosphere, and intimately dependent on its conditions, but

its most important material effects appear to lie in its modification

of the constitution of the atmosphere.

=The non-atmospheric factors.=—The atmospheric constituents are not,

however, the only elements intimately connected with the life function.

Compounds of sulphur, phosphorus, potassium, sodium, chlorine, iron,

calcium, magnesium, silicon, and other elements are more or less

essential to the life of many organisms, or are employed by them for

their skeletons, coverings, etc. Incidentally, nearly all the common

elements become intimately related to living organisms either in the

relations of active elements in their physiological functions, or of

passive elements in their structure or in their auxiliary parts.

_Three Classes of Effects._

Out of life processes grow three rather distinct classes of results:

(1) changes in the amounts and proportions of the constituents of

the atmosphere and, to some slight extent, of the hydrosphere and

lithosphere; (2) aid or hindrance to inorganic processes, such as

disintegration, erosion, and deposition; and (3) distinctive products,

either (_a_) of organic matter that would not have come into the

existing combination but for life, such as peat, lignite, amber, etc.,

or (_b_) of special forms of inorganic matter that would not have

arisen but for life, such as coral deposits, shell-marl, diatom ooze,

etc.

(1) _Changes in the composition of the atmosphere._

The succession of modifications which the atmosphere has undergone

from time to time through the action of life will be discussed as the

earth’s history is followed in the second volume. It may suffice here

to note briefly the chief ways in which the atmosphere has probably

been modified by the agency of life, not only as regards its quantity

but also as regards the proportions of its constituents.

=The consumption and restoration of carbon dioxide.=—As the fundamental

food of the organic world, carbon dioxide has suffered enormous

consumption in the course of the geological ages, and is now reduced

to the very small proportion of .0004 or .0003 of the whole. At the

outset it was probably one of the most abundant constituents; possibly

even the chief one. It has been partially restored, concurrently with

its consumption, by animal respiration, by certain classes of plant

action, and by combustion and other forms of inorganic combination.

This restorative action has been incomplete at all known stages of

the earth’s history, and hence there has been constant loss of carbon

dioxide. The inorganic processes which have also profoundly affected

both the consumption and restoration of carbon dioxide are here

neglected and discussed elsewhere.

=The freeing and consumption of oxygen.=—The oxygen of the atmosphere

is actively consumed by animals and by plants, but on the other hand,

it is set free abundantly by green plants, and hence its amount has

probably fluctuated from time to time according to the state of

balance between the organic processes of its production, and those of

its consumption. The consumption of oxygen by organic processes is,

however, little more than a reversal of the previous process by which

it was set free; for instance, green plants in forming their food set

free the oxygen of the carbon dioxide used for the purpose. When the

organic substance so formed is ultimately consumed through plant or

animal action or by inorganic means, an equivalent amount of oxygen

reunites with the carbon to again form carbon dioxide. And so if the

whole of the organic matter is returned to the inorganic state, no more

oxygen is consumed than had been before set free in the process of

forming the organic matter. But, as a matter of fact, a large amount

of organic matter has not gone back completely to the inorganic state,

and this residue constitutes a factor of no small importance in the

geological record.

=The organic residue.=—There is a certain portion of vegetation that

is not consumed by animals or by other plants, and that escapes

combustion and all kinds of ordinary decay, and this constitutes a

part of the organic residue. Animals never completely oxidize all

the organic matter they take into their systems; their bodies never

entirely consume themselves. A like statement may be made respecting

those plants that feed on organic matter. That which animals and plants

leave unoxidized is indeed more or less preyed upon by other animals

and plants, and relatively little escapes final reoxidation, but there

is a remnant, and this constitutes another part of the organic residue.

The more conspicuous forms of the organic residue are found in the

mucks, peats, lignites, coals, organic oils, and gases, but in addition

there is not a little disseminated organic matter in nearly all the

sedimentary rocks; in the aggregate, this probably amounts to more than

the distinct organic deposits.

=The meaning of the organic residue.=—All the unoxidized, or

incompletely oxidized, carbon in the organic residue implies that

oxygen has previously been separated from this residual carbon by

plants and given to the atmosphere, and hence has been a source of

atmospheric enrichment in oxygen. The amount thus contributed is

equal to that which is required to restore the residual carbon to its

original state of oxidation. So, in a similar way, the unoxidized

hydrogen in the organic hydrocarbons and like compounds implies that

oxygen has been separated from the hydrogen of water and given to the

atmosphere, and hence this also is a source of atmospheric enrichment

in oxygen. It seems safe, therefore, to conclude that the action of

life, taken as a whole, has increased the free oxygen of the atmosphere.

While not here under consideration, it is not to be forgotten that

inorganic processes involving the same atmospheric constituents have

been in operation concurrently with the organic processes, and that

they have also affected the amounts and proportions of the atmospheric

constituents. Rocks have been oxidized in greater or less measure

at the expense of the atmospheric oxygen, and hence when the total

atmospheric problem is considered, there arises the question whether

the amount of oxygen in the atmosphere has been increased or diminished

during geological history, when the balance is struck between the

inorganic and the organic actions. The probabilities seem to us to

strongly favor the view that organic action has preponderated, and that

the oxygen has been increased beyond its primitive amount, but that it

has fluctuated during known geological history. The reasons for this

view will appear in the historical chapters.

The disintegration of the crystalline rocks and the solution of

limestone have consumed much carbon dioxide, and this is to be added to

the loss through organic action. On the other hand, there are inorganic

processes that supply carbon dioxide, and hence when the larger problem

of the atmosphere is raised, the factors become so complicated that

their consideration is best deferred to the historical chapters. This

passing reference may stand us in good part lest we forget, for the

moment, the inorganic factors in the atmospheric problem.

=The more inert factor.=—Nitrogen in the free state is relatively inert

chemically, and it does not appear that it can be used directly by the

higher plants and animals in appreciable amounts. Certain bacteria,

and perhaps certain algæ[288] and other low forms of plants, have the

power of using free nitrogen, and this is a principal way in which it

is put within the reach of higher plants. Nitrogen is also combined

in small quantity in the atmosphere by electric action, and thus made

available for plants. On account of the inertness of nitrogen and

of the relatively limited amount required for organic purposes, the

nitrogen of the atmosphere has been less consumed than the carbon

dioxide. Besides this, the nitrogen compounds are very decomposable,

and are very generally and completely returned to their original state.

Deposits of nitrates or other nitrogenous compounds are relatively rare.

It is obvious that if there is any considerable source of supply

concurrent with this slight loss, the amount of nitrogen in the

atmosphere must have been increasing. We have seen that volcanoes

give forth considerable quantities of nitrogen, and that this _may_

be a real addition to the atmosphere, and not merely a return of

the atmospheric nitrogen that had been carried down previously by

underground-water. It has also been noted that crystalline rocks

contain occluded nitrogen, which is doubtless freed by their

disintegration. It is, therefore, not improbable that the nitrogen of

the atmosphere has been increasing, both actually and relatively.

=Probable fluctuations of atmospheric composition.=—With this general

sketch of the interplay of the atmospheric elements under organic

influence, we are prepared for the further conception that if one

or another of these actions was relatively more vigorous than usual

for a period, it would bring about a variation in the proportions of

the atmospheric constituents. If, for example, vegetation flourished

luxuriantly for a long period, but was measurably protected from the

organisms that preyed upon it and from inorganic decomposition, as by

falling into water or by prompt burial under sediment, the atmosphere

might be growing richer in oxygen. If, on the other hand, vegetation

were being relatively reduced, as _perhaps_ it is being reduced now

by man, and if previous organic products were being reoxidized at an

unusual rate, as they are now in the burning of timber, coal, natural

oil and gas, the carbon dioxide of the atmosphere might be relatively

increasing, while the oxygen might be relatively diminishing. The

possible fluctuations of the atmosphere as the result of organic action

are, therefore, matters of vital importance, and invite attention in

the historical study of the earth and in the outlook into its future.

=The climatic effects of organic action.=—Interest does not, however,

rest at this point. The researches of physicists have made it probable,

if they have not altogether demonstrated, that the composition of

the atmosphere has much to do with the climatic conditions at the

surface of the earth. The atmosphere blankets the earth and equalizes

its temperature. Acting as a screen, it subdues in some measure the

intensity of the sun’s rays by day, while it retards the radiation

of the earth’s acquired heat at night. This is in some measure the

function of all the constituents of the atmosphere, but by no means

of all equally. The oxygen and nitrogen are relatively diathermous,

letting the sun’s rays pass in freely, and the earth’s rays pass

out freely; but carbon dioxide and the vapor of water are much less

diathermous, particularly to rays of low intensity, such as are thrown

out by non-luminous bodies like the earth. It follows that while the

solar rays come in rather freely and heat the surface of the earth,

the dark rays which the earth radiates back are measurably arrested by

the carbon dioxide and vapor of water, and serve to keep the air warm.

The influence of the vapor of water is vividly shown in the different

degrees to which cooling takes place at night in a dry and in a moist

atmosphere, respectively, where other conditions are the same. Ice is

said to form at night in desert regions where the air is extremely dry,

even within the tropics, while in humid regions of the same latitude

and altitude oppressively hot nights are common. The influence of the

carbon dioxide is not thus familiarly demonstrated, since its amount

varies but slightly in different localities, but physical experiment

indicates that it has a similar function.

If the amount of carbon dioxide in the atmosphere varies from age to

age, the climate of the earth must apparently vary accordingly, and on

this is built one of the hypotheses of climatic variation subsequently

to be considered. We shall find that there have been great changes in

the climate of the earth during its history. There is good evidence

of former glaciation, not only in the northern United States and in

England, Germany, and central Russia, but in India, Australia, and

South Africa. At other times, figs and magnolias grew in Greenland

and Spitzbergen, and corals flourished in the Arctic seas. There is

good evidence of arid periods where humidity now prevails, and of

humid periods where aridity now prevails. It is not assumed that the

influence of organic action on the atmosphere has been the sole, or

perhaps even the main, cause of these great climatic changes, but it is

believed that it has been an important contributing factor. It is even

possible that the climate of the future is much dependent on the agency

of man, as implied above, however little ground there may be to suppose

that he will, with altruistic purpose, control his action with a view

to its bearing on the generations that may live tens of thousands of

years hence.

(2) _Aid and hindrance to inorganic action._

=The promotion of disintegration.=—While the influence of organic

action on the lithosphere is quite superficial, and far less radical

than that on the atmosphere, it is still important. Plants promote

both disintegration and disaggregation under certain conditions,

and hinder them under others, as already set forth. Chemical action

of a decomposing and solvent nature takes place in connection with

the roots of plants, while their growth sometimes rends rocks into

whose crevices they have insinuated themselves. The acids and other

products of organic growth and of organic decomposition attack some

of the constituents of the rocks and contribute to their solution and

disintegration. On the other hand, organic matter entrapped in the

sediments, and so introduced into the strata at various depths, often

acts as a reducing agency, causing the deposit of substances carried in

solution in the underground-waters. Ores are sometimes thus formed, as

explained in the discussion of ore-deposits (p. 476). Organic action

on the whole promotes solution and disintegration at the surface, and

prepares the way for deposition below.

=Protection against erosion.=—Another important function of vegetation

is the protection of the land surface against erosion, as already noted

in the discussion of erosion. A mantle of grass, especially if it forms

a turf, or a carpet of leaves protected by bush and forest, greatly

retards surface wash. It does this not only because it directly covers

the soil, but because it holds back the run-off and tends to prevent

those violent floods which give to erosion its greatest intensity.

There is a marked difference between the erosive work which a given

amount of water will do if, in the one case, it runs off gradually, and

in the other, precipitately. By way of offset, it is to be noted that

the disintegrating action of vegetation prepares the rock material for

easy erosion, and to this extent helps in its removal by the drainage;

but on the average this is greatly overbalanced by the protection

afforded by the vegetal covering, though this is not true in every

instance.

=The influence of land vegetation on the character of the

sediments.=—The presence or absence of a vegetal covering influences

the kind of deposit which is derived from the land, particularly

if the surface be occupied by crystalline rocks. If the surface be

well clothed with vegetation, the crystals of the complex silicates,

such as the feldspars, micas, and ferromagnesian minerals, are

usually disintegrated into clayey products before they are removed,

so that, when borne away and deposited, the result is common shale.

Concurrently, the relatively undecomposable quartz-grains are rounded

into sand, and deposited as common quartzose sandstone, while the

calcareous material is borne away in solution and deposited as

limestone. But if the surface be bare of vegetation, the crystalline

rocks are usually _disaggregated_ before they are decomposed, for

destructive action works best at the junctions of crystals, and along

cleavage lines, and hence the crystals are usually separated from one

another before they are fully decomposed. In the absence of a covering

to hold them in place until they are decomposed, they are apt to be

washed away, and the resulting deposit consists in considerable part

of grains of feldspar, mica, hornblende, and other minerals, which

do not usually occur in well-decomposed sediments. The deposits are,

therefore, of the nature of arkose, if the original rocks are granitic,

or of the nature of wacke, as the term is used in this book, if they

are of the basic type. On this is based the inference that a vegetal

covering of the land extended as far back in the history of the

earth as clay shales, quartzose sandstones, and limestones form the

prevailing sediments.

(3) _Distinctive deposits._

=Organic rocks.=—In the chapter on the origin and descent of rocks,

a group of rocks formed directly from organic matter is recognized

and described. The chief of these are peat, lignite, bituminous

coal, anthracite, and graphite. It is the belief of many geologists

that natural gases, oils, and asphalts are also mainly derived

from animal and vegetal remains. An alternative view, advocated by

Mendelejeff and Moissan, assigns the oils, gases, etc., in part at

least, to deep-seated carbides to which water has gained access and

developed hydrocarbons, after the analogy of acetylene.[289] Whatever

may be the truth relative to inorganic action, it is clear from

geological conditions that some of the natural gases and oils are

organic products. Besides the more common organic deposits, there

is a long list of minor products, among which are amber, copalite,

paraffine, ozocerite, camphene, etc. Guano and coprolites represent the

excrementitious class.

=Inorganic rocks due to life.=—Besides these deposits of organic

matter, or of its decomposition products, there is a large class formed

from the inorganic matter that served auxiliary functions in the

economy of life, such as shells, skeletons, etc. For the greater part

these are composed of calcium carbonate, and give rise to limestones,

marls, chalk, etc. Not a few, however, are silicious, and give rise

to flints, cherts, and silicious earths. Some are formed of calcium

phosphate, and a few of other inorganic material. The deposits formed

in these ways have been defined in the chapter on rocks.

_Fossils._

The term _fossil_ is used so comprehensively as to include not only

the remains of plants and animals themselves, but their tracks,

impressions, casts, replacements, and all other distinct traces.

It also embraces nests, borings, implements, and other distinctive

products. These enter into the formation of the two classes of rocks

just considered, but they have an independent function. They constitute

the specific record of life, and their study not only reveals much of

the past history of plants and animals, but furnishes one of the most

important means by which the ages of formations are determined. In the

early development of the science it was found that the uppermost and

hence the latest beds of rock contain fossil forms either identical

with those now living, or closely similar to them; that beds below

these bear life relics that depart somewhat more from the living forms,

and are somewhat less highly developed; that beds still lower bear

fossils that depart still more from the living types, and are more

primitive in general, and so on down as far as fossils are found.

=The general order of life succession determined by stratigraphy.=—Thus

it appeared from the evidence of the strata that there was a general

order of life succession. It was also found that this was, _in its

main features_, the same for all the continents. By continued and

close studies, the particulars of the succession were worked out more

and more fully, and the work is still being pushed forward to greater

and greater degrees of refinement. At the same time, it was found

that there were different faunas and floras in different parts of the

world in past times, much as there are now; that there were shiftings

and migrations as now; that given species were increasing in some

regions and dying out in others, and that innumerable variations and

complications entered into the evolution and distribution of the life

forms. But under and through all these there run a sufficient number

of common features to show beyond reasonable question the order of

succession of life.

Throughout all this study, _the chief guide was the actual order_ in

which the fossils were found in the succession of strata, because

there is no evidence so conclusive of the order of events as the

superposition of the sedimentary beds when they are normal and

undisturbed. By the study of the fossils in the successive beds, it was

found that there was a more or less progressive evolution of plants and

animals brought about by modifications of their forms, and that these

modifications assisted in determining the order of succession when

the evidence of the strata was defective; and so the biological and

stratigraphical factors reacted helpfully on each other.

=Fossils as means of correlation.=—While stratigraphy was thus, in

the earliest stages, the main reliance in determining the order of

events, and biology was the chief gainer, in the end stratigraphy

received ample compensation, if indeed it did not become the greater

beneficiary; for at no known and accessible place is there a complete

succession of sedimentary beds. There are great series here and there,

but their connections with one another are more or less concealed

by surface formations or water-bodies. So also at many places the

stratified series has been broken up by deformation, or cut away by

erosion. Hence there was need for some reliable means of matching the

beds of separated series, and of making up a complete ideal series.

This means is found in the fossils they contain. While the variations

of the faunas and floras in different regions, and their migrations,

introduce some minor difficulties, the relations of the fossiliferous

beds of one region to those of another can be determined with great

satisfaction, and often with great precision. This is particularly so

when abundant floating or free-swimming species lived in the seas and

were freely fossilized, for they were deposited on the coasts of all

the continents at practically the same time, and no uncertainties from

migration or local differences in rate of evolution intervened to throw

doubt upon the correlation. Without the aid of fossils, the correlation

of the deposits on the separate continents would be attended with

grave obstacles and much uncertainty, if not with quite prohibitive

difficulties.

+B. Special Modes of Aggregation and of Movement.+

Inorganic solid matter is chiefly crystalloidal; organic matter is

chiefly colloidal; but there are colloidal states of inorganic matter

and there are crystalloids among the organic products. In the inorganic

world, solids very generally tend to organize in the form of crystals;

in the organic world, they as generally tend to organize in the form of

cells. Neither tendency is complete or exclusive, but each is dominant

in its own sphere.

Still more distinctive than the formation of cells is the growth of

complex organized bodies, the differentiated members of which perform

special functions for one another, and are mutually dependent on one

another. This is a profound departure from the habitual modes of the

inorganic world.

Still more so is the power of voluntary motion in more or less

disregard of outside physical influences. Through this power,

distribution may take place contrary to current and wind, and

to gravitation itself. From the view-point of past geologic

transportation, this is perhaps more singular than important, for no

great mass of matter has been transported contrary to the influences

of gravity, wind, and current, by the exercise of this peculiar

power of animals, but it is not without geologic importance in the

migrations and in the redistributions of organic influences that arise

from migrations. When the influence of man is included, the geologic

effects require consideration, but here the third distinctive factor,

the mental element, comes into effective play, and we pass to its

consideration.

+C. The Mental Element.+

Current opinion does not recognize a mental element as residing in the

plant world, and it is divided as to the degree of its development in

the lower animal kingdom, but its influential presence in the higher

animal orders and in man is beyond legitimate question. Two phases

are to be recognized: (1) the material work done under the stimulus

and direction of mental impulses, as, for example, excavations,

transportations, changes of drainage, removal of forests, cultivation

of soil, etc., and (2) the intellectual work of the faculties

themselves irrespective of material changes. In one view, geology

is a purely material science concerned solely with the formation of

the earth and with the physical development and relations of its

inhabitants. In another, geology is a comprehensive historical science

concerned with every phase of the world’s history, and certainly

not least with the higher forms of life development, with their

psychological, sociological, and other phases of mental attainments,

since these are the highest output of the earth’s evolution. The latter

seems to us the more comprehensive view.

(1) =The material effects of the mental element.=—Lyell long since

urged that the direct work of man in changing the face of the earth was

slight compared with that of the contemporaneous inorganic agencies.

He called attention to the relative insignificance of the quarries,

pits, cellars, and other excavations of man, compared with the work

of streams, waves, and other inorganic agencies. There is justness in

this view, but it needs qualification. It is to be observed that the

mental era has but just begun, and that its effects are increasing

with a rapidity quite phenomenal when measured by the slow pace of

most geologic events. The excavations and transportations of material

to-day show an enormous advance on those of Lyell’s day, which was,

geologically speaking, but a moment ago. The mile-tons of industrial

freightage in the Mississippi basin are to-day not wholly incomparable

with the drainage transportation of the same area a century ago. A

century ago is named, because the surface was then covered with natural

vegetation, and the normal effect of surface erosion, independent of

man, was then experienced. At present the indirect effects of man’s

action are mingled with those of natural processes, and these indirect

effects are probably much more important than the direct ones. The

removal of the native vegetation and the cultivation of the soil

expose the surface to wash to a degree far beyond that prevalent when

the surface was prairie sod, or leaf-carpeted forest, and denudation

and transportation have been greatly multiplied in consequence. Not

only has this cultivation increased the exposure to erosion, but, by

increasing the rate of run-off, it has added to the erosive power

of the streams. The ditching of swamps and other tracts of retarded

drainage has contributed to this acceleration. The naked, soil-less

uplands of some of the once populous kingdoms of the Orient, notably

portions of Syria and Greece, are sad witnesses of the accelerated

erosion that attends cultivation. The erosion of certain southern

fields of the United States in the last forty years is another striking

illustration. It is doubtful whether some parts of this region suffered

as much erosion in the preceding five centuries as they have during

the last one. On the other hand, some compensation is found in the

reservoirs established for water-power, and in artificial devices for

retarding and steadying stream flow.

In the light of considerations such as these, man may well be regarded

not only as a potent geological agent, but as dangerously so to

himself. The hope is that the intelligence that has wrought a change

of surface conditions serviceable for the present, but dangerous to

the future, will be so enlarged as to inspire a still more intelligent

control of surface conditions which shall compass the future welfare as

well as transient benefit.

=Human modification of the animal and vegetal kingdoms.=—Man’s agency

is also coming to be felt powerfully in the modification of the plant

and animal life of the land and even to some extent of the sea. The

larger animals that are not propagated by man are fast approaching

extinction. At the present rate of extension of man’s dominion, a

century or so will see the disappearance of nearly every large mammal

and reptile that he does not choose to protect or propagate. By way

of compensation, certain selected animals are increasing and will

doubtless continue to increase. The result is, therefore, likely to

be a peculiar assemblage of animal life dependent strictly on the

choice of a dominant type, a state of things that has apparently never

occurred in an equal degree in the past history of the earth. How far

the minor forms of life, especially the insect life, and the denizens

of the sea, may be brought under this monopolistic control may not be

predicted so easily.

A similar profound transition in vegetation is being forced by man. The

native vegetation is rapidly being replaced by selected varieties, and

by varieties that take advantage of conditions furnished by man. As the

agricultural control of the earth becomes more complete and effective,

a result toward which very rapid progress is being made, a new flora

of man’s selection will very generally prevail over the whole land

surface of the globe. It is doubtful whether at any time in the history

of the earth changes of flora and of fauna, and of surface, have been

more rapid than those that are now taking place under the accelerating

influence of man’s action, and this accelerating influence springs

not mainly from automatic or instinctive reaction, but from conscious

impulse and intelligent direction.

(2) =The psychological factors as such.=—Are the introduction and

the evolution of the psychological factors themselves to be regarded

as subjects of geological study? We shall find that, at the outset,

the geologic record is a complete blank so far as clear evidence of

terrestrial organisms actuated by their own intelligence is concerned;

that later, organisms with some apparent consciousness and intelligence

appeared, and that the mental element increased apace unto its present

attainment. We know that relationships of a sociological nature arose

in apparent feebleness, and gradually evolved into more definite,

higher, and more complex forms. By sociological factors we mean merely

those conscious relations which one organism bears to another, of

which the parental and the gregarious impulses are two fundamental

expressions. For manifest reasons, the introduction and evolution of

the psychological and sociological factors themselves have received

little direct recognition as a portion of geological studies. The

record of such factors in the fossils of past ages is necessarily

obscure and imperfect, and the interpretation of what there is lacks

certainty and precision. None the less, this psychological record,

with all its imperfections, is beyond valuation, and must, we think,

come to be an indispensable factor in the study of psychological and

sociological evolution, for it shows, what nothing else can show

equally well, the extremely prolonged history of that evolution, and

it gives hints of modes and means which no study of existing stages

can equally reveal. The organization of the Cambrian trilobites,

for example, implies no small development of the senses and of the

coordinating faculties even at that early stage, and a study of the

relations of these to their fellow creatures opens up the first

known chapter in the sociological record of the earth’s inhabitants.

From this stage onward the progress in the development of the higher

faculties, and of the sociological relations of the leading forms, is

one of the most instructive phases of the great history. Such a study

reveals the fact that many questions, narrowly supposed to be purely

human, have had their prototypes in the earlier experiences of the

animal kingdom. Some of these questions have found solutions, temporary

or permanent, which passed under the test of ages to whose length human

experience affords no parallel, and have received the sanction or

disapproval of such tests according as they were well or ill adapted to

the actual conditions involved. If one seeks the lessons of history in

the largest sense, he cannot wisely neglect the prolonged record of the

great biological family.

II. SPECIAL CONTRIBUTIONS OF THE ORGANIC KINGDOMS.

An essential part of the historical chapters of the second volume will

consist of the description and illustration of the life progress of the

successive periods. It will suffice here to give a preliminary synopsis

of the kinds of record made by the several groups of plants and animals.

+A. Contributions of the Plant Kingdom.+[290]

The record of plants in the early geological ages is extremely

imperfect. In the very earliest times the conditions seem to have been

wholly unsuited to the preservation of any relics of life; but even

after animal remains were abundantly preserved in the sea sediments,

the plant record was still very meager for a long period. This was

probably due in the main to two chief causes: (1) the probable

softness and perishability of the early types of vegetation, and (2)

the fact that vegetation is preponderantly terrestrial. At no time

has marine vegetation reached a high development. Land conditions

favor decomposition, transportation, and erosion, and through these,

destruction; and only under rather occasional and exceptional

conditions did the old lands leave a good record of their life.

Nevertheless all the great groups of plants, viz. the Thallophytes

(algæ, fungi), the Bryophytes (mosses, liverworts), the Pteridophytes

(ferns, horsetails, lycopods), and the Spermatophytes (gymnosperms,

angiosperms) have left some record.

REFERENCE TABLE OF THE PRINCIPAL GROUPS OF PLANTS.

{ { Cyanophyceæ, blue-green algæ.

{ { Chlorophyceæ, green algæ.

{ { Rhodophyceæ, red algæ.

{ Algæ and { Phæophyceæ, brown algæ.

{ algoid forms { Diatomaceæ, diatoms.

{ { Coccospheres }

{ { Rhabdospheres } Pelagic algæ(?).

+Thallophytes+ { { Charophyta, stoneworts.

(Thallus {

plants) { { Phycomycetes, algæ-fungi, water-molds.

{ { Ascomycetes, ascus-fungi, mildews.

{ Fungi and { Basidiomycetes, basidium-fungi, mushrooms.

{ fungoid { Æcidiomycetes, æcidium-fungi, “rusts.”

{ forms { Schizomycetes, “fission-fungi,” bacteria.

{ { Myxomycetes, “animal fungi,” slime-molds.

{

{ Lichens Symbiont algæ and fungi.

+Bryophytes+ { Hepaticæ, liverworts.

(Moss plants) { Musci, mosses.

{Filicales { Filices, true ferns.

{ { Cycadofilices, cycad-ferns.

{

{Equisetales { Equisetæ, scouring-rushes, horsetails.

+Pteridophytes+ { { Calamites.

(Fern plants) {Sphenophyllales.

{ { Lycopodiaceæ, club-mosses.

{Lycopodiales { Lepidodendra.

{ { Sigillaria and stigmaria.

{ { Cordaiteæ, cordaites.

{ { Cycadales { Bennettiteæ.

{ Gymnospermæ { (cycads) { Cycadaceæ.

{ (Naked seed) { Coniferæ, evergreens.

+Spermatophytes+{ { Ginkgoaceæ, ginkgo.

(Seed plants) {

{ Angiospermæ { Monocotyledoneæ, cereals, grasses, etc.

{ (Covered Seed) { (one-leafed seed).

{ (Flowering { Dicotyledoneæ, oaks, poplars, peas, etc.

{ plants) { (two-leafed seed).

=The contribution of the Thallophytes (algæ, fungi, bacteria).=—The

Thallophytes embrace the simplest types of plants, and are probably the

nearest present representatives of the ancestral forms. Some of them

are minute one-celled organisms, as simple as an organism can well be

conceived to be. The simple blue-green algæ of our fresh waters well

represent this class. The most are, however, multicellular, and some

(as the great seaweeds) rise to a degree of complexity and of a bodily

segmentation resembling that of the higher plants. The various species

are adapted to an extremely wide range of conditions; some live in hot

springs at 170° Fahr., and some in Arctic seas at the freezing-point;

some flourish in fresh water, some in brackish, some in salt water, and

some even out of the water. This wide adaptation implies an ancient

and plastic type. The fact that they flourish in waters so hot and

sometimes also so sulphurous as to be fatal to most plants, suggests

the possibility of their introduction during the very early volcanic

stages of the earth, while conditions were yet uncongenial for other

plants.

The geologic work of the thermal algæ is well shown in the beautiful

travertine and sinter deposits of the Yellowstone Park (Figs. 215 and

218). At the Mammoth Hot Springs the deposits are calcareous, while

at most of the other hot springs silicious deposits are formed, in

both cases partly, but not wholly, by the aid of algæ. The beautiful

yellows, reds, browns, and greens of these springs are not mineral

coloring, but living plants.[291] In the calcareous waters, the algæ

are believed to cause the deposition of calcium carbonate from calcium

bicarbonate by consuming the second equivalent of carbon dioxide

that rendered the carbonate soluble.[292] In the silicious waters,

the process of deposition is not understood. Similar deposits by the

aid of algæ take place in the geyser regions of Iceland and of New

Zealand, in the hot springs of Carlsbad, where they have been well

studied by Cohn,[292] and in most other hot springs. The same, or very

similar, forms of algæ abound in nearly all waters, fresh and salt,

but the question whether they make calcareous and silicious deposits

in notable quantity appears not to have received as yet the critical

investigation its importance deserves, except in a few special cases.

It is clear, however, that in the cool waters such deposits do not

reach the conspicuous amounts that they attain in the thermal springs.

In the shallow waters of the ocean, especially in the warmer regions,

lime-secreting algæ are abundant and make large contributions to the

lime deposits.

Among the higher algæ are the lime-secreting corallines or nullipores

(Rhodophyceæ, red algæ), once regarded as animals, which contribute

a notable part of the calcareous substance of coral reefs. They are

important geologic agents in the temperate and tropical seas, and have

been traced as far back in time as the early Paleozoic era.

The Challenger reports[293] describe two forms of minute calcareous

spherical organisms, Rhabdospheres and Coccospheres, as very

abundant in the surface-waters of the temperate and tropical seas,

and as important in contributing to the calcareous deposits of the

sea-bottoms. The affinities of these bodies are in doubt, but they are

regarded by Murray as probably pelagic algæ.

The stoneworts (Characeæ), an aberrant group of algæ inhabiting fresh

and brackish water, secrete notable quantities of calcium carbonate

in and around their tissues, and the accumulation of these gives rise

to marl or limestone. It has recently been urged that our so-called

shell-marls are mainly due to Charæ,[294] the molluscan shells being

incidental rather than essential constituents.

In very ancient and also in some of the later strata, there are

limestones that do not carry any visible fossils, and their origin is,

therefore, debatable. There are also not a few limestones that are made

up of a fine-grained base through which are scattered molluscan shells,

corals, etc., in a fine state of preservation. The condition of these

fossils bears rather adversely on the view that shells, etc., have been

powdered in sufficient numbers and to a sufficient degree to form the

compact base. In all these cases the usual explanations leave something

to be desired. It is worth considering whether low forms of plants may

not be among the undemonstrated agents in forming these apparently

unfossiliferous limestones or parts of limestones. The calcium

carbonate deposited by the algæ is in minute and delicate form, and is

usually crystalline while yet in the living tissues. It is, therefore,

easily subject to comminution and to such further crystallization as

would obscure the minute features that constitute the evidences of

algal origin.

The more complex and conspicuous algæ, the seaweeds, have left

impressions of their stems and fronds on the marine beds of most of the

periods, but they are usually obscure. Seaweeds are perhaps the source

of the vegetal matter in certain carbonaceous shales and limestones. As

seaweeds extract bromine and iodine and certain metallic ingredients

from the sea-water, some of the iodine and bromine springs issuing from

ancient marine deposits, and certain ores, may owe their origin to

ancient seaweeds.

Diatoms, minute plants of the Thallophyte group, secrete a delicate

framework of silica which becomes a contribution to the silicious

deposits. Diatoms have sometimes contributed the material for very

considerable beds, such as those of the ooze-bogs now forming in the

marshes of the geyser basins of the Yellowstone Park,[295] and the

diatom oozes of the deep sea (Fig. 353, p. 425).

Fungi, for obvious reasons, have left but scant traces of themselves.

Bacteria are believed to be recognizable as far back as the Paleozoic

era. They are now the chief agents in the decomposition of organic

matter, and may be regarded as the prime enemies of the fossil record.

It is probable that similar decomposition took place actively in the

earliest ages, for otherwise the remains of the ancient organisms

should be more abundant. There is hence a theoretical probability that

bacteria flourished as far back as the stratigraphic record goes.

Not unlikely they were originally simple algæ that turned from the

primitive habit of making their own food, to living on other organisms

or their remains, and in so doing lost their power of manufacturing

chlorophyll and of using inorganic carbon compounds. Their remarkable

adaptation to the most varied conditions, and their extraordinary

ability to endure the greatest vicissitudes of environment, support the

view that they are a very ancient and plastic form.

At present certain bacteria are important to higher vegetation because

of their ability to use the free nitrogen of the atmosphere and to

combine it into forms available for the higher plants. It is not

improbable that they have subserved this important function through

all the known ages. Some experiments seem to show that certain of the

existing algæ have this power, and possibly the ancestral forms of

plants possessed it. The bacteria, being a derived and not an original

form, could not have performed the function for the first plants. It is

possible, of course, that the inorganic supply of nitrogen compounds

was sufficient for plant life at the outset.

=The contribution of the Bryophytes (liverworts, mosses).=—The mosses

and liverworts have left no certain record of their work in the

earlier and middle geologic eras, and, if they existed at all, their

contributions were unimportant. Although low forms of plant life,

they are not primitive ones, as they are characterized by a definite

alternation of generations implying a considerable time antecedent to

the attainment of their present forms; hence there are no very cogent

theoretical reasons for assigning them a place in early geologic

history, though their absence cannot be affirmed. Some botanists think

the Pteridophytes were derived from some ancestral form of liverwort,

which, if true, would require the presence of the latter in an early

geologic period; but the negative geological evidence relative to

their presence favors the alternative view that the Pteridophytes were

derived from some form of the Thallophytes by an independent line. In

recent times, certain of the mosses, especially the sphagnum mosses,

have played a notable part in the formation of peat accumulations. For

this, their habit of growing in bogs, and of dying below while they

continue to grow above admirably fits them.

=The contribution of the Pteridophytes (ferns, horsetails, lycopods,

Sphenophyllum).=—The Pteridophytes include the most important fossil

plants of the earlier and middle geologic eras. To them we owe chiefly

the great carbonaceous deposits of the Coal Measures and probably

most of the disseminated carbons of the early and middle eras;

perhaps also much of the natural oil and gas. Their special work is

so conspicuous that it will be noted at length in the chapters on

the Devonian and Carboniferous periods, and hence may be passed here

with brevity. The ferns, now known more for their beauty than their

importance, are the representative type of the group, and are really

a wonderful family, having preserved their characteristic leaf-forms

with a persistence attained by no other group of plants. The Paleozoic

ferns are recognizable as such by every one, irrespective of botanical

knowledge; indeed it is the detection of the differences, rather than

the resemblances, between the ancient and modern forms, that requires

expert knowledge. This continuity shows that since their introduction

the changes of climate have never been so great as to prevent their

propagation, without radical modification, in some part of the globe,

and this fact rather narrowly limits the range of surface temperatures,

and of other climatic vicissitudes. The persistence of the Equisetæ

(horsetails, scouring-rushes) and the lycopods (club-mosses) bears

like testimony, as does the persistence of life in general; but the

rather delicate ferns are perhaps more obviously significant than most

organisms.

=The contribution of the Spermatophytes (seed plants, including

gymnosperms or “evergreens” and angiosperms or “flowering

plants”).=—The angiosperms, the dominant group to-day, make their

appearance in the record in the latter part of the Mesozoic era, and

their contribution is, therefore, relatively modern. They contributed

to the coals, lignites, oils, and organic gases of the late geological

periods, as did the Pteridophytes in the earlier periods, the

latter participating, however, in the late deposits. Perhaps the

most important function of the Spermatophytes lay in their superior

serviceability as food for the higher land animals, by virtue of their

seeds, fruits, and foliage. Neither the Thallophytes, Bryophytes, nor

Pteridophytes, nor all combined, approach the Spermatophytes in food

value for the higher types of animal life, and it is doubtful whether

the higher evolution of the land animals could have taken place without

the previous introduction of the seed plants. It will be noted in the

historical narrative that the great placental group of mammals came in

and deployed with marvelous rapidity, as geological progress goes, soon

after the Spermatophytes became the dominant form of vegetation.

=Plant life terrestrial rather than marine.=—It is to be noticed that

the chief development of all the great groups of plants took place

on the land, or in the land-waters, rather than in the sea. This is

preeminently true of the higher types, and appears also to be true of

even the Thallophytes, although the number of individual algæ and their

total mass is very much greater in the sea than on the land and in the

land-waters. But the fresh-water algæ appear to possess in a higher

degree than the marine forms those plastic and germinal characters

from which new forms spring, and are probably to be regarded as the

parental type. These are facts to be pondered on, since it has been

the current opinion of geologists that life arose in the sea and was

propagated thence to the land. The alternative view that life developed

primarily on the land and in the land-waters and migrated to the sea is

not, however, without its support in the plant world, as we thus see,

and the plant world was the primitive one; the dependent animal world

necessarily followed its development. The hypothesis of a terrestrial

origin of life throws a very suggestive cross-light on many geological

problems, as will be seen later, and it may well be entertained as an

alternative working hypothesis until the facts are more fully developed.

B. +Contributions of the Animal Kingdom.+[296]

As already noted, animal life is dependent on the decomposition of

matter organized by green plants, and the conversion of its potential

energy into active forms. Animals are, therefore, dynamic rather than

constructive agencies. Nevertheless they transform organic vegetal

matter into organic animal matter, and this is sometimes really an

advance in organization. The organized animal matter is subject to

preservation in some small degree, though it usually perishes. Some

contribution is, therefore, made to the organic deposits, chiefly in

the form of hydrocarbons. It is the view of some geologists that the

natural oils and gases have an animal origin in the main.

REFERENCE TABLE OF THE PRINCIPAL GROUPS OF ANIMALS.[297]

+Protozoa+ { Rhizopoda { Foraminifera.

(The simplest { { Radiolaria.

animals) {

{ Flagellata }

{ Infusoria } Unknown in fossil state.

{ Gregarina }

+Cœlenterata+ { Porifera Spongiæ { Calcareous sponges.

(Sponges, { { Silicious sponges.

corals, { Cnidaria { Anthozoa, coral polyps.

jellyfishes) { { Hydrozoa, hydroids and medusæ.

+Echinodermata+ { Pelmatozoa { Cystoidea, cystids.

(Crinoids, { { Crinoidea, stone lilies.

starfishes, { { Blastoidea, blastids.

sea-urchins) {

{ Asterozoa { Ophiuroidea, brittle-stars

{ { Asteroidea, starfishes.

{

{ Echinozoa { Echinoidea, sea-urchins.

{ { Holothuroidea, sea-cucumbers.

+Vermes+ { Platyhelminthes }

(Worms) { Rotifera }

{ Nemathelminthes } Rare as fossils.

{ Gephyrea }

{

{ Annelida, sea-worms.

+Molluscoidea+ { Bryozoa, sea-mosses.

(Mollusc-like forms) { Brachiopoda, lamp-shells.

+Mollusca+ { Pelecypoda, lamellibranchs, bivalves.

(Molluscs) { Scaphopoda, tusk-shells.

{ Amphineura, chiton.

{ Gastropoda, univalves, snails, etc.

{ Cephalopoda, nautilus, cuttlefish.

+Arthropoda+ { Branchiata { Crustacea.

(The { { Trilobita, trilobites.

articulates) { { Gigantostraca, horse-shoe crabs.

{ { Entomostraca, ostracoids, barnacles.

{ { Malacostraca, lobsters, crabs.

{

{ Tracheata { Myriapoda, centipedes.

{ Arachnoidea, spiders, scorpions.

{ Insecta, insects.

{ Cyclostomata, lampreys.

{

{ Pisces { Selachii, sharks.

{ (fishes) { Holocephali, spook-fishes.

{ { Dipnoi, lung-fishes.

{ { Teleostomi, ganoids and teleosus.

{ { (common fishes).

+Vertebrata+ {

{ Amphibia, amphibians, batrachians.

{ Reptilia, reptiles.

{ Aves, birds.

{ Mammalia { Prototheria, monotremes.

{ (mammals){ Metatheria, marsupials.

{ { Eutheria, placentals.

As dynamic organisms animals have need for supporting- and

working-frames, for protective covering or housing, and for offensive

and defensive weapons, and these have been constructed chiefly out of

inorganic matter, and subordinately of indurated organic matter. It is

through these that animals have made their chief contribution to the

material of the geologic record. Skeletons and other hard parts to give

internal stiffness or firmness; shells, plates, indurated integuments,

and various other forms of external protection; teeth, spines, horns,

and other means of gathering and masticating food, and of attack and

defense, contribute material to the deposits, and form a record of the

life activities and of the physiographic environment. All of the eight

groups of animals, viz. Protozoa, Cœlenterata, Echinodermata, Vermes,

Molluscoidea, Mollusca, Arthropoda, and Vertebrata, have left some

record, but it is in all cases a very imperfect one.

=The contribution of the Protozoa.=—The Protozoa are related to the

animal kingdom much as the Thallophytes are to the vegetable, and

the two bear a close structural resemblance to one another. So near,

indeed, do the Protozoa and the Thallophytes approach one another in

their minuteness and simplicity, that the place of not a few organisms

is in doubt, and the two kingdoms, in general so different, seem here

to blend in the group Flagellata. The Protozoa are usually very minute

one-celled organisms with very little differentiation of tissue or

organs. Of the four classes of Protozoa, only one, the Rhizopoda, is

found in the fossil state. The rhizopods secrete silicious skeletons,

and calcareous, silicious, and chitinous tests of a great variety of

forms, and this gives them geologic importance. The deep-sea oozes and

the chalk deposits are their best-known contributions at present. They

have probably played a more important rôle in the formation of ordinary

limestones and silicious silts than can be demonstrated, because of the

delicacy of their relics and the ease with which these are pulverized

by wave-action in the shallow seas, or changed by recrystallization

or by concretionary aggregation. The globigerina oozes are formed

largely from the calcareous shells of Foraminifera (Fig. 351), one

of the orders of rhizopods, among which the genus Globigerina is a

leading form. Those forms which make the deep-sea oozes live, not on

the bottom, but near the surface of the open sea, and on the death of

the organisms, the shells, tests, and skeletons sink to the bottom.

Chalk is formed in a similar way from calcareous Foraminifera, but not

necessarily in very deep water. Foraminifera live in shallow water as

well as in the open sea, and in this case they sometimes creep on the

bottom or are attached to algæ, but their deposits in shallow water are

usually much obscured by other kinds of deposition and by destructive

action. Some of the foraminiferal shells are divided into chambers and

assume various spiral forms, of which the _Nummulites_, named from

their resemblance to coins, are notable examples. These formed an

important part of the nummulitic limestone of the Eocene period.

The radiolarian ooze is characterized by the silicious tests of various

members of the silica-bearing order, Radiolaria. The “Barbadoes earth”

and “Tripoli” are notable deposits of fossil radiolarians.

=The contribution of the Cœlenterata.=—The Cœlenterata embrace the

sponges, the coral polyps (Anthozoa), and the hydroids and medusæ

(Hydrozoa). The contribution of coral polyps to the formation of

limestone is most important, and is too familiar to require elaboration

here. The corals range throughout nearly the whole fossiliferous

series, and their development will be followed and illustrated in the

historical chapters.

The sponges are widely represented by their spicules, and not

uncommonly their aggregate form is preserved even in very ancient

strata. Their contribution is largely silicious, but is partly

calcareous. The hydroids and medusæ have left little trace of

themselves in the rocks, although impressions supposed to represent

medusæ are found in strata as early as the Cambrian. Certain coral-like

forms, as the Millepores, Tubularia, and Stromatopora, are classed as

Hydrozoa. The graptolites, delicate leaf-like floating forms, very

serviceable in marking exact horizons on different continents because

of their free distribution, are also classed here.

=The contribution of the Echinodermata.=—Under the echinoderms are

grouped the crinoids (sea-lilies), cystoids, blastoids, ophiuroids

(brittle stars), asteroids (starfishes), echinoids (sea-urchins),

and holothuroids (sea-cucumbers). This is one of the marked groups

of ancient as well as modern life, and its beautiful fossils grace

every period in which life relics are well preserved. The cystoids and

crinoids, and later the blastoids, were prominent in the Paleozoic

ages, while the remaining forms were more conspicuous later, though

early introduced. All divisions, except the holothuroids, whose

softness prevented, have left a good record, as fossil records go.

Their relics are chiefly calcareous, and they most abound in the

limestones, some of which are largely made up of their remains, as the

encrinital limestone (Fig. 349). They will be subjects of frequent

comment and illustration in the historical chapters.

=The contribution of the Vermes.=—Most of the worms are ill adapted to

fossilization and are not known in the fossil form. The segmental worms

of the sea, the annelids, however, left some traces of themselves in

tubes and borings and in tracks and sometimes by fossil jaws and teeth.

They range from the earliest fossil-marked horizons onward, but seem to

have always been an inferior group.

=The contribution of the Molluscoidea.=—This group includes the

bryozoans, whose fossil products closely resemble the minute-celled

corals, and the brachiopods, whose shells closely resemble those of

the molluscs. Both are calcareous and make important contributions

to the formation of limestone (Fig. 350). A few brachiopods secrete

calcium phosphate instead of calcium carbonate. Both classes have a

great geologic range and their fossils are valuable aids in identifying

and correlating formations. Probably the brachiopods are more utilized

for this purpose than any other single class. They are the symbol of

conservatism and persistence, ranging from the Cambrian to the present

time, and embracing some forms that have scarcely changed to the extent

of generic difference in that time.

=The contribution of the Mollusca.=—The molluscs have also ranged from

the earliest well-recorded times, and some divisions, as the pelecypods

(lamellibranchs, embracing clams, oysters, etc.) and gastropods

(snails, etc.), have undergone no very marked change beyond a rather

ample and progressive development; but others, as the cephalopods

(nautilus, squids, cuttlefish, etc.), mark out the progress of the ages

by distinct and striking changes of form. Their shells are chiefly

calcareous and they have contributed materially to the formation of

limestone. Muddy and sandy bottoms are, however, more congenial to the

pelecypods and gastropods than to the corals, crinoids, and many other

limestone-forming types, and hence fossils of these molluscs frequently

abound in shales and sandstones and give them a calcareous element. In

sandstones, however, the calcareous matter is often dissolved out and

only the casts of the shells remain. The molluscs will be much cited

and illustrated in the historical chapters.

=The contribution of the Arthropoda.=—This group embraces the

crustaceans, myriopods, spiders, and insects. The hard parts of their

bodies are mainly horny or chitinous forms of organic matter, and hence

their relics differ notably from the inorganic calcareous and silicious

remains of most of the preceding forms. The Arthropoda did not at

any time form a notable stratum of rock. Their geologic value lies

chiefly in what they teach of the progress of life and its relations,

and the aid they render in correlation and identification. In these

respects the group is a notable one. It was represented in the early

fossiliferous strata by the trilobites, one of the most interesting of

all types of fossils. These were probably the most highly developed

organisms of their times and give the clearest hints of the stage of

psychological and sociological development that had been reached when

first the record of life is opened to us. The record of the myriopods,

spiders, and insects dates from the middle Paleozoic, and gives the

first clear hints of animal life on the land.

=The contribution of the Vertebrata.=—In the vertebrates the dynamic or

working organism may be said to reach its highest expression, unless

it be in the flying insects, and their inorganic residue becomes

relatively unimportant in rock formation. Although the greatest of all

animal types in most respects, it has never formed more than trivial

beds of rocks. There are occasional “bone beds,” but they are thin and

limited in extent, and only partially formed of vertebrate matter. The

geological importance of the vertebrates lies in the higher field of

life evolution and in its mental accompaniment. Fishes excepted, the

vertebrates are mainly land types, and have for their chief colleagues

plants and insects. The other groups of animals are mainly, though not

wholly, marine. The vertebrates have little place in the Paleozoic

record, except near its close, but they dominate the Mesozoic and

Cenozoic eras, and are conspicuously the master type to day.

III. THE ASSOCIATIONS AND ECOLOGICAL RELATIONS OF LIFE.

A. +The Basis of Floras and Faunas.+

Geologic interest is not confined to the kinds of plants and animals

that have lived and the contributions they have made to the deposits,

but embraces also their assemblage into floras and faunas, and the

relations of these assemblages to the prevailing physiographic

features. These assemblages and relationships are among the most

suggestive factors of the earth’s evolution, and are the most

instructive for purposes of comparison with human history, and for

forecasting the future of man and of the whole biological kingdom.

Moreover, floras and faunas, as such, are used in the correlation

of formations, and in this application they give surer results than

correlations by individual species. A particular species may live

far beyond the usual period of a species, and if fossilized in one

region in its early history and in another in its late history, the

two formations might be referred erroneously to the same stage. This

is far less likely to happen with a whole assemblage of forms. There

is a similar liability to error in interpreting migrations on the

basis of a single or a few species, for a single species or a few

species may be transported by unusual or accidental means, so to speak,

when there is no normal pathway for general migration, and when no

systematic migration takes place. In most of the great questions that

arise concerning the connections and disseverances of the continents,

and concerning the unions and separations of the oceans, which are the

fundamental causes of the migrations and of the isolations of plants

and animals, typical floras and faunas are to be studied, rather than

isolated species or sporadic forms. A brief sketch of the leading

causes and consequences of these special assemblages of plants and

animals may aid in appreciating the underlying significance of floras

and faunas, and in interpreting their meaning as they are met in the

study of the strata. A part of these grow out of the relations of

the organisms to one another, and a part out of the relations of the

organisms to their environment.

(1) _Assemblages Influenced by the Mutual Relations of Organisms._

(_a_) =Food relations.=—The relations of food-supply are among the most

obvious reasons for assemblages. As animals are dependent directly

or indirectly on plants for their food, they must gather where the

plants grow, or in the currents in which the plant products are borne.

Whatever determines an assemblage of plants also causes, or at least

invites, an assemblage of animals. Whatever causes an assemblage of

particular plants, invites an assemblage of the particular animals

that use these plants. Animals that feed on plants are in turn preyed

upon by other animals, and these in turn by others. A whole train of

organisms may, therefore, be gathered into a region by the conditions

that foster a certain kind of vegetation there. In interpreting the

physical significance of such a train, it is obvious that the head of

the train carries the fundamental meaning. The dependent creatures

that follow the primary forms may be only incidentally, and perhaps

very slightly, adapted to the physical environment.

(_b_) =Adaptive relations.=—Organisms depending on other organisms

for food or other necessary conditions of life, present many forms

of adaptation the better to secure their food and to use it. These

adaptations are the consequences and the signs of the assemblage, and

are of the greatest service in interpreting the place and significance

of the organisms in the assemblage. Teeth usually reveal the food

of their possessors, and hence teeth are among the most significant

of fossils. Fortunately their functions require them to be hard and

durable, and hence well suited to fossilization. The growth of low

plants into trees forced a notable series of adaptations in the animals

that fed upon them in the matter of height, of reaching members, of

climbing, and probably at length of parachuting and flying. In these

and similar ways the floras and faunas took on special phases because

of the mutual relations of their members.

(_c_) =Competitive relations.=—The assembling of plants and animals,

with their prodigious possibilities of multiplication, brought

competition, and with it a struggle for food which often became a

struggle for existence, and out of this grew innumerable modifications

of form and habit. These have become so familiar since the great

awakening caused by the doctrines of Darwin and Wallace that they need

no elaboration here.

(_d_) =Offensive and defensive relations.=—Within limits, plants are

benefited by the feeding of animals and respond by developing seeds and

fruits that especially invite such action, their compensation being

found in planting and distribution. It is obvious that, on the whole,

the continued growth of plants is largely dependent on the renewal of a

supply of carbon dioxide through the agency of animals and some plants,

bacteria in particular. Otherwise the supply would become so reduced as

to greatly limit plant life. It has been estimated[298] that the whole

of the present supply of carbon dioxide would be consumed by plants in

one hundred years if the consumption continued at the present rate and

no carbon dioxide was returned. It is now well known that the so-called

decay by which carbon dioxide is freed is due more to microscopic

organisms than to inorganic processes. It seems clear, therefore, that

the continued activity of plants is largely due to their consumption

by animals and other plants. But still, though the larger good of

plants is conserved by the predaceous action of animals, and of certain

parasitic and saprophytic plants, their individual preservation

is often conserved by defensive devices, such as thorns, poisons,

bitter compounds, etc. This is notably true in desert regions where

the conditions are hard and the total extinction of plants would be

threatened if animals were permitted to feed freely upon them. Within

the animal world, the preying of one form upon another is the main

source of that great struggle for existence which has characterized

the whole known history of life, and has been one of the influential

factors in shaping the evolution of life and in modifying the special

aspects assumed by the floras and faunas of each period.

=Implied forms of life.=—The full meaning of the fossils of any period

can only be gathered by duly considering these relationships in their

interpretation. The existence of animals implies the existence of

plants in supporting abundance, whether the record contains their

relics or not; an animal with a protective covering implies an enemy;

a tooth of a specific kind implies the appropriate class of food,

etc. While inferences of this kind are subject to error, they are at

present the only means by which the faunas and floras of most ages

can be rounded out into a rational assemblage of organisms, that is,

an assemblage that affords the necessary food for its members and

an adequate function for the offensive and defensive devices which

its members present. Only a small part of the life that lived was

fossilized, and only a small part of the fossils actually carried

in the strata have been collected, because only a small part of the

strata are exposed at the surface. The direct record now accessible

is, therefore, very incomplete and hence the need—and in the need the

excuse—for adding the forms that are implied by the character of the

known fossils.

(2) _Assemblages Influenced by Environment._

It has been noted that some animals depend for existence on other

animals; that ultimately all animals depend on plants, and that green

plants alone can make food directly from inorganic material. Green

plants, therefore, head the train of dependencies, and their relations

to the physical conditions that surround them are the primal relations.

=Plant societies.=[299]—The control of physical conditions has been

sufficient to develop special associations or societies of plants by

fostering those adapted to these conditions and eliminating those

that are not. Among these are (1) the hydrophytes (“water plants”),

embracing those that grow in water or in very wet situations; (2)

xerophytes (“drought plants”), embracing the opposite class, which are

adapted to very dry situations; (3) mesophytes, including those suited

to conditions lying between these extremes, the great middle class to

which the prevailing upland vegetation belongs; and (4) the halophytes

(“salt plants”), which are dependent on the presence of certain salts,

and embrace such plants as are found on the seacoast, around salt

springs, on alkaline flats, etc. The characters which distinguish the

xerophytes from the hydrophytes and mesophytes have special geological

interest, as they aid in determining the climatic conditions, a feature

whose interest increases as the variability of the ancient climates is

more fully recognized.

Within these greater groups there are special minor associations

determined by soil, temperature, topography, subjacent strata, and

by the relations of the plants to one another.[300] These natural

groups are valuable indications of the agricultural capabilities of

the districts occupied by them. They may be regarded as the outcome of

Nature’s experiments in crop-raising, running consecutively through

thousands of years. They are natural correlations of compatible members

into communities of plants. Some members of the society are obviously

dependent on others, as certain forms of undergrowth on the shadowing

of the upper growth, as of vines upon supporting-trees, etc. There is

probably a more occult relation in some cases, the effects of certain

plants on the soil being sometimes advantageous to other plants, and

sometimes harmful, as illustrated in the conditions that require a

rotation of crops.

The chief point of geologic interest lies in the fact that floras

are not mere miscellaneous mixtures of plants that happen to live

in a given area at a given period, but are organized communities,

in a more or less definite sense. They therefore imply more or less

definitely the physical conditions which are congenial to them, and

thus furnish the basis for interpreting such conditions in the past, so

far as the floras are well preserved. The faunas, especially the land

faunas, being primarily dependent on the floras, furnish a basis for

interpretations of like import.

B. +The Influence of Geographic Conditions on the Evolution of

Floras and Faunas.+

The geographic features of the earth impose on organisms a complex

series of influences which modify the evolution of life and produce

faunal and floral variation on a large scale. The larger assemblages of

life, which inhabit a continent or dwell in a great sea, are designated

faunas and floras, as well as the smaller assemblages just discussed,

but obviously in a broader and in a different sense. The disseverance

of the land by the sea, or of the sea by the land, isolates the life

and forces independent development. The introduction of cold zones,

desert tracts, or other potent climatic belts has somewhat the same

effect. So, measurably, does the raising of a mountain range or a

plateau, or the sinking of critical portions of the sea-bottom.

=The development of provincial and cosmopolitan faunas.=[301]—If a

region is isolated from other regions by the cutting off of all ready

means of intermigration, as by the formation of an island from what

had been a peninsula, or of an inland sea from what had been a bay,

the flora and fauna are developed by themselves without much influx of

other forms, and hence become local or provincial. This is usually more

marked in the case of the fauna than of the flora, because the latter

has more ample means of dispersion, on the whole, and so the fauna may

for convenience be taken as the type. A good illustration is the native

fauna of Australia which was once connected with Asia, but has long

been separated from it. Previous to importations by man, this continent

had a very peculiar and distinct fauna, descended from its Mesozoic

inhabitants. Most of the isolated islands have peculiar faunas, but in

many cases they were isolated from the beginning, having been built up

by volcanic action from the bottom of the sea, and their faunas are

due to the accidents of transportation and to the development of these

sporadic forms in isolation.[302]

It is evident that whenever any geographic change introduces a

barrier to migration, the faunas of the dissevered portions will, in

all probability, develop along different lines, and will diverge into

provincial faunas. On the other hand, any geographic change that unites

areas and leads to intermigration, tends to a community of fauna or to

cosmopolitanism. These tendencies have been markedly felt all through

the geologic ages, and constitute one of the most vital features of

their history. When continents are connected, their faunas intermingle

and the exchange gives rise to common forms. They tend to blend into

one great fauna except so far as the local differences develop those

minor assemblages previously discussed. When continents are separated,

they tend to develop peculiar faunas, as do islands, but on a larger

scale. This is very obvious in the case of the land life, but needs

more special statement for the oceans.

The oceans constitute a single body of water with ample connections and

stirred by a system of constant circulation. Probably this has been

true for most of known geologic time. A single cosmopolitan fauna of

the largest type might be expected. This is in a measure realized in

the pelagic fauna of the open ocean, though this is somewhat modified

by the climatic zones. But the marine faunas that are fossilized in

the known strata, and have most geologic interest, are, with rare

exceptions, not those of the open ocean, but those of the shore zones

and of the shallow seas. Now, although these shore belts and shallow

seas are broadly connected with the great ocean body, and are usually

regarded as a part of it, they are singularly separated from it,

or rather they are singularly separated by it, so far as the life

dependent on shallow-water conditions is concerned. To this life, the

deep sea is a barrier not quite as effective as the land, but still a

barrier. The key to this important fact may be found in a consideration

of _the vertical distribution of life_.

The great horizon of life is at or near the contact zone of the

atmosphere with the hydrosphere and lithosphere. Life declines with

increasing altitude, partly because of the lowering temperature,

and partly because of the increasing tenuity of the atmosphere.

The successive changes of plant and animal life with the ascent of

mountains and plateaus is familiar. Life declines in descent into the

sea chiefly from lack of light, and secondarily from the lowering of

temperature. Light is essential to the formation of chlorophyll and,

through it, of all other organic compounds. The chlorophyll-forming

plants are, therefore, limited to such depths as are penetrated by the

rays necessary for the photosynthesis of organic matter. Vision is cut

off within 200 to 300 feet, and most plant growth takes place above

that depth. Photographic effects become feeble or inappreciable at 1000

to 1200 feet.[303] The photosynthesis of plants is chiefly aided by the

lower and middle part of the spectrum, while the ordinary photographic

work is chiefly done by the upper end, so that the photographic limit

is below the photosynthetic limit. Microscopic plants are sometimes

found lower than these limits, but they may have been carried below

their working limits by currents or other incidental agencies. For

all general purposes, the limiting depth of living carbon-compounding

plants may be set at 100 fathoms, as a generous figure—about the

average depth of the border of the continental shelf—while the vast

majority flourish only in the upper third of this depth.

Life does not cease here, for the products of this surface-life sink

to greater depths and are fed upon by forms of sea animals that have

become adapted to the dark and cold abyss of the ocean. Obviously,

these deep-sea forms are a very distinct type of life, and constitute a

fauna of the most pronounced kind, the abysmal fauna. Another distinct

fauna occupies the open-ocean surface, the pelagic fauna. Still a

third fauna occupies the shallow-water tract, whose bottom lies within

the light zone—_the photobathic zone_—and embraces the animals that

are dependent on the plants of this zone, or on its light and warmth,

and that are more or less fixed to the bottom or confined to the zone

because their food is there.