nature corresponding to the acidic glasses, obsidian and pitchstone.
=Till= (bowlder clay)—a stony or bowldery clay or rock rubbish formed
by glaciers.
=Trachyte=—a name formerly applied to a rock possessing a peculiar
roughness due to its cellular structure; but at present mainly confined
to a compact, usually porphyritic igneous rock, consisting mainly of
sanidine associated with varying amounts of triclinic feldspar, augite,
hornblende, and biotite.
=Trap=—a general term for igneous rocks of the darker basaltic types.
=Travertine=—a limestone deposited from calcareous waters, chiefly
springs; usually soft and cellular, and hence also called calcareous
tufa, calc sinter.
=Tuff= (tufa)—a term including certain porous granular or cellular
rocks of diverse origins; the volcanic tuffs embrace the finer kinds
of pyroclastic detritus, as ashes, cinders, etc.; the calcareous tufa
embrace the granular and cellular deposits of springs; the better usage
limits the term to volcanic clastics.
=Water-lime=—an impure argillaceous limestone possessing hydraulic
properties.
=Wacke=—a dark earthy or granular deposit formed from basic tuffs or
from the disaggregation of basaltic and similar rocks; a term which may
well come into more general use to distinguish the silicate sands that
arise from the disaggregation, but only partial decomposition, of basic
rocks, as arkose does, the like products of the acidic or granitoid
rocks, and as sandstone does, the granular products of complete
chemical decomposition.
ORE-DEPOSITS.[207]
Ore-deposits are but a special phase of the rock-forming processes
already discussed. They have peculiar interest because of their
industrial value. An ore is simply a rock that contains a metal that
can be profitably extracted, though for convenience the term is used
more broadly to include unworkable lean ores and ore material. The
metal need not preponderate or form any fixed percentage of the whole,
for the criterion is solely economic and not petrologic. A gold ore
rarely contains more than a very small fraction of one percent. of the
precious metal, while high-grade iron ore yields sixty-odd percent.
of the metal. In iron ore, the metallic oxide or carbonate makes up
nearly the whole rock; in gold ore, the metal is the merest incidental
constituent, from the petrologic point of view.
=Concentration.=—The essential fact in the formation of ores is the
unusual concentration of the metal. There are vast quantities of
all the metals disseminated through the rock substance of the earth
and even throughout the hydrosphere, but they do not constitute
ores because they have no economic value. They become ores when
concentrated in accessible places to a workable richness. The degree
of concentration required is measured by the value of the metal. The
essential elements for consideration are, therefore, (1) the original
distribution of the metallic materials through the rocks, (2) their
solution by circulating waters (or, rarely, by other means), (3)
their transportation in solution to the place of deposit, (4) their
precipitation in concentrated form, and (5) perhaps their further
concentration and purification by subsequent processes.
=Exceptional and doubtful cases.=—There are a few cases where
ore-deposits are made by volcanic fumes or vapors, but these may
be neglected here. Formerly, ores were often attributed to vapors
supposed to arise from the hot interior, but this mode of origin seems
incompatible with physical conditions. Ores have been attributed to
water originally contained as steam in lavas, and to waters escaping
from the interior of the earth, these waters being supposed to be
especially mineralized. Direct evidence on this point is obviously
beyond reach. Segregation in the molten state is recognized as a source
of ores, but its function is probably confined chiefly to partial
enrichment as stated below. There are other occasional methods, but the
chief process of concentration, immeasurably surpassing all others,
consists in the leaching out of ore materials disseminated through the
country rock and their redeposition in segregated forms, as an incident
of the recognized system of water circulation.
=Original distribution.=—The original distribution of ore material
through the primitive rocks is beyond the ken of present science,
for even the nature of the true primitive rocks is unknown. For
present purposes it is sufficient to regard all rocks concerned in
ore-deposition as either igneous or sedimentary, and to inquire, as
a first step, how far ordinary igneous and sedimentary processes
contribute to the segregation of ore material, leaving for a second
stage of inquiry the subsequent processes of concentration.
=Magmatic segregation.=—In a few instances workable masses of ore seem
to have arisen from lavas by direct segregation in the molten state,
without the aid of subsequent concentration by water action, on which
most ores are dependent. It is not improbable that the segregation
of metallic iron and nickel, and perhaps other metals, in the deeper
parts of the earth may be a prevalent process, giving rise to masses
like the native iron found in basalt in Greenland. This iron closely
resembles the nickel-irons of meteorites, which may be illustrations
of similar action in small planetary bodies that have been disrupted.
Metallic masses so segregated presumably gravitate toward the planetary
center and hence, whatever their inherent interest, have little
relation to a subject whose basal criterion is economic. It is not at
all improbable, however, that in the magmatic differentiation of the
lavas that come to the surface, there is some metallic segregation that
may make the enriched parts effective ground for the concentrating
processes of water circulation, and so determine the location of
ore-deposits. Igneous rocks are not equally the seats of ore-deposits,
even when the circulatory conditions seem to be equally favorable.
These conditions may not really be equally favorable, but there is
good ground to believe that some igneous masses constitute a richer
field for concentration than others. No definite rule, however,
for distinguishing rich varieties of rock from lean ones has been
determined. The basic igneous rocks are, on the whole, perhaps somewhat
richer in ores than the acidic class, but there is no established law.
Many acidic rocks bear more and richer ores than many basic ones. The
view here entertained is that both classes are subject to regional
enrichment through conditions connected with their origin, as yet
little known.
=Marine segregation and dispersion.=—In the formation of the
sedimentary rocks from the primitive and igneous rocks there was
notable metallic concentration in some cases, and even more notable
depletion in others. The ground-waters of the land, after their
subterranean circuits, carried into the water-basins various metallic
substances in solution. These were either precipitated early in the
marine or lacustrine drift of the waters, or became diffused throughout
the oceanic body. In the main they appear to have been widely diffused,
and either to have remained long in solution, or to have been very
sparsely deposited through the marine or lacustrine sediments. As a
rule, these sediments seem to contain less of valuable ore material
than igneous rocks, and this is rational, for, as we shall see, the
ground-water circulation of the land tends to concentrate and hold back
a part of the metallic content of the land rocks so that only a residue
reaches the sea. But there are important exceptions to this general
rule of sedimentary leanness.
The iron-ore beds of Clinton age ranging from New York to Alabama, and
appearing also in Wisconsin and Nova Scotia, form a stratum in the
midst of the ordinary sediments, and contain marine fossils. The great
ore beds of Lake Superior were originally of similar type, and so are
most other important iron deposits. It cannot be said, in most cases,
that these iron deposits are marine as distinguished from lacustrine or
lodgment deposits, but they are at least sedimentary. The ferruginous
material was originally disseminated widely through antecedent land
rocks, but was concentrated in the course of the sedimentary processes.
Limestone appears to have been sometimes enriched locally in lead and
zinc, and more rarely in copper, in the course of its sedimentation.
The lead and zinc regions of the Mississippi basin have been regarded
as dependent on such regional enrichment as a primary condition. This
localized enrichment has been attributed to solutions brought into the
sea from neighboring metal-bearing lands and precipitated by organic
action in the sea-water,[208] this organic action being more effective
in some areas than in others because of the unequal distribution of
life and the concentration of its decaying products. It is assumed
that such precipitates were at first too diffuse to be of value, and
further concentration was required to bring them together into workable
deposits; but the further processes appear to have been effective only
where the preliminary enrichment had taken place. At any rate, the
workable deposits are singularly localized, while the concentrative
processes are very general.
Metallic material is sometimes partially concentrated in sandstones
and shales in the process of sedimentation, though more rarely. The
copper-bearing shale (Kupferschiefer) of the Zechstein group in
Germany, so extensively worked along the flanks of the Harz Mountains,
is a striking example.
It is in every way reasonable to suppose that land-waters, on reaching
the margins of the water-basins, must occasionally find conditions
favorable for the precipitation of their metallic contents, and that
the ratio of these precipitates to other material might be relatively
high in the more favorable situations, and that this enrichment of the
country rock may be a condition precedent to a sufficient subsequent
concentration to yield workable accumulations.
It is, therefore, inferred that while the processes of sedimentation
tended on the whole to leanness, they gave rise to (1) some very
important ore-deposits, notably the chief iron ores, the greatest of
all ores in quantity and in real industrial value, and (2) a diffuse
enrichment of certain other areas which made them productive under
subsequent concentrative processes, while the sedimentary formations in
general were left barren.
=Origin of ore regions.=—From these considerations it appears that for
the fundamental explanation of “mining regions” we must look mainly (1)
to magmatic differentiation, so far as the country rock is igneous, and
(2) to sedimentary enrichment, so far as the rock is secondary. The
determining conditions in both cases are obscure and unpredictable, but
the recognition of such regions, and of the function of preliminary
diffuse regional enrichment, contributes to a comprehensive view of
the complex processes of ore concentration. The subsequent processes
consist in the further concentration of the ore material into sheets,
lodes, veins, and similar aggregations by ground-water circulation,
or else in the purification of the ores by the removal of useless or
deleterious material, or in both combined.
=Surface residual concentration.=—The simplest of all modes of
concentration takes place in the formation of mantle-rock. An insoluble
or slightly soluble metallic substance sparsely distributed through a
rock may be concentrated to working value by the decay and removal of
the main rock material, leaving the metallic material in the residuary
mantle. The tin ores of the Malay peninsula[209] are especially good
examples. The crystals of tin oxide were originally scattered sparsely
through granite and limestone, but by their decay and partial removal
it has accumulated in workable quantities. Certain gold fields and
certain iron ores have acquired higher values in the same way. Such
residuary material may be further concentrated by wash into gulches
or alluvial flats, in the course of which the lighter parts of the
mantle-rock are largely carried away, and the heavier, including the
metal or its compounds, are mainly left behind. Gold placers are the
best example. The mining of placers by hydraulic processes is but a
further extension of the natural process of concentration.
Such concentrates in past ages have in some cases been buried by later
deposits, and hence certain ancient sandstones, conglomerates, and
mantle-rocks have become ore-bearing horizons. The Rand of South Africa
appears to be of this type.
=Purification and concentration.=—A somewhat different mode of
concentration and purification has affected certain of the great iron
deposits. As already explained, the iron compounds were originally
dissolved from the iron-bearing constituents of the primitive or of
igneous rocks, or their derivatives, and were deposited in beds as
chemical stratiform deposits. In some cases they were sufficiently
pure, as first precipitated, to be worked profitably, but in most cases
they were seriously affected by undesirable mineral associates. When,
however, such impure deposits are subjected for long periods to the
percolation of waters from the surface under favorable conditions,
the impurities are often dissolved and the ores concentrated. The
great Bessemer ore-deposits of Lake Superior are examples. Originally
impure carbonates or silicates, they have been converted into rich and
phenomenally pure ferric oxides along certain lines of ground-water
circulation, and in certain areas of free leaching. Van Hise has shown
the definite relation between the water circulation and the production
of the high-grade ores.[210] Vast quantities of unconcentrated lean
ores lie in the tracts not thus purified and enriched by circulating
waters. This does not appear to be simply residual concentration. The
waters seem to have added ferric oxide brought from above, while they
carried away the “impurities,” silica, carbon dioxide, etc. Perhaps
this is an instance of mass action in which the ore present aided in
causing additions to itself.
=Concentration by solution and reprecipitation.=—By a process almost
the opposite of residual concentration, ore material is often
leached out of the surface-rock by water circulating slowly through
its pores, cleavage planes, and minute crevices, and is carried on
with the circulation until it reaches some substance which causes a
reaction that precipitates the ore material. This substance may be a
constituent of some rock which the circulating water encounters, such
as organic matter. More commonly, the precipitation seems to be due
to the mingling of waters charged with different mineral substances,
the mingling inducing reaction and the precipitation of the ore.
Precipitation, however, does not necessarily follow such commingling.
The junctions of underground waterways are sometimes characterized by
barrenness instead of richness. In the expressive phraseology of the
miners, a tributary current sometimes “makes” and sometimes “cuts out.”
In chemical phrase, when the mingling waters reduce the solubility
of the appropriate substance sufficiently, an ore-deposit is formed;
when they increase its solubility, they promote barrenness. Changes of
pressure and temperature may enter into the process, and mass action
may lend its aid when once a deposit is started.
More concretely stated, the general process of underground ore
formation appears to be this: the permeating waters dissolve the ore
material disseminated through the rock and carry it thence into the
main channels of circulation, usually the fissures, broken tracts,
porous belts, or cavernous spaces. If precipitating conditions are
found there, deposition takes place. The precipitating conditions
may be merely changes of physical state, such as cooling or relief
of pressure, but probably much more generally they consist in the
commingling and mutual reaction of waters that have pursued different
courses and become differently mineralized, as implied above. In these
cases the metal-bearing current may be scarcely more important than the
precipitating current.
Since the solvent action is a condition precedent to deposition, the
location of the greatest solvent action first invites attention. At
present it must be treated in general terms, for it is not known what
solutions must be formed beyond the fact that they must include the
ore material. Probably they must include much besides. Furthermore,
it is not known that deep-seated rocks carry more ore material than
similar rocks at or near the surface or at any other horizon. Fantastic
conceptions of deep-seated metallic richness are to be shunned as quite
beyond practical consideration. The water circulation is probably very
slight below a depth of two or three miles at most, and above that
depth there is little ground to suppose that the rocks of one horizon
are inherently more metalliferous than others of their kind. There is
no assignable reason why the igneous rocks at the surface are not as
rich in ore material as the igneous rocks two or three miles below,
since all are probably eruptive and of much the same nature on the
whole, being in many cases parts of the same eruptions.
=Location of greatest solvent action.=—Solvent action is probably most
_intense_ where the temperature and pressure are highest, that is, in
the deeper reaches of water circulation; but the _amount_ of water
passing in and out of the deeper zone is but a small fraction of that
which courses through the upper horizons, and the total solvent action
is quite certainly much greater in the upper zone than in the lower. At
the same time the solutions in the upper zone are quite certainly more
dilute than those below. The horizon of greatest solution lies between
the surface and a level slightly below the ground-water surface, or, in
other words, in the zone where atmosphere and hydrosphere coöperate.
Surface-waters are charged with atmospheric and organic acids and
other solvents, and their general effect upon the rocks is markedly
solvent down to or often below the permanent water-level. In this zone
concentration by residual accumulation may take place, as already
noted, if the metallic compounds resist solution; otherwise this zone
is depleted of its ore material by solution, and preparation is made
for deposition elsewhere.
Solution also continues to take place varyingly as the water descends
below this zone of dominant solution, and extends probably to the full
depth of water circulation, but in the deeper circuit, precipitation
also takes place and the action becomes complex. With the waters
taking up and throwing down material at the same time, it is difficult
to estimate the balance of results.
When waters that have been mineralized near the surface descend, they
often take on a precipitating phase at no great depth below the upper
level of the ground-water; thus sulphides that were oxidized and
dissolved near the surface are reprecipitated, often at horizons not
greatly below the permanent water-level. Waters that dissolve metallic
substances in the upper levels often become charged with sulphuretted
hydrogen and other precipitants within a few scores or a few hundreds
of feet of the surface, as deep wells abundantly prove. The freshness
of surface which metallic sulphides often exhibit at these levels is
fair ground for inferring recency of deposition and absence of solvent
action. Actual demonstrations of depositions in progress are not
wanting.
=Short-course action.=—The concentration which thus takes place by
solution in the upper zone, followed closely by reprecipitation within
a few score or a few hundred feet, may well be termed the _short-course
mode_ of ore concentration. It finds its most important illustration in
what is commonly known as the “secondary enrichment” of ore-deposits.
The ores in the outcropping edge of the vein or lode are dissolved
by the surface-waters, carried a short distance down the ore tract
and redeposited, causing enrichment at that point. This is only a
special case of what takes place generally at this horizon. It is
effective in this case because it has a previous partial concentration
to work upon. Secondary enrichments of this kind often contain most
or all the workable values of the ore tract. If instead of a previous
concentration in a vein, lode, or similar ore tract, there had been
partial concentration in the country rock by sedimentation, as in the
case of iron-ore beds and perhaps lead-, zinc-, and copper-impregnated
sediments, the short-course method may give working values not before
possessed. In some of the more obscure cases of previous partial
concentration in the crystalline and other rocks, it is probably this
short-course action that brings the concentration up to working value.
It is probably effective also in concentrating the metallic contents
of certain igneous rocks that were rich in metallic material when
extruded. How far this is true has been, and still remains, a mooted
question.
=Long-course action.=—After the surface-waters have once passed through
a cycle of dissolving and precipitating action, as they are apt to
do within the first few hundred feet of their courses below the
water-level, they are liable to pass through a succession of dissolving
and depositing stages, each reaction resulting in a state that makes
a new reaction possible. This is especially true if the waters pursue
deep courses. Strictly speaking, the precipitations usually concern
only a part of the substances dissolved. New substances are often
taken up in the very act of throwing down those already held, and the
way thus prepared for further changes. If the water pursues a deep
and devious course, it may receive additions by solution and suffer
losses by precipitation at many points in its course, both descending
and ascending. The changes are very complex, and in the case of a deep
or long circuit where various rocks, pressures, and temperatures are
encountered, the history becomes one long succession of complexities,
the full nature of which is not yet revealed.
In the deeper circuits, each individual current usually takes on a
descending, a lateral, and ascending phase, the three being necessary
to complete a circuit. The chemical conditions of the waters in the
three phases are probably not sharply distinguished from one another,
and hence there seems to be no defined horizon of concentration
comparable to that near the water-level already described. The chief
distinctions in the deeper regions relate to pressure, temperature,
length or depth of penetration, and duration of contact. It seems safe
to assume, as a general truth, that, other things being equal, the
solutions become more complex and more nearly reach general saturation
the farther and the deeper the waters penetrate.
It has long been a mooted question whether ore-deposits are due chiefly
to descending, to lateral, or to ascending currents. The question in
its usual form is too undiscriminating for advantageous discussion, but
if the ore-deposits due to surface or short-course concentrations and
reconcentrations be set aside, as in some sense a separate class, the
relative functions of the descending, the lateral, and the ascending
portions of the deeper circulations become a measurably definite
question. Two great working factors enter into the comparison: (1) much
greater circulation in the upper zone, where lateral movement most
prevails; (2) much greater heat and pressure in the lower zone, where
the circulation must be chiefly vertical.
Heat and pressure in general favor solution, and hence so far as this
factor goes, descending water is likely to be increasing its mineral
content, rather than diminishing it by deposition. But this is
only general; particular elements of the solution may be deposited.
In ascending, as the same water must later, it is predisposed to
deposition from loss of solvent power through reduction of pressure
and temperature. The theoretical balance is here clearly in favor
of preponderant deposition by the ascending portion of the current.
So far as precipitation is dependent on the mingling of differently
mineralized waters, descending and ascending currents seem to be
situated much alike, in general, for both are subject to accessions and
mutual unions.
The amount of water that circulates in the deeper horizons is much less
than that nearer the surface. Allowing a few hundred, or at most one
or two thousand feet for the special short-circuit zone next below the
water-level (it is known to reach 1000 to 1500 feet in some cases),
the water circulating through the next 1000 or 2000 feet is probably
several times greater than all that circulates at greater depths, and
this greater circulation above doubtless offsets, in greater or less
measure, the intensified action of the deeper circulation. Much of the
upper and more rapid circulation is lateral, being actuated by the
sloping surface of the ground-water, which in turn is determined by
topography, precipitation, and other surface conditions. Theoretical
considerations, therefore, favor the view that lateral flow is
an important factor in the concentration of ore material. But as
descending and lateral currents almost inevitably meet and mingle
with ascending currents, it is difficult to distinguish, in the
ore-deposits, the special functions of each phase of action. It is even
more difficult to determine whether the different phases are not alike
essential to the mutual reactions on which the deposition depends.
It may be as necessary to have a precipitant as to have a metallic
constituent in solution to be precipitated, and what is more, this
precipitating agency may be a substance of no economic value in itself
and of no obvious relations to the substances that form the ores. If
the deposition is due solely to a physical state, as relief of pressure
or lowering of temperature, these considerations do not hold.
=Summary.=—The general results are probably these: In the deeper
circuits, more ore material is brought upward and deposited than is
carried downward and deposited, so that metallic values are shifted
toward accessible horizons. In the lateral currents, more metallic
values are shifted toward the trunk-lines of circulation—the great
crevices and other waterways—than are carried from these into the rock
and distributed, and lateral segregation results. At the same time
the atmospheric waters acting at or near the surface concentrate ore
values downwards. _The sum total of these processes is to promote the
development of the higher ore values in accessible horizons, and along
the main lines of circulation._
=The influence of contacts.=—As ore-deposits depend on a dissolving
state followed by a depositing state of the waters, and perhaps on a
complex succession of these alterations, it is obvious that conditions
which favor changes of state and the commingling of different kinds
of water are apt to be favorable to ore production. At any rate it is
observed that many important ore-deposits occur at the contact between
formations of different character. The contact of igneous rock with
limestone is a rather notable instance. It is not to be inferred that
such contacts are generally accompanied by workable ore-deposits, but
merely that a notable proportion of workable ore-deposits occur at such
junctions. It is rational to suppose that where the chemical nature of
the two formations is in contrast, the waters that percolate through
the one are likely to be mineralized very differently from those that
course through the other, and hence that on mingling at the contact,
reactions are specially liable to take place, and that when a valuable
metallic substance is present it is liable to be involved and by chance
to suffer precipitation. Reactions are the more probable because the
contact is likely to be a plane of crustal movement, and hence more or
less open and accompanied by fractures, zones of crushed rock and other
conditions that facilitate circulation and offer suitable places for
ore formation.
=The effect of igneous intrusions.=—A special case of much importance
arises when lavas are intruded into sediments that have previously been
partially enriched in the ways above described. The igneous intrusion
not only introduces new contact zones, and more or less fracturing,
but it brings into play hot waters with their intensified solvent
work, their more active circulation, and the reaction between waters
of different temperatures. The special efficiency of these agencies is
believed to be the determining factor in many cases.
=The influence of rock walls.=—The rock walls themselves are thought
sometimes to be a factor in ore-precipitating reactions. By mass
action, they may withdraw a constituent of the solution and destroy
its equilibrium in such a way as to cause the precipitation of the
metallic constituent. Once deposited on the walls ores aid, by mass
action, the further accretion of ores.
The special forms which ores assume in deposition, as beds, veins,
lodes, stockworks, disseminations, segregations, etc., are chiefly
incidental to the local situation in which the essential chemical or
physical change takes place.
CHAPTER VIII.
STRUCTURAL (GEOTECTONIC) GEOLOGY.
=The structural phases which rocks assume.=—In the previous chapters,
the general method by which rocks are formed has been set forth, and
many of their structural features have been touched upon incidentally.
It remains to assemble the structural features already mentioned, and
to consider certain additional structural phases which rocks assume.
STRUCTURAL FEATURES OF SEDIMENTARY ROCKS.
In the deposition of sediments in the sea, or in other bodies of
standing water, the coarser portion of the material is usually
deposited in the shallow water near the shore where the wave-action
is strongest, and the less coarse of various grades is deposited at
greater and greater distances from the land, while only extremely
fine silt is usually carried out to abysmal depths (see p. 380). To
this general law of distribution there are important exceptions. Fine
sediments are sometimes deposited near the shore, and where currents,
tidal agitation, or floating ice are effective, coarse deposits are
occasionally carried far out from the shore.
=Stratification.=—Sedimentary rocks are usually arranged in more
or less distinct layers; that is, they are _stratified_. The
stratification consists primarily in the superposition of layers of
different constitution or different compactness on one another. Layers
of like constitution or compactness are often separated by films of
different material which cause the partings between them. The bedded
arrangement of stratified rocks is due to various causes, but primarily
to the varying agitation of the waters in which the sediment was laid
down. Where the depositing waters are agitated to the bottom, coarse
sediment is likely to be deposited. Where the waters are quiet at the
bottom, fine sediment is the rule. Since the agitation of the waters is
subject to frequent change, it follows that coarser material succeeds
finer, and finer coarser, in the same place. Hence arise _beds_,
_layers_, and _laminæ_. The terms _layer_ and _bed_ are generally used
as synonyms, while _laminæ_ are thinner divisions of the same sort.
The term stratum is sometimes applied to one layer and sometimes to
all the consecutive layers of the same sort of rock. For the latter
meaning the term _formation_ is often used. Sometimes bedding seems
to have been determined by strong currents which temporarily not only
prevented deposition over a given area, but even cut away the loose
surface of deposits already made, giving a firm surface from which
succeeding deposits are distinct. This sequence of events is sometimes
shown by the truncation of laminæ, and by other signs of erosion. The
commoner sorts of bedded rock are limestones, shales, sandstones, and
conglomerates.
The bedding of _limestones_ is often caused by the introduction of thin
films of clayey material which interrupt the continuity of the lime
accumulation and cause natural partings. Sometimes, however, bedding
arises from variations in the physical condition of the lime sediment
itself. Lamination is not usually conspicuous in pure limestone, though
it may be well developed in the shaly phases of this rock. _Shales_ are
normally laminated as well as bedded, and the lamination is often more
notable than the thicker bedding. Bedding in shale may arise from the
introduction of sandy laminæ, or by notable changes in the texture of
the shale material. Similarly, _sandstones_ are sometimes divided into
beds by shaly (clayey) partings, but more often by variations in the
coarseness of the sand itself, or by the presence of laminæ that are
less coherent than those above and below. Sometimes the layers appear
to be determined by the compacting of the surface of sand already
accumulated before it was buried by later deposits. Sandstones may
be thick- or thin-bedded, and their bedding passes insensibly into
lamination.
Sand deposits usually take place in relatively shallow water, and
the sand is subjected to much shifting before it finds a permanent
lodgment. In the course of this shifting, bars are formed which usually
have a rather steep face in the direction in which they are being
shifted. The sand carried over the top of the bar finds lodgment on
the sloping terrace face. The inclined laminæ thus formed constitute
a kind of bedding, but since its planes do not conform to the general
horizontal attitude of the formation as a whole, it is called _false-_
or _cross-bedding_ or, more accurately, _cross-lamination_ (see Fig.
368). The same structure is developed on delta fronts and generally in
water shallow enough to be subject to frequent agitation at the bottom.
Sandstone is cross-bedded more commonly than other sorts of sedimentary
rock.
The bedding of _conglomerate_ is due chiefly to variations in
coarseness. Laminæ or thicker layers of sand are frequently found
between layers of coarser material. Conglomerate is likely to be
thick-bedded, and cross-bedding is common.
=Lateral gradation.=—When the varying nature of the agitation of
the sea at different depths and along the different parts of the
coast-border, and during different phases of the sea-currents, is
considered, it will be readily understood that sedimentary beds are
affected by many irregularities, and that deposits of one kind grade
into others horizontally with great freedom. Thus a bed of conglomerate
(gravel) may grade laterally into sandstone, and this into shale or
limestone. It is indeed rather more remarkable that the sedimentary
strata should be as regular and persistent as they are, than that they
sometimes grade into one another.
[Illustration: +Fig.+ 368.—Cross-bedding in sandstone. Dells of the
Wisconsin near Kilbourn, Wis. (Bennett.)]
=Special markings.=—The rhythmical action of waves gives rise to
undulatory lodgment, known as _ripple-marks_ (Fig. 324). They are
usually not the direct product of the surface-waves, since they are
much too small. They are produced mainly by the vibratory movement of
the undertow, but they apparently result from various other phases of
vibratory agitation of the bottom waters. They are sometimes made by
streams and stream-like currents. Ripple-marks are apparently preserved
indefinitely under proper circumstances. They are sometimes found, for
example, on very ancient quartzites. Ripples are also made by wind
(p. 37). Ripple-marks are usually only an inch or two from crest to
crest, but in rare instances they attain much greater size. Examples
of ripple-marks 30 feet across are known.[211] Occasional ridges
and depressions of much greater dimensions are produced which are
attributable to the formation of successive bars, or to the building
of wave-cusps.[212] _Rill-marks_ are not infrequently produced by the
undertow and other currents passing over pebbles, shells, etc. (Figs.
325 and 326).
[Illustration: +Fig.+ 369.—Mud-cracks in Brunswick Shale, N. J.
(Kümmel.)]
Sediments are sometimes exposed between tides, or under other
circumstances, for periods long enough to permit drying and cracking at
the surface. On the return of the waters, the cracks may be filled and
permanently preserved. These are known as _sun-cracks_ or _mud-cracks_
(Figs. 328 and 369). They chiefly affect shales, but are occasionally
seen in limestones and fine-grained sandstones. During the exposure of
the sediments a shower may pass and _rain-drop impressions_ (Fig. 370)
be made which are subsequently filled by fine sediment and preserved.
The size and depth of rain-drop impressions give some hint as to the
meteorological conditions of far-off ages. _Wave-marks_, which consist
of the faint line-ridges developed on a sandy beach at the limit of the
incoming wave, are sometimes preserved and may be seen occasionally on
layers of rock deposited millions of years ago.
[Illustration: +Fig.+ 370.—Rain-drop impressions. (Brigham.)]
=Concretionary structure.=—Various sedimentary formations contain
nodules or irregularly shaped masses of mineral matter unlike the
rock in which they occur. When these nodules consist of matter
aggregated about some center, they are called _concretions_. They
are common in sedimentary rocks, and here it may sometimes be
seen that the aggregation has taken place about a shell, a leaf,
or some other organic relic. The nuclei are, however, not always
organic. The material of the concretion may have come from the
immediately surrounding rock, having been first dissolved by water
and then deposited about the nucleus, or it may have been introduced
from without, likewise by the agency of water. In the first case,
the mineral matter of the concretion is usually one of the minor
constituents of the rock. Thus the commonest concretions in limestone
are composed of impure silica (chert, Fig. 361); in shale, of lime
carbonate or iron sulphide; in sandstone, of iron oxide. The concretion
may be made up almost wholly of concentrated matter, in which case
the matter originally in the place of the concretion has been crowded
aside; or it may involve much of the material of the imbedding rock.
Thus the concretion of lime carbonate in shale may be nearly pure,
or it may involve much of the earthy matter of the shale, while the
concretion of iron oxide in sandstone commonly includes much sand. In
extreme cases, indeed, the concentrated matter of the concretion merely
cements the material involved into distinct nodules. Occasionally the
rock substance itself takes on a concretionary form, all or most of its
material being involved.
[Illustration: +Fig.+ 371.—Discoid calcareous concretions from
post-glacial clays. Ryegate, Vt. (Photo. by Church.)]
[Illustration: +Fig.+ 372.—Irregular calcareous concretions. Ryegate,
Vt. (Photo. by Church.)]
[Illustration: +Fig.+ 373.—Calcareous concretions, some of them showing
bilateral symmetry. Ryegate, Vt. (Photo. by Church.)]
[Illustration: +Fig.+ 374.—Irregular tubular silicious concretions
in Arikaree clays. Northwest of Wildcat Mountain, Banner Co., Neb.
(Darton, U. S. Geol. Surv.)]
In size, concretions may vary from microscopic dimensions to huge
masses, 8, 10, or even more feet in diameter. The variations in shape
are also great. They may be spherical, elliptical, discoid, or they
may assume more irregular and complex forms (Figs. 371 and 372). The
conditions of growth have much to do with the form. Thus a concretion
which starts as a sphere may find growth easier in one plane than
another, when it becomes discoid. Two or more concretions sometimes
grow together, giving rise to complicated forms. Some of the most
complex and fantastic forms are perhaps to be explained in this way.
Concretions sometimes take the form of tubes. Some minute tubular
concretions were formed about rootlets, but the larger ones appear to
owe their form to other influences (Fig. 374).
[Illustration: +Fig.+ 375.—Section of a concretion (_septarium_) the
cracks of which have been filled by matter deposited from solution.
About half natural size. (Photo. by Church.)]
[Illustration: +Fig.+ 376.—Section of a concretion, the cracks in which
have been filled by deposition from solution. The filling appears to
have wedged the parts of the original concretion apart. The fillings
are veins. Some of them show that the vein-material was deposited on
both walls. About half natural size. (Photo. by Church.)]
One of the most extraordinary features of some concretions of
complex form is their symmetry. This may be of various phases; in
exceptional cases there is a bilateral symmetry almost as perfect as
in the higher types of animals. This is especially true of certain
calcareous concretions developed in plastic clays (Fig. 373).
[Illustration: +Fig.+ 377.—Septarium from Cretaceous clays near the
east base of the Rocky Mountains in Montana. (Photo. by Church.)]
Concretions sometimes develop cracks within themselves, and these may
then be filled with mineral matter differing in composition or color
from that of the original concretions (Figs. 375 and 376). Concretions
the cracks of which have been filled by deposition from solution,
are called _septaria_. They are especially abundant in some of the
Cretaceous shales and clays. In not a few cases the filling of the
cracks appears to have wedged segments of the original concretion
farther and farther apart, until the outer surface of the septarium is
made up more largely of vein-matter than of the original concretion
(Fig. 377). Such concretions are often popularly known as “petrified
turtles.”
Concretions of the sort indicated above often develop after the
enclosing sedimentary rock was deposited. This is shown, among other
things, by the fact that numerous planes of lamination may sometimes be
traced through the concretions.
Concretions also form in water during the deposition of sedimentary
rock. Exceptionally, sedimentary rock is made up chiefly of
concretions. The chemical precipitates from the concentrated waters
of certain enclosed lakes sometimes take the form of minute spherules
which resemble the roe of fish. From this resemblance the resulting
rock is called _oolite_ (Fig. 357). Oolite is now forming about some
coral reefs, presumably from the precipitation of the lime carbonate
which was temporarily in solution. Considerable beds of limestone
are sometimes oolitic. The calcium carbonate of such rock may be
subsequently replaced by silica, so that the oolitic structure is
sometimes found in silicious rock. If the concretions become larger,
say as large as peas, the rock is called _pisolite_ instead of oolite
(Fig. 378).
[Illustration: +Fig.+ 378.—Pisolite. Half natural size. (Photo. by
Church.)]
[Illustration: +Fig.+ 379.—Columnar structure, “Devil’s Post Pile.”
Upper San Joaquin Canyon, Sierra Nevada Mountains.]
Beds of iron ore are likewise sometimes concretionary. Thus in the
Clinton formation there are widespread beds of “flaxseed” ore made up
of concretions of iron oxide which, individually, resemble the seed
which has given the ore its name. The nucleus in this case is usually
too small for identification.
=Secretions.=—When cavities in rock are filled by material deposited
from solution, the result is sometimes called a _secretion_. Secretions
therefore grow from without toward a center, while concretions follow
the opposite order. Crystal-lined cavities (_geodes_, Fig. 359) and
agates (Fig. 358) are examples of secretions. Crystal-lined cavities
and veins are the same in principle.
STRUCTURAL FEATURES OF IGNEOUS ROCKS.
Certain structural features of igneous rocks have been mentioned in
treating of their origin in the previous chapter. When a great flow
of lava spreads out upon the surface, there is no internal lamination
or stratification, and the resulting rock is usually classified as
massive rather than stratified; but when a succession of flows occur,
each individual flow forms a layer, and the series as a whole becomes
stratiform. The successive flows are not usually coextensive. If the
later flows of the closing stages of a period of vulcanism fail to
reach as far as the earlier ones, a terraced or step-like aspect is
given to the region, whence the name _trap-rock_ (_trappe_, steps)
is derived. Such lava sheets, especially if of basalt, often assume
a _columnar_ structure in cooling, the columns being rude six-sided
prisms standing at right angles to the cooling surfaces (Figs. 379
and 380). This phenomenon is usually best developed where the sheet
is intruded between layers of preexisting rock in the form of sills.
The formation of the columns is sometimes regarded as a variety of
concretionary action, but more commonly as a result of contraction. The
former is suggested by the ball-and-socket ends of the sections of some
columns (Fig. 382). The development of the columns by contraction may
be explained as follows: The surface of the homogeneous lava contracts
about equally in all directions on cooling. The contractile force may
be thought of as centering about equidistant points. About a given
point, the least number of cracks which will relieve the tension in all
directions is three (Fig. 383). If these radiate symmetrically from the
point, the angle between any two is 120°, the angle of the hexagonal
prism. Similar radiating cracks from other centers complete the columns
(Fig. 384). A five-sided column would arise from the failure of the
cracks to develop about some one of the points (Fig. 385).
[Illustration: +Fig.+ 380.—Columnar structure, obsidian cliff,
Yellowstone Park. (Iddings, U. S. Geol. Surv.)]
When lava is forced into crevices or rises to the surface through
fissures, and the residual portion solidifies in them, it gives rise
to _dikes_, as illustrated in Figs. 2 and 417 (not a true dike). Dikes
are sometimes affected by columnar structure. In this case, as in all
others, the columns are likely to be at right angles to the cooling
surface. Lava solidifying in the passageway leading from the interior
of a volcano gives rise to a _neck_ or _plug_. If the lava is forced
between beds of rock in the form of a sheet, and solidifies there, it
is called a _sill_. If, after rising to a certain point in the strata,
the lava arches the beds above into a dome, and forms a great lens-like
or cistern-like mass, it constitutes a _laccolith_ (Fig. 334). If an
intrusion of the laccolithic type faults the overlying beds instead
of arching them, and especially if the vertical dimension of the
intruded mass be great in comparison with its lateral dimensions, its
shape is more like that of a plug or core. Such an intruded core is a
_bysmalith_[213] (Fig. 124). Between the bysmalith and the laccolith
there are various gradations, just as between the laccolith and the
sill. When lava forces aside the rocks at considerable depths or
absorbs them by solution or by “stoping,” and then solidifies in great
masses of irregular or undetermined forms, these masses are called
_batholiths_.
[Illustration: +Fig.+ 381.—Sections of columns from Giant’s Causeway,
coast of Ireland.]
[Illustration: +Fig.+ 382.—Ball-and-socket joints in columns of basalt.
(Scrope.)]
[Illustration: +Fig.+ 383.—Diagram to illustrate the first stages in
the formation of hexagonal columns by contraction.]
[Illustration: +Fig.+ 384.—The completion of the hexagonal columns.]
[Illustration: +Fig.+ 385.—Diagram to illustrate the development of
five-sided columns.]
_Volcanic cones_ are familiar structures built up about the vents of
active volcanoes, and will be discussed under vulcanism.
STRUCTURAL FEATURES ARISING FROM DISTURBANCE.
=Inclination and folding of strata.=—The original attitude of beds,
whether formed by water or by lava-flows, is normally horizontal, or
nearly so. Both kinds of deposits, however, occasionally take place
on considerable slopes. Modifications of the original attitude result
from earth movements, and the measurement of these modifications is
an important feature of field study. It is recorded in terms of _dip_
and _strike_. The dip is the inclination of the beds referred to a
horizontal plane, as illustrated in Fig. 386, and is usually measured
by a clinometer, the principle of which is shown in Fig. 387. In
measuring the dip, the maximum angle is always taken. In Fig. 386, for
example, the angle would be less if the direction were either to the
right or left of that indicated by the arrow. The direction as well as
the amount of the dip is always to be noted. This must be determined
by the compass, to which the clinometer may be conveniently attached.
Dip 40°, S. 20° W. gives the full record of the position of the bed of
rock under consideration. The strike is the direction of the horizontal
edge of dipping beds, or more generally, the direction of a horizontal
line on the surface of the beds. This is illustrated in Fig. 386. Since
the strike is always at right angles to the dip, the strike need not be
recorded if the direction of the dip is. Thus dip 40°, S. 20° W. is the
same as dip 40°, strike N. 70° W.
[Illustration: +Fig.+ 386.—Diagram illustrating dip and strike.
(Geikie.)]
[Illustration: +Fig.+ 387.—The clinometer.]
[Illustration: +Fig.+ 388.—Open anticline, near Hancock, Md. (Russell,
U. S. Geol. Surv.)]
[Illustration: +Fig.+ 389.—Closed anticline, near Levis Station,
Quebec. (Walcott, U. S. Geol. Surv.)]
[Illustration: +Fig.+ 390.—Inclined anticline. (Van Hise, U. S. Geol.
Surv.)]
[Illustration: +Fig.+ 391.—Recumbent anticline. (Van Hise, U. S. Geol.
Surv.)]
[Illustration: +Fig.+ 392.—Syncline, C. & O. canal, 3 miles west
of Hancock, Md. The beds are shale and sandstone near base of the
Silurian. (Walcott, U. S. Geol. Surv.)]
When the beds incline in a single direction, they form a _monocline_.
When beds are arched so as to incline away from one another, they
form an up-fold or _anticline_ (Figs. 388 to 391). The anticline may
depart from its simple form, as shown in Figs. 390 and 391. When beds
are curved downward so as to incline towards one another, they form
a _syncline_ (Fig. 392). When beds assume the position shown in Fig.
393, the folds are said to be _isoclinal_. When they are arched so as
to form a cone or dome, and incline in all directions from a central
point, they are said to have a _quaquaversal_ dip. When considerable
tracts are bent so as to form great arches or great troughs with many
minor undulations on the flanks of the larger, they are designated
as _geanticlines_, or _anticlinoria_ (Figs. 394 and 395), and
_geosynclines_ or _synclinoria_ (Figs. 396 and 397). Folding is often
accompanied by the development of slaty cleavage (p. 440).
[Illustration: +Fig.+ 393.—Isocline. (Van Hise, U. S. Geol. Surv.)]
[Illustration: +Fig.+ 394.—Anticlinorium: diagrammatic. (Van Hise, U.
S. Geol. Surv.)]
[Illustration: +Fig.+ 395.—Anticlinorium. General section in the
central massif of the Alps. (Heim.)]
[Illustration: +Fig.+ 396.—Synclinorium: diagrammatic. (Van Hise, U. S.
Geol. Surv.)]
[Illustration: +Fig.+ 397.—Synclinorium, Mt. Greylock, Mass. (Dale, U.
S. Geol. Surv.)]
[Illustration: +Fig.+ 398.—A series of diagrams illustrating actual
field relations in regions of folded strata. Westchester Co., N. Y.
(Dana.)]
[Illustration: +Fig.+ 399.—Diagram to show how dip and strike are
recorded.]
[Illustration: +Fig.+ 400.—Map record of dip and strike, showing
synclinal structure.]
[Illustration: +Fig.+ 401.—Map record of dip and strike showing
anticlinal structure.]
[Illustration: +Fig.+ 402.—The structure of the area shown in Fig. 401,
in cross-section.]
[Illustration: +Fig.+ 403.—Map record of dip and strike showing
plunging (dipping down at ends) anticline.]
As found in the field, folds are usually much eroded, and often
completely truncated (Fig. 398). The determination of anticlinal or
synclinal structure is then not based on topography, or even on such
sections as shown in Figs. 394 to 397, for such sections are relatively
rare. The structure is determined by a careful record of dips and
strikes. On the field map, the record may be made as shown in Figs.
399 to 401, where the free ends of the lines with but one free end
point in the direction of dip, while the other lines represent the
directions of strike. Applying this method, the structure shown in
Fig. 400 represents a syncline, and that in Fig. 401 an anticline. In
cross-section, the structure presented by Fig. 401 would appear as
in Fig. 402. Fig. 403 shows a doubly plunging anticline; that is, an
anticline the axis of which dips down at either end. Fig. 404 shows a
combination of synclines and anticlines, and Fig. 405 a cross-section
along the line _ab_ of Fig. 404. The outcrops of rock where the dip and
strike may be determined may be few and far between, but when they
are sufficiently near one another, the structure of the rock, as shown
in Fig. 405, may be worked out, even though the surface be flat.
[Illustration: +Fig.+ 404.—Map record of dip and strike showing complex
structure.]
[Illustration: +Fig.+ 405.—Cross-section of Fig. 404 along the line
_ab_.]
[Illustration: +Fig.+ 406.—Complex folding. Section across the Alps
from the neighborhood of Zürich toward Como: about 110 miles. (Heim
and Prestwich.)]
[Illustration: +Fig.+ 407.—Generalized fan fold of the central massif
of the Alps. (Heim.)]
[Illustration: +Fig.+ 408.—Intimate crumpling of beds near head of
Sperry glacier, Mont. (Meyers.)]
[Illustration: +Fig.+ 409.—Intimate crumpling in detail, accompanied by
faulting. Jasper Hill, Ishpeming, Mich. (Meyers.)]
Much the larger portion of the earth’s surface is occupied by beds that
retain nearly their original horizontal attitude; but in mountainous
regions the beds have usually suffered bending, folding, crumpling, and
crushing, in various degrees, in the course of the deformations that
gave rise to the mountains. Distortion is on the whole most intense
and characteristic in the most ancient rocks known, the Archean, in
which a distorted condition is nearly universal, so far as observation
goes. Distortion is assigned chiefly to lateral thrust arising from
the shrinkage of the earth, as explained in the chapter on Earth
Movements. The simpler, and some rather complex forms of deformation,
are shown in the preceding figures, but the folding is sometimes much
more complex (Fig. 406), the folds sometimes “fan” (Fig. 407), and the
beds of which they are composed are sometimes intricately crumpled
(Figs. 408 to 410). Among these various phases of deformation there are
all gradations and combinations. Overturned folds reverse the order of
the strata in the under limb of the fold. After such folds have been
greatly eroded, so that their outer form is lost and their relations
have become obscure, the reversed beds are likely to be interpreted as
though they lay in natural order. In such a case as that represented in
Fig. 411, a complex structure may be interpreted as a simple one. Thus
the strata of Fig. 411 may have the structure shown in Fig. 412, 413,
or 414, so far as dip and strike show.
[Illustration: +Fig.+ 410.—Plicated layers of thin-bedded chert in
limestone, etched by erosion. Lower Cambrian (?), two miles southwest
of Big Pine, Inyo Co., Cal. (Walcott, U. S. Geol. Surv.)]
=Joints.=—The surface rocks of the earth are almost universally
traversed by deep cracks called _joints_ (Figs. 415, 138 and 140). In
most regions there are at least two systems of joints, the crevices of
each system being roughly parallel to one another, while those of the
two systems, where there are two, are approximately at right angles.
In regions of great disturbance, the number of sets of joints is often
three, four, or even more. The joints of each set may be many yards
apart, or in exceptional cases, but a few inches, or even a fraction of
an inch.
Generally speaking, there are more systems of joints, and more
frequent joints in each system, where the rocks are much deformed than
where they have been but little disturbed. In undisturbed rocks the
joints approach verticality, but in regions where the rocks have been
notably deformed, the joint planes may have any position. Not rarely
they simulate bedding planes, especially in igneous and metamorphic
rocks (Fig. 416). In the latter case especially, the cleavage due to
jointing is often mistaken for bedding. They do not ordinarily show
themselves at the surface in regions where there is much mantle rock,
but they are readily seen in the faces of cliffs, in quarries, and,
in general, wherever rock is exposed (Figs. 138 and 140). Though some
of them extend to greater depths than rock has ever been penetrated,
joints are, after all, superficial phenomena. They must be limited to
the zone of fracture, and most of them are probably much more narrowly
limited. Joints frequently end at the plane of contact of two sorts
of rock. Thus a joint extending down through limestone may end where
shale is reached. Joints are frequently offset at the contact of
layers or formations, and a single joint sometimes gives place to many
smaller ones. All these phenomena are to be explained on the basis of
the different constitution and elasticity of various sorts of rock.
Generally speaking, rigid rock is more readily jointed than that which
is more yielding.
Joints may remain closed, or they may gap. In the latter case, they may
be widened by solution, weathering, etc., but they are quite as likely
to be filled by detritus from above, or by material deposited from
solution (veins). It is along joint-planes that many rich ore-veins are
developed (pp. 478–484).
[Illustration: +Fig.+ 411.—This diagram might represent either
isoclinal or monoclinal structure. In the former case the strata might
have the structure shown in any one of the following Figures, 412 to
414, so far as dip and strike show. (Dana.)]
[Illustration: +Fig.+ 412.—A possible interpretation of Fig. 411.
(Dana.)]
[Illustration: +Fig.+ 413.—A possible interpretation of Fig. 411.
(Dana.)]
[Illustration: +Fig.+ 414.—A possible interpretation of Fig. 411.
(Dana.)]
[Illustration: +Fig.+ 415.—Jointed rocks. Cayuga Lake, N. Y. (Hall.)]
Joints have been referred to various causes, among which tension,
torsion,[214] earthquakes,[215] and shearing[216] are the most
important. Most of them may probably be referred to the tension or
compression developed during crustal movements.[217] In the formation
of a simple fold, for example, tension-joints parallel with the fold
will be developed, if tension goes beyond the limit of elasticity
of the rock involved. If the axis of a fold is not horizontal, that
is, if it “plunges,” as it commonly does, a second set of joints
roughly perpendicular to the first will be developed. If the uplift be
dome-shaped and sufficient to develop joints, they will radiate from
the center. It is true that joints affect regions where the rocks have
not been folded, and where they have been deformed but little, but
deformation to some extent is well-nigh universal.
[Illustration: +Fig.+ 416.—Jointing in granite. The surface of the rock
is a joint plane. Northwest boundary of the United States. The edges
of other joint planes normal to the surface are also shown. (Ransome,
U. S. Geol. Surv.)]
[Illustration: +Fig.+ 417.—Sandstone dike. Northern California.
(Diller, U. S. Geol. Surv.)]
A minor cause of tension-jointing is shrinkage, due (1) to cooling, as
in the development of the columnar structure of certain lavas, and (2)
to dessication, as shown by the cracks developed in mud when it dries.
These causes, however, are not believed to affect rock structures to
any considerable depth. Torsional joints and joints due to earthquake
vibrations appear to be special phases of tension-joints.
Two or more sets of joints may also be produced by compression,
the number being dependent on the complexity of the folding. Many
compression-joints correspond in direction with planes of shearing.
They are often associated with minor faulting and with slaty cleavage.
Tension-joints appear to be much more widely distributed than
compression-joints.
=Sandstone dikes.=—Exceptionally, open joints are filled by the
intrusion of sedimentary material from beneath. Thus have arisen the
remarkable sandstone dikes[218] of the West, especially of California
(Fig. 417). Such dikes are sometimes several miles (nine at least) in
length. The sand of these dikes was forced up from beneath either by
earthquake movements or by hydrostatic pressure.
[Illustration: +Fig.+ 418.—Diagram of a normal fault.]
=Faults.=—The beds on one side of a joint-plane or fissure are
sometimes elevated or depressed relative to those on the opposite side,
and the displacement is known as a _fault_ (Figs. 418 and 419). The
joint-planes may have any position, and hence fault-planes may vary
from verticality to approximate horizontality. The angle by which the
fault-plane departs from a vertical position is known as the _hade_
(_bac_, Fig. 418). The vertical displacement (_ac_) is the _throw_ and
the horizontal displacement (_bc_) the _heave_. The heave and the throw
are to be distinguished from the _displacement_, which is the amount of
movement along the fault-plane (_ab_, Fig. 418).
The cliff above the edge of the downthrow side is a _fault-scarp_. In
many, probably in most cases, the scarp has been destroyed, or at any
rate greatly obscured by erosion; but occasionally fault-scarps of
mountainous heights, as along the east face of the Sierras and along
many of the basin ranges of Utah, Nevada, etc., are found though much
modified by erosion (Fig. 419).
Faults sometimes arise from over-intense folding (Fig. 420). A
deformation which at one point results merely in a bending of the beds,
may occasion a fault at another. Faults may pass into folds either
vertically (Figs, 421 and 422) or horizontally (Fig. 423). In such
cases, thickening and thinning, and stretching and shortening of the
beds is often involved (see Figs. 421 and 422). Faults are often due to
the greater settling of the beds on one side of a fissure than on the
other, without special disposition to fold.
[Illustration: +Fig.+ 419.—A fault-scarp; the triangular faces rising
abruptly above the plain at the ends of the spurs. (Davis.)]
[Illustration: +Fig.+ 420.—Diagrams showing relations of faults and
folds.]
[Illustration: +Fig.+ 421.—The fault above grades into a fold below.
Thickening and thinning of layers next the fault-plane evident. Based
on experimental results of Willis (13th Ann. Rept., U. S. Geol.
Surv.)]
[Illustration: +Fig.+ 422.—Fault below grading into fold above.
Stretching and thinning, and shortening and thickening of beds under
pressure is involved. Based on experimental results of Willis.]
[Illustration: +Fig.+ 423.—Diagram showing a fault grading into a
monocline horizontally.]
[Illustration: +Fig.+ 424.—Slickenside surface. (Prestwich.)]
The rock on either side of a fault-plane is often smoothed as the
result of the friction of movement. Such surfaces are _slickensides_
(Fig. 424). A slickenside surface has some resemblance to a glaciated
surface, but generally gives evidence of greater rigidity between the
moving surfaces.
Faults are of two general classes, _normal_ and _reversed_. In the
normal fault (Fig. 418) the overhanging side is the downthrow side,
i.e., the downthrow is on the side towards which the fault-plane
inclines, as though the overhanging beds had slidden down the slope.
Normal faults, as a rule, indicate an extension of strata, this being
necessary to permit the dissevered blocks to settle downwards. In
the reversed fault, the overhanging beds appear to have moved up
the slope of the fault-plane, as though the displacement took place
under lateral pressure. This is clearly shown to be the case where an
overfold passes into a reversed fault (Fig. 420). Reversed faults are
further illustrated by Figs. 425, 426, and 427. Where the plane of the
reversed fault approaches horizontality, the fault is often called a
_thrust-fault_, or an _overthrust_. In such cases the throw is to
be distinguished from the _stratigraphic throw_ (see Fig. 426). In
thrust-faults, the heave is often great. The eastern face of the Rocky
Mountains near the boundary-line between the United States and Canada
has been pushed over the strata of the bordering plains to a distance
of at least eight miles.[219] Overthrusts of like gigantic displacement
have been detected in British Columbia,[220] Scotland,[221] and
elsewhere.
[Illustration: +Fig.+ 425.—Perspective view and vertical section of a
thrust-fault. (Willis, U. S. Geol. Surv.)]
[Illustration: +Fig.+ 426.—Diagram of a thrust-fault illustrating the
several terms used in describing faults. The distinctions between
heave and displacement, and between throw and stratigraphic throw,
are to be especially noted. (Willis, U. S. Geol. Surv.)]
[Illustration: +Fig.+ 427. —Step-fold showing (in 1) break in the
massive limestone bed which determines the plane of the break-thrust
along which the displacement shown in 2 takes place. (Willis, U. S.
Geol. Surv.)]
Sometimes a fault branches (Fig. 428) and sometimes the faulting is
distributed among a series of parallel planes at short distances from
one another,[222] instead of being concentrated along a single plane,
thus giving rise to a _distributive fault_ (Fig. 429). This is perhaps
more common in normal than in reversed faulting.
[Illustration: +Fig.+ 428.—Branching-fault. (Powell.)[
[Illustration: +Fig.+ 429.—Diagram showing a series of small
faults—distributive faulting.]
[Illustration: +Fig.+ 430.—Fault in Gering series. Near Rutland Siding,
near Crawford, Neb. (Darton, U. S. Geol. Surv.)]
The amount of throw occasionally reaches several thousand feet.
Occasionally faults of incredible dimensions are reported, but
these are perhaps misinterpretations. Faults are observed to die
out gradually when traced horizontally, sometimes by passing into
monoclinal folds, and sometimes without connection with folding. In
depth they probably die out in similar ways in most cases. Where the
throw is great, they probably give place to folds below (Fig. 421).
Other phenomena of faulting are illustrated by Figs. 430–435. A fault
of thousands, or even hundreds of feet is probably the sum of numerous
smaller slippings distributed through long intervals of time. Faulting
is probably one of the common causes of earthquakes.
[Illustration: +Fig.+ 431.—Contorted and faulted laminated rock. Cook
Inlet. (Gilbert, U. S. Geol. Surv.)]
[Illustration: +Fig.+ 432.—Faulting shown in a cobblestone. The
fault-planes have become veins by deposition from solution. The
figure shows how the relative ages of crossing-faults may be
determined. (Schrader, U. S. Geol. Surv.)]
[Illustration: +Fig.+ 433.—Figure showing minute faulting. The length
of the specimen is 8 inches. The number of faults is nearly 100.
(Photo. by Church.)]
[Illustration: +Fig.+ 434.—Diagram illustrating common phenomena of a
faulted region. (Dana.)
[Illustration: +Fig.+ 435.—Diagram showing a fault, the plane of which
forms an open fissure and has been filled with débris from above.
(Powell.)]
=The significance of faults.=[223]—Faults afford a valuable indication
of the conditions of stress to which a region has been subjected, but
some caution must be exercised in their interpretation. Normal faults
usually indicate an extension of the surface sufficient to permit the
fault-blocks to settle down unequally. Reversed faults usually signify
a compression of the surface which requires the blocks to overlap
one another more than they did before the faulting. In other words,
normal faulting usually implies tensional stress, and reversed faulting
compressional stress. It is not difficult to see, however, that in an
intensely compressed and folded region there might be cases of normal
faulting on the crests of folds where local stretching took place, and
that reversed faults might occur even in regions of tension. But such
cases must usually be local, and capable of detection and elimination
by a study of the phenomena of the surrounding region. These
exceptional cases aside, the general inference from prevailing normal
faults is that the regions where they occur have undergone stretching,
while the inference from the less widely distributed reversed faults is
that the surface where they occur has undergone compression.
In view of the current opinion that the crust of the earth has been
subjected to great lateral thrust as a result of cooling, it is
well to make especial note of the fact that the faults which imply
_stretching_ are called _normal_ because they are the more abundant;
and that the faults which imply thrust are less common, and are styled
_reversed_. The numerical ratio of normal to reversed faults has never
been closely determined, but normal faults very greatly preponderate,
and are estimated by some writers to embrace 90-odd per cent. of the
whole. The testimony of normal faults is supported by the prevalence
of gaping crevices, and of veins which are but crevices that stood
open until they were filled by deposition. All these phenomena seem to
testify to a _stretched_ condition of the larger part of the surface of
the continents. This will again claim attention in the study of Earth
Movements.
[Illustration: +Fig.+ 436.—Diagram showing an area of rock with
monoclinal structure. One layer notably unlike the others.]
=Effect of faulting on outcrops.=—Faulting may bring about numerous
complications in the outcrop of rock formations. In a series of
formations having a monoclinal structure (Fig. 436), many changes may
be introduced. Let it be supposed in the following cases that, after
faulting, the surface has been reduced to planeness by erosion. If
the fault-plane be parallel to the strike of the beds (_ab_, Fig.
436), and hence a _strike fault_, the outcrop of a given layer may be
duplicated (_H_, Fig. 437), or it may be eliminated altogether (Fig.
438). If the fault-plane be parallel to the direction of dip (_cd_,
Fig. 436), a _dip fault_, the layer _H_ will outcrop, as in Fig.
439, if the downthrow was on the far side, or as in Fig. 440 if the
downthrow was on the opposite side. In both cases the outcrop _H_ is
_offset_, the amount of the offset decreasing with increasing angle of
dip and increasing with increasing throw of the fault. If the fault
be oblique to the direction of dip and strike (_ef_, Fig. 436), an
_oblique fault_, the outcrop of such a layer as _H_ will have the
relations shown in Fig. 441 if the downthrow was to the left, and that
shown in Fig. 442 if the downthrow was to the right. In the former
case, it is said that there is _offset with overlap_; in the latter,
_offset with gap_. The amount of the overlap and gap, respectively,
increases with the increase of throw and hade, and decreases with
increase of dip. In all cases the outcrop (after the degradation of the
upthrow side) is shifted down dip.
[Illustration: +Fig.+ 437.—Same as Fig. 436, after (1) displacement by
a strike fault and (2) base-leveling. The outcrops of certain beds
are repeated.]
[Illustration: +Fig.+ 438.—Diagram illustrating how a strike fault in
such a structure as that shown in Fig. 436 may cause the outcrop of
certain beds to disappear.]
[Illustration: +Fig.+ 439.—Diagram illustrating how a dip fault in the
structure shown in Fig. 436 affects the outcrop when the downthrow
was on the farther side of the fault-plane.]
[Illustration: +Fig.+ 440.—Same as Fig. 439, except that the downthrow
was on the opposite side.]
[Illustration: +Fig.+ 441.—Oblique fault in the structure shown in Fig.
436. The downthrow was on the left side. The outcrop of layer _H_ is
offset with _overlap_.]
[Illustration: +Fig.+ 442.—Same as Fig. 441, except that the downthrow
was on the right side, and the offset is with a _gap_ instead of an
overlap.]
[Illustration: +Fig.+ 443.—Diagram showing effect of faulting on the
outcrops of synclinal beds.]
[Illustration: +Fig.+ 444.—Diagram showing effect of faulting on
outcrops of anticlinal beds.]
[Illustration: +Fig.+ 445.—Diagram illustrating the effect of
diminishing throw on outcrops in regions of folded rocks.]
If a fault crosses folds at right angles to their axes, the effect is
to change the distance between the outcrops of a given bed on opposite
sides of the fault, after the truncation of the folded beds. The
distance is decreased on the upthrow side of a syncline (Fig. 443) and
increased on the upthrow side of an anticline (Fig. 444). If the throw
of a fault in tilted beds diminishes in one direction, it may cause
beds to outcrop, as shown in Fig. 445. Various other complications
arise under other circumstances. Since faults rarely show themselves in
the topography of the surface, except under special circumstances (see
p. 151), their detection and measurement is usually based on the study
of the relations of the beds involved, as illustrated by Figs. 436–445.
CHAPTER IX.
THE MOVEMENTS AND DEFORMATIONS OF THE EARTH’S BODY
(DIASTROPHISM).
The body of the earth is subject to an infinite variety of movements,
ranging from the almost inconceivably rapid to the almost imperceptibly
slow, and from the almost immeasurably minute to the enormously
massive; but, for practical treatment, they fall mainly into two
couplets: (1) the minute and rapid, and (2) the slow and massive.
Sudden movements of local masses, giving rise to intense vibrations,
are put in the first class. There are innumerable minute and slow
movements, but unless they rise to appreciable magnitude by long
continuance, they are neglected.
MINUTE AND RAPID MOVEMENTS.
The crust of the earth is in a state of perpetual tremor. For the
most part, these tremors are too minute to be sensible, but are
revealed by delicate instrumental devices. Some of them are but the
declining stages of sensible vibrations, but others are minute from
their inception. Many of them spring from the ordinary incidents of
the surface, and claim attention chiefly as obstacles to the study of
more significant oscillations. Winds, waves, waterfalls, the tread of
animals, the rumble of traffic, the blasts of mines, the changes of
temperature, the variations in atmospheric pressure, the weighting of
rainfall and the lightening of evaporation, the rupture of rock or ice
or frozen earth, and many other processes, make their contributions
to local and minute movements. For the greater part, these vibrations
are superficial in origin, and are soon damped beyond recognition by
dispersal and by the inelastic and discontinuous nature of the looser
material of the surface. When a temporary rigid crust is formed by
freezing, as in winter, these surface vibrations are transmitted with
much less loss, and the distances at which the rumble of winter traffic
is heard, is a good illustration of the function of continuity and
solidity in the conveyance of vibrations.
_Earthquakes._
When the tremors spring from sources within the earth itself and are of
appreciable violence, they are recognized as earthquakes. The sources
of earthquake tremors are various. The most prevalent is probably the
fracture of rocks and the slipping of strata on each other in the
process of faulting. The interpretation of movements of this class
has now been so far perfected that the length and depth of the fault,
the amount of the slip, and the direction of the hade are capable of
approximate estimation.[224] To the same class belong the movements
due to slumping. They are illustrated by the sliding and arrest of
great masses of sediment along the steep fronts of deltas, and of
the accumulations of deep-sea oozes on steep submarine slopes. Such
slumping is, in reality, superficial faulting. Seismic tremors often
attend volcanic eruptions, and are then probably attributable to the
sudden fracture and displacement of rock by the penetration of lava,
or by rapid and unequal heating. They are perhaps also due sometimes
to the sudden generation or cooling of steam in underground conduits,
crevices, and caverns, the action possibly being in some cases of the
“water-hammer” type. In rare instances, probably, the bursting of beds
overlying pent-up non-volcanic gases may give origin to earthquakes. A
more superficial source of earthquake vibrations is the collapse of the
roofs of subterranean caverns.
Seismic vibrations seem to be in part compressional, in part
distortional, in part (on the surface) undulatory, and in part
irregular. The distortional are especially significant, as they seem to
imply a solid medium of transmission.
=Points of origin, foci.=—It is probable that nearly or quite all
earthquake movements start within the upper ten miles of the crust,
and most of them within the upper five. Some of the earlier estimates
indeed placed the points of origin as deep as 20 or 30 miles, but in
these cases the necessary corrections, discussed below, were neglected.
Most of the recent and more accurate estimates fall within the limits
given.
The method of estimating the depth of the centers of disturbance
consists in observing the directions of throw or thrust of bodies
at the surface, and in regarding these as representing the lines
of emergence of the earthquake-waves. By plotting these lines of
emergence, and projecting them backwards to their underground
crossings, a first approximation to the location of the focus is
reached (the lines _EF′_, Fig. 446). From the nature of the case, the
observations of the angles of emergence cannot be very accurate, but an
effort is made to limit the error by making the number of observations
great.
Two systematic corrections are to be applied to all such estimates,
the one for varying elasticity and density, and the other for
varying continuity. Both reduce the estimated depth. In making the
correction for varying elasticity, it must be noted that the velocity
of vibrations varies directly as the square root of the elasticity,
and inversely as the square root of the density. The velocity is also
accelerated by increase of temperature. The elasticity, temperature,
and density all increase with depth. Theoretically, the increase of
velocity due to the increasing elasticity and temperature of increasing
depths, overbalances the retardation due to increasing density, and
recent observations on the transmission of seismic waves through deep
chords of the earth have confirmed this conclusion. The path of the
vibration will, therefore, be curved toward the surface, as pointed out
by Schmidt and illustrated in Fig. 446, taken from his discussion.[225]
From this it is clear that the focus is not so deep as implied by the
simple backward projection of the lines of emergence.
[Illustration: +Fig.+ 446.—Diagram illustrating by closed curves the
different rates of propagation of seismic tremors from a focus _F_,
and, by lines normal to these, the changing directions of propagation
of the wave-front. It will be seen that the paths of propagation
curve upwards in approaching the surface. If the lines of emergence,
as at _E_ and _E_, be projected backwards, as to _F′_, the points of
crossing will be below the true focus.]
A second correction must be made for the differences of continuity
of the upper rock in the vertical and horizontal directions. In the
outer part of the earth, the continuity in horizontal directions is
interrupted by vertical fissures. Were these not usually filled with
water, they would soon kill the horizontal component of the seismic
wave, and the residual portion would be directed almost vertically to
the surface, for the width of the fissures is almost always greater
than the amplitude of the seismic vibrations. The water restores the
continuity, in a measure, but not perfectly, for the elasticity of
water is much less than that of rock. It is clear that in horizontal
movement there must be a constant transfer from rock to water and from
water to rock, and this must retard, as well as partially destroy, the
vibrations. In a vertical direction, however, the rocks rest firmly
upon one another, and this gives measurable continuity, the only change
being from one layer or kind of rock to another. It seems certain,
therefore, that the vertical component of the seismic wave will be less
damped and less retarded in transmission than the horizontal. It will,
therefore, reach the surface sooner and will have the greater effect on
bodies at the surface, not only for the reasons given, but also because
it emerges more nearly in the line of least resistance and of freest
projection. On this account, a second correction must be added to the
correction for elasticity, and this must further reduce appreciably the
first estimate of the depth of the focus.
Observation shows that in some way a seismic wave becomes separated in
transmission into portions of different natures and speeds, but their
interpretation is yet uncertain. These separated portions probably
consist of the compressional, the distortional, and the undulatory
waves, and perhaps of refractions and reflections of these (see Fig.
448).
A most important recent achievement is the detection and investigation
of seismic tremors that appear to have come _through_ the earth. The
transmission of such waves promises to reveal much relative to the
nature of the deep interior, when enough data are gathered to warrant
conclusions. The rate of propagation in the central parts is found to
be greater than in the outer parts, implying high elasticity within.
=The amplitude of the vibrations.=—From the very disastrous effects
of severe earthquakes, it is natural to infer that the distinctive
oscillations must have large amplitude, but in fact it is the
suddenness of the vibration, rather than its length, that is
effective. Instrumental investigations indicate that the oscillations,
after they have left their points of origin, are usually only a
fraction of a millimeter in amplitude; at most they seldom exceed a few
millimeters. A sudden shock with an amplitude of 5 or 6 millimeters is
sufficient to shatter a chimney. It is true that estimates assigning
amplitudes of a foot or more have been made, but their correctness is
open to serious doubt. It should be understood that it is the length
of oscillation of the particles of the subsurface rock transmitting
the vibrations that is referred to, not the movement of the free
surface, or of objects on the surface. The throw at and on the surface
is much greater. Just as a slight, quick tap of a hammer on a floor
is sufficient to make a marble lying on it bound several inches, so
a sufficiently sudden rise of the surface of the earth, though but a
fraction of an inch, may project loose bodies many feet.
[Illustration: +Fig.+ 447.—Illustration of the destructive effects of
the Charleston earthquake, showing definite direction of throw. (W.
J. McGee.)]
=Destructive effects.=—The interpretation of the disastrous results
of earthquake shocks has, therefore, its key in the suddenness and
strength of rather minute vibrations of the earth-matter, but it is
also dependent on the freedom of motion of the bodies affected. The
rocks of the deeper zones, where the matter is sensibly continuous,
transmit the seismic vibrations without appreciable disruptive effect,
so far as known, though the origin of crevices has been assigned to
this cause; but bodies at the surface are fractured, overturned, and
hurled from their places. The reason is doubtless this: Within a great
mass firmly held in place by cohesion and pressure on all sides, the
forward motion of a particle develops an equal elastic resistance,
and it is quickly thrown back again and the wave passes on. At the
surface, where bodies are freer to move, the stroke of the vibration
projects the body, and so, instead of vibratory resilience, the chief
energy is converted into mass-motion. The tap of a hammer sends an
almost imperceptible vibration along the floor, but this vibration may
throw a glass ball, beneath which it runs, into the air. So the minute
vibrations of earth-matter may travel miles from their origin through
continuous substance with little result, and then so suddenly thrust a
loose body on the surface, or the base of a column, or the foundation
of a house, as to rack it with differential strains, or even to hurl
it to destruction. So, too, earth-waves striking the sea-border may
thrust the waters off shore by their sudden impact, and the reaction
may develop a wave which overwhelms the coast. Such waves may doubtless
arise from a sudden stroke of seismic vibrations on the sea-bottom.
The great gaping fissures that sometimes open during earthquakes occur
oftenest where the surface on one side is less well supported than on
the other, as on a slope, or near a bluff-face or a river-channel. When
in such situations the earth is once suddenly forced in the direction
of least resistance, it is not always met by sufficient elastic
resistance to throw it back. Sometimes, however, there is an elastic
return, and the fissure closes forcibly an instant after it is opened.
=Direction of throw.=—Immediately above the point of origin,
technically the _epicentrum_ or _epi-focal point_, bodies are projected
upwards. When crushing takes place in such a case, it is due to the
upthrust or to the return downfall. At one side of the epicentrum the
thrust is oblique in various degrees, and is usually more destructive,
if not too far from the epicentrum. The destructiveness commonly
increases for a certain distance from the epi-focal point, and then
diminishes. Under ideal conditions, the greatest effects are found
where the vibration emerges at an angle of about 45°, but various
influences modify this result. Lines drawn through points of equal
effect (isoseismals) are not usually regular circles or ellipses about
the epicentrum, as they would be under ideal conditions. The various
divergencies represent differences of effective elasticity, of surface,
and of other influences. As most earthquakes originate from lines,
planes, or masses, rather than points, there are doubtless differences
of intensity of vibration at different points on the lines, planes,
or areas of origin, and these differences introduce inequalities in
propagation and in surface effects.
[Illustration: +Fig.+ 448.—Illustrations of the records made by
earthquake tremors after distant transmission through the earth. The
four diagrams represent the same set of tremors as received at Shide,
Kew, Bidston, and Edinburgh in Great Britain. The movement was from
left to right. (Milne.)]
=Rate of propagation.=—The progress of a seismic wave varies very
greatly. Both experimental tests and natural observations give very
discordant results. At present, they justify only the broad statement
that the velocity of propagation varies from several hundreds to
several thousands of feet per second at the surface. The rate seems to
be greater for strong vibrations than for weak ones, and hence it is
faster near the origin than farther away. The strength of a vibration
dies away, theoretically, according to the inverse square of the
distance from the point of origin. Practically there is to be added to
this the partial destruction of the vibrations by conversion into other
forms of motion.
=Sequences of vibrations.=—Near the source, the main shocks are apt to
come suddenly and to be followed by minor tremors. At a distance there
are usually “preliminary” vibrations followed by the main tremors,
and these by others of gradually diminishing value. This development
is assigned to different rates of propagation, and to refractions and
reflections not unlike the prolongation of thunder (see Fig. 448).
This deployment of the vibrations is notably developed in the shocks
that pass long distances through the earth. The vibrations of the
first phase are regarded as compressional, those of the second as
distortional, while the largest oscillations which arise still later
perhaps come around the surface, and may be undulatory, though their
nature is not yet determined.
There is often, however, a true succession of original shocks caused by
a succession of slips or ruptures at the source. Sometimes these are
exceedingly persistent, running through days, weeks, or even months. In
such cases a slow faulting is probably in progress, and little slips
and stops follow in close succession. In one instance as many as 600
shocks in ten days have been reported.[226]
=Gaseous emanations.=—Vapors and gas frequently issue from earthquake
rents, and are popularly made to serve as causes, but they are usually
merely the earth gases that are permitted to escape by the rending
of the ground, or are forced out by readjustment of the shaken beds.
Like other subterranean gases, they are often sulphurous, and they are
sometimes hot, especially in volcanic regions. Where the shocks are
connected with eruptions, the gases may be truly volcanic.
=Distribution of earthquakes.=—Over large portions of the globe,
severe earthquakes are exceedingly rare, but in certain regions they
are unfortunately frequent. For the most part, these are volcanic
districts, but this is by no means a universal relation. Earthquakes
and volcanoes are only in part associates. In general, it may be said
that earthquakes are frequent where geologic changes are in rapid
progress, as along belts of young mountains, where the stresses are
not yet adjusted, or at the mouths of great streams, where deltas are
accumulating, or about volcanoes, where temperatures and strains are
changing, or on the great slopes, particularly the submarine slopes,
where readjustments in response to inequalities of surface stress are
in progress. Not a few, however, occur where the special occasion is
not at all obvious.
_The Geologic Effects of Earthquakes._
Earthquakes are of much less importance, geologically, than many
gentler movements and activities. Disastrous as they sometimes are to
human affairs, they leave few distinct and readily identifiable marks
which are more than temporary.
=Fracturing of rock.=—During the passage of notable earthquake waves,
the solid rock is probably often fractured (see p. 509), though where
it is covered by deep soil the fractures are rarely observable at the
surface. Elsewhere the crevices are readily seen, especially if they
gape. In a few instances surface-rock has been seen to be thoroughly
shattered after the passage of an earthquake, as in the Concepcion
earthquake of 1835.[227] Joints which were before closed are often
opened during an earthquake. Thus in northern Arizona, not far from
Canyon Diablo, there is a crevice traceable for a considerable
distance, which is said to have been opened during an earthquake.
Locally, it gaps several feet. Other notable earthquake fissures
have been recorded in India,[228] Japan, and New Zealand. During an
earthquake which shook the South Island of New Zealand in 1848, “a
fissure was formed averaging 18 inches in width, and traceable for a
distance of 60 miles, parallel to the axis of the adjacent mountain
chain.”[229] The development of fractures or the opening of joints is
sometimes accompanied by faulting. This was the case in Japan during
the earthquake of October 28, 1891, when the surface on one side of a
fissure, which could be traced for 40 miles, sank 2 to 20 feet. In this
case there was also notable horizontal displacement, the east wall of
the fissure being thrust locally as much as 13 feet to the north.[230]
=Changes of surface.=—Circular surface openings or basins are sometimes
developed during earthquakes. This was the case during the Charleston
earthquake of 1886,[231] and similar effects have been noted elsewhere.
These openings often serve as avenues of escape for ground-water,
gases, and vapor. They are commonly supposed to be the result of the
collapse of caverns, or other subterranean openings, the collapse often
causing the forcible ejection of water. Such openings are likely to be
formed only where the surface material is incoherent. Sandstone dikes
(p. 514) may perhaps be associated in origin with earthquakes.
Earthquakes are likely to dislodge masses of rock in unstable
positions, as on slopes or cliffs. They may also occasion slumps and
landslides.[232]
=Effects on drainage.=—The fracturing of the rock may interfere with
the movement of ground-water. After new cracks are developed, or old
ones opened or closed, the movement of ground-water adapts itself
to the new conditions. It follows that springs sometimes cease to
flow after an earthquake, while new ones break out where there had
been none before. The character of the water of springs is sometimes
changed, presumably because it comes from different sources after the
earthquake. Joints may be so widened as to intercept rivulets, and
the waters thus intercepted may cause the further enlargement of the
opening. Illustrations of this sort are furnished by the earthquakes
of the Mississippi valley (Lat. 36° to 38°) in 1811–12. Where faults
accompany earthquakes, they occasion ponds or falls where they cross
streams. Illustrations of both were furnished by the Chedrang River of
India after the earthquake of 1897.[233]
=Effects on standing water.=—Some of the most destructive effects of
earthquakes are felt along the borders of the sea. Thus the great
sea-wave of the Lisbon earthquake (1755) and that of the earthquake
which affected the coasts of Ecuador and Peru in 1868 are examples.
Such waves have been known to advance on the land as walls of water 60
feet in height. They are most destructive along low coasts, for here
the water may sweep much more extensively over the land. The great
loss of life during an earthquake has usually been the direct result
of the great waves. Lakes are also affected by earthquakes, their
waters sometimes rising and falling for several hours after the initial
disturbance, but lake-waves are much feebler than those of the sea, and
are not often destructive.
Earthquake shocks are sometimes remarkably destructive to the life
of lakes and seas. Thus during the Indian earthquake of 1897, “fishes
were killed in myriads as by the explosion of a dynamite cartridge ...
and for days after the earthquake, the river (Sumesari) was choked
with thousands of dead fish ... and two floating carcasses of Gangetic
dolphins were seen which had been killed by the shock.”[234] This
wholesale destruction of life is of interest, since the surfaces of
layers of rock, often of great age, are sometimes covered with fossils
of fish or other animal forms, so numerous and so preserved as to
indicate that the animals were killed suddenly and in great numbers,
and their bodies quickly buried. It has been suggested that such rock
surfaces may be memorials of ancient earthquake shocks.[235]
=Changes of level.=—Permanent changes of level sometimes accompany an
earthquake. Thus after the earthquake of 1822 “the coast of Chili for a
long distance was said to have risen 3 or 4 feet.”[236] Similar results
have occurred on the same coast at other times, and on other coasts at
various times. Depression of the surface is perhaps even more common
than elevation. Thus on the coast of India all except the higher parts
of an area 60 square miles in extent were sunk below the sea during
an earthquake in 1762. Widespread depression in the vicinity of the
Mississippi in Missouri, Arkansas, Kentucky, and Tennessee accompanied
the earthquakes of 1811 and 1812. Some of the depressed areas were
converted into marshes, while others became the sites of permanent
lakes. Reelfoot Lake, mainly in Tennessee, is an example. Change of
level is involved wherever there is faulting, and faulting is probably
rather common in connection with earthquakes.
Changes of level are not confined to the land. Where earthquake
disturbances affect the sea-bottom in regions of telegraph cables, the
cables are often broken. In such cases notable changes have sometimes
been discovered and recorded when the cables were repaired. Striking
examples are furnished by the region about Greece.[237] In one instance
(1873) the repairing vessel found about 2000 feet of water where about
1400 feet existed when the cable was laid. In another instance (1878)
the bottom was “so irregular and uneven for a distance of about two
miles, that a detour was made and the cable lengthened by five or
six miles.” In still another case (1885) the repairing vessel found a
“difference of 1500 feet between the bow and stern soundings.” These
records point to sea-bottom faulting on a large scale.
It is probably no nearer the truth to say that changes of level result
from earthquakes than to say that earthquakes result from changes of
level. The two classes of phenomena are probably to be referred to a
common cause.[238]
SLOW MASSIVE MOVEMENTS.
It is a far cry from the intense and inconceivably rapid oscillations
of the earthquake, to the excessively slow subsidences of continents,
or even the slow wrinkling of mountain folds. Not infrequently rivers
wear down their channels across a mountain range as fast as it rises
athwart them. The movements of continents are even more deliberate.
But, far apart as these contrasted movements are, in rate and method,
they are associated in ultimate causation, and the earthquake shock is
often merely an incident in the formation of a mountain range or in the
subsidence of a continent.
The great movements are usually classed (1) as continent-making
(epeirogenic) and (2) mountain-making (orogenic). They may also be
classed as (1) vertical movements and (2) horizontal movements, and
dynamically, as (1) thrust movements and (2) stretching movements.
It is to be understood that these distinctions are little more than
analytical conveniences, for continental movements are often at the
same time mountain-making movements; vertical movements are usually
involved in horizontal movements, and stretching usually takes part in
the processes in which thrust predominates, and _vice versa_. But where
one phase greatly preponderates, it may conveniently give name to the
whole.
=Present movements.=—Critical observations on seacoasts show that
some shores are slowly rising and some slowly sinking _relative to
the ocean-level_. We do not certainly know what their movements are
relative to the center of the earth; very possibly all may be sinking,
one set faster than the other, the ocean-surface also going down at an
intermediate rate. Theoretically, all might possibly be rising, one
set faster than the other, the ocean also rising at an intermediate
rate, though this is extremely improbable. One set may be actually
rising relative to the center of the earth, and the other sinking,
while the ocean-level is stationary, or nearly so. This is the way in
which we are accustomed to interpret them. A general shrinkage of the
earth, however, is probably going on, carrying down land-surface and
sea-surface. It has been urged by Suess[239] that the general shrinkage
is so great that the local upward warpings and foldings never equal
it, and that the real movements are all downward, though in different
degrees. This is probably the general fact at least. Over against this
is the popular disposition to regard earth movements generally as
“upheavals.” There is also a predilection for regarding the rigid land
as moving and the mobile sea-level as fixed. In reality, the sea is an
extremely adaptive body that settles into the irregular hollows of the
lithosphere, and is shifted about with every warping of the latter.
Whatever change affects the capacity of its depressions affects also
the sea-level. If they are increased, the sea settles more deeply into
them; if they are decreased, the sea spreads out more widely on the
borders of the land. The one thing that gives a measure of stability
to the sea-level is the fact that all the great basins are connected,
and so an _average_ is maintained. A warping down in one part of the
sea-bottom may be offset by an upward warping somewhere else in the
72% of the earth’s surface covered by the ocean, and so it is only the
_sum total of all changes_ in the sea-bottoms and borders that effects
the common level. Thus it happens that, notwithstanding its instability
and its complete subordination to the lithosphere, the sea-level is
the most convenient basis of reference, and has become the accepted
datum-plane. If there were some available mode of measuring the
distance of points from the center of the earth, it would give absolute
data and absolute terms, and would reveal much that is now uncertain
respecting the real movements of the surface. For convenience, however,
since absolute terms are impracticable, the ordinary language of
geology, which represents movements as upward and downward, according
to their relations to the sea-level or to the average surface, will be
employed. Notwithstanding this concession to convenience in the use
of terms, it is of the greatest importance to form, and to constantly
retain, true fundamental views.
=Fundamental conceptions.=—The existence of any land at all is
dependent on the inequalities of the surface and of the density of the
lithosphere, for if it were perfectly spheroidal and equidense, the
hydrosphere would cover it completely to a depth of about two miles.
Not only are inequalities necessary to the existence of land, but these
inequalities must be _renewed from time to time_, or the land area
would soon, geologically speaking, be covered by the sea. The renewal
has been made again and again in geological history by movements that
have increased the inequalities in the surface of the lithosphere.
With each such movement, apparently, the oceans have withdrawn more
completely within the basins, and the continents have stood forth more
broadly and relatively higher, until again worn down. This renewal of
inequalities appears to have been, in its great features, a _periodic
movement_, recurring at long intervals. In the intervening times,
the sea has crept out over the lower parts of the continents, moving
on steadily and slowly toward their complete submersion, which would
inevitably have been attained if no interruption had checked and
reversed the process. These are the great movements of the earth, and
in them lies, we believe, the soul of geologic history and the basis
for its grand divisions. The reasons for this will appear as the
history is followed, and its most potential agencies are seen unfolding
themselves. At the same time, there have been numerous minor surface
movements in almost constant progress. While these two classes of
movements have been associated, and are perhaps due in the main to the
same causes, they are sufficiently different in some of their dynamic
aspects to be separated in treatment.
_Nearly Constant Small Movements._
Innumerable gentle warpings have affected nearly every portion of the
surface of the globe at nearly all stages of its history. Not only
during the periods of great movements were there countless minor and
gentler movements, but at times of relative quiescence there were
slow swellings and saggings of the surface of the lithosphere. They
sometimes affected small areas and sometimes large ones, and they
were sometimes of upward phase and sometimes of downward. They were
the immediate agencies in locating and controlling the deposition
of stratified rocks, though they rest back on the great movements
for their working conditions. Very slow sinkings of sea-borders have
permitted deposition to go on in shallow water for long periods
without being interrupted by the local filling of the sea. Very slow
swellings of land tracts, relative or absolute, have permitted erosion
to supply material for such sedimentation for long periods without
exhausting the sources. Very slow upward warpings in one region and
downward warpings in another have shifted the borders of the land
and sea, and with them the areas of erosion and deposition. Thus
have arisen overlaps and unconformities of strata and diversities
in their distribution from stage to stage. Such movements may have
amounted to a few inches, or a few feet, or a few fathoms per century.
Downward movements have sometimes affected a considerable section of a
continent, letting in a shallow _epicontinental_ sea upon it, such, for
example, as the North Sea upon the northwestern border of the continent
of Europe, and Hudson Bay upon the northeastern part of North America.
Similar movements seem to have extended the seas even more widely
upon the surface of the land in times past, as attested by the great
transgressions of the ocean-borders and the great epicontinental spread
of strata. Notwithstanding their great breadths, the epicontinental
seas were generally shallow. Similar gentle warpings of upward phase
rescued the bottoms of shallow seas from submersion, and inaugurated
erosion; or they bowed base-leveled lands upward, and rejuvenated
their streams and inaugurated a new cycle of denudation. Often they
connected continents previously separated by shallow straits, and thus
inaugurated inter-continental migrations of land life, while they
stopped inter-oceanic migration.
The gentleness and frequency of these movements is attested by the
character of the sediments and by their relations to one another, as
will be seen in the study of the sedimentary series.
=Reciprocal features.=—These minor warpings show a notable tendency
to be reciprocal. If one area is bowed up, another near by is bowed
down. If the continents settle, the oceans rise on their borders. If
the land is cut down, the sea is filled up. There is an important
phase of this deserving especial note. Certain tracts have been slowly
bowed upwards into long land swells, the streams being rejuvenated and
degradation hastened. Adjacent tracts have been slowly bowed downwards
into long parallel troughs which received the wash from the adjacent
swells, and thus became tracts of exceptional sedimentation. Such a
tract of parallel swell and sag, if our interpretation be correct,
developed along the Atlantic border of North America in the Paleozoic
era. By the slow upward warping of the swells, the feeding-grounds of
the streams were maintained, and the sags were filled about as fast
as they sank. Thus a great depth of sediment was laid down in the
course of an era measured by millions of years. So in other regions,
especially near the borders of the continents, there have been similar
reciprocal movements, giving at once feeding-grounds for the streams
and lodgment-grounds for the sediments, side by side in parallel
belts. It is a common view that these belts of deep sedimentation were
the forerunners of mountain formation, and that they determined the
formation of the mountains. In view of the grounds for doubting the
efficiency of so superficial an agency in mountain formation, which
will appear as we go on, it may be well to hold this view in abeyance,
and to dwell on the reciprocal nature of the action, in which the
upward bowing that gave the feeding-grounds is as vital a factor as
the sagging that accommodated the sedimentation. It is important to
recognize that in so far as the crust was weak enough to yield to
these gentler forces, it was not strong enough to accumulate the great
stresses necessary to form mountain ranges, and further, that in so far
as the stresses were eased by the gentle warping, they could not be
accumulated for the later work of mountain-folding. It is nevertheless
probable that the conditions which located the gentle swelling and
sagging also located the mountain-folding.
_The Great Periodic Movements._
=Mountain-forming movements.=—Along certain tracts, usually near the
borders of the continents, and at certain times, usually separated by
long intervals, the crust was folded into gigantic wrinkles, and these
constitute the chief type of mountains, though not the only type. The
characteristic force in this folding was lateral thrust. The strata
were not only arched, but often closely folded, and sometimes intensely
crumpled. In extreme cases, like the Alps, the folds flared out above,
giving overturn dips and reversed strata, as illustrated in the chapter
on Structural Geology, pp. 501–511. In these cases there was an upward
as well as a horizontal movement, for the folds themselves were
lifted; but the horizontal thrust so much preponderated, and was so
much the more remarkable, that the upward movement was overshadowed.
It is well to note, however, that these mountain ranges are crumpled
_outward_ and not _inward_, as might be expected if they resulted
simply from the shrinkage of the under side of a thin shell. The folds
are sometimes nearly upright and symmetrical, and sometimes inclined
and asymmetrical, as illustrated in the chapter referred to. Where the
folds lean, the inference has been drawn that the active thrust came
from the side of the gentler slope, the folds being pushed over toward
the resisting side, and this seems to be commonly true. The original
attitude of the beds, however, has much to do with the character of
the folds.[240] By a slight change in the mode of thrust, sheets of
paper may be so pushed as to lean forward or backward at pleasure.
The leaning of the folds seems, therefore, a doubtful criterion for
determining the direction of the active movement. Mountains of the
thrust type usually consist of a series of folds nearly parallel to
each other, the whole forming an anticlinorium.
[Illustration: +Fig.+ 449.—The great Eurasian mountain tract. Jones
Relief Globe. (Photo. by R. T. Chamberlin.)]
=Distribution of folded ranges.=—The prevailing location of this
class of mountains is so generally near the borders of the continents
that the relation is probably significant. Dana[241] long ago called
attention to the fact that the greatest mountain ranges stand opposite
the greatest ocean-basins, and he connected the elevation of the one
with the depression of the other. One of the most notable exceptions
to this relation is the complex system of southern Europe, from the
Pyrenees to the Caucasus, and another is the Altai and connected ranges
(Fig. 449). The Urals and not a few minor ranges are also exceptions.
It is probably better to regard the crumpled tracts as lying on the
borders of great segments of the earth that acted essentially as units,
and to regard the relationship to the sea as a coincidence that is only
in part causal.[242]
=Plateau-forming movements.=—Another leading phase of crustal movement
is the settling or rising of great blocks of the crust, as though by
vertical rather than horizontal force. The western plateau of North
America and the great plateau of Thibet are gigantic examples. The
American plateau embraces numerous blocks which, while they have been
elevated together, are individually tilted in their own fashion. At
the surface, they are separated by fault-planes, but below, some of
them, and perhaps most of them, pass into flexures. Most of these
flexures are of the monoclinal type (p. 516), which dynamically means
much the same as a fault; but some of them may be of the compressive
type, without inconsistency with vertical fault-relief above. Research
has not yet covered thoroughly any great plateau, and knowledge of
this class of movements is less complete than that of folding by
lateral thrust, and it has a less ample place in the literature of the
subject. The plateau-forming movements are, however, much more massive
than the mountain-folding movements, and stand next in magnitude to
the continent-forming movements. Plateaus may be regarded as smaller
platforms superposed on the continental platforms.
In the ocean-basins, there appear to be raised platforms of the
plateau type, and there are remarkable “deeps” that have the aspect of
anti-plateaus.
=Continent-forming movements.=—True continent-forming movements
appear to have antedated the earliest known sediments. As far back
as we can read the sedimentary record, the continents seem to have
been well established, and there is little evidence that they have
since been fundamentally changed. It is true that some very eminent
geologists have rather freely connected formations on one continent
with formations having similar faunas on an opposite continent, by a
hypothetical conversion of the intervening ocean-bottoms into land
or shallow water; but most such faunal relations can be explained
almost equally well by migration around the coasts, or at most by
mere ridge-connections. The paucity, if not total absence, of abysmal
deposits in the strata of the continents, taken with the persistence
of terrestrial and coastal faunas, leaves little room for assigning
an interchange of position between abysmal depths and continental
elevations, and _vice versa_. Dynamic considerations also offer grave
difficulties. The doctrine of the persistence of continents probably
ought not to be pushed so far as to exclude shallow water, or even
land, connections between South America, Antarctica, Australia,
India, and South Africa, directly or indirectly, at certain stages
of geological history. Without forming final conclusions as to the
measure of the change which the continents have suffered during known
geological history, it is safe to conclude that the continents and
ocean-basins were in the main formed very early in the earth’s history,
and that subsequent changes have consisted chiefly in the further
sinking of the basins and the further protrusion of the land, save as
the latter has been cut down by erosion. Incidentally, the ocean-basins
have probably been extended and the continents restricted. On the other
hand, the continents have been built out on their borders by wash
from the land, and the waters of the ocean have been somewhat lifted
by the deposition of sediment in their basins. It is estimated that
the cutting away of the present continents, and the deposition of the
material in the ocean-basins, would raise the sea-level about 650 feet.
(R. D. George.)
=Relations of these movements in time.=—The folding movements seem
to have had extraordinary prevalence in the earliest ages, for the
Archean rocks are almost universally crumpled, and often in the most
intricate fashion. There is no sign that the folding was then limited
to the borders of the continents; it seems rather to have affected
the whole continental surface. After the beginning of the well-known
sedimentary series, crumpling appears to have taken place chiefly at
long intervals, thus marking off great time-divisions, and to have been
confined at any given stage to certain tracts, chiefly on the borders
of great segments of the earth’s crust.
Concerning the plateau-forming movements in the past, knowledge is very
meager, as the detection of plateaus of ancient times is more difficult
than the detection of folds. Gentle warpings have apparently been in
progress at all times.
=Relations of vertical to horizontal movements.=—The downward movements
are unquestionably the primary ones, and the horizontal ones are
secondary and incidental. The fundamental feature is doubtless central
condensation actuated by gravity, and the master movements are the
sinkings of the ocean-basins. The great periodic movements that made
mountains and plateaus, and changed the capacity of the ocean-basins,
probably started with the sinking of part or all of the ocean-bottoms.
In the greater periodic movements, probably all the basins participated
more or less, but some seem to have been more active than others. For
example, in the last great mountain-making period, the Pacific basin
seems to have been more active than the Atlantic, while in the similar
great event at the close of the Paleozoic, the opposite seems to have
been true. The squeezing up of the continents doubtless took place
simultaneously with the settling of the basins. The true conception is
perhaps that the ocean-basins and continental platforms are but the
surface forms of great segments of the lithosphere, all of which crowd
toward the center, the stronger and heavier segments taking precedence
and squeezing the weaker and lighter ones between them. The area of the
more depressed or master segments is almost exactly twice that of the
protruding or squeezed ones. This estimate includes in the latter about
10,000,000 square miles now covered with shallow water. The volume
of the hydrosphere is a little too great for the true basins, and it
runs over, covering the borders of the continents. The amount of the
overflow fluctuates from time to time, and may be neglected in a study
of the movements and deformations of the lithosphere.
=The squeezed segments.=—The great protruding segments show a tendency
toward rude triangularity. They are (1) the Eurasian, now strongly
ridged on the south and east, and relatively flat on the northwest;
(2) the African, rather strongly ridged on the east, but less abruptly
elevated on the west and north; (3) the North American, now strongly
ridged on the west, more gently on the east, and relatively flat at the
north and in the interior; (4) the South American, strongly ridged on
the west and somewhat on the northeast and southeast.
The foregoing form the major group. The minor group embraces (5) the
Antarctic segment, not as yet sufficiently known to be well defined,
and (6) the Australian, broadly reniform rather than triangular. To
these are perhaps to be added (7) the largely submerged platform that
stretches from Sumatra and Java on the southwest to the Philippines
on the northeast, and is attached to India on the northwest; and (8)
Greenland, which, though closely associated with North America, is
partially separated by a rather deep depression.
=The depressed or master segments.=—The great sunken segments show
a tendency to assume roughly polygonal, rather than triangular,
forms. This accords with the primary place assigned them, since, in a
spherical surface divided into larger and smaller segments, the major
parts should be polygonal while the minor residual segments are more
likely to be triangular. The major segments are (1) the Pacific, (2)
the Indian, (3) the North Atlantic, and (4) the South Atlantic. These
form the principal group, while (5) the Arctic deeps (not including
the shallow epicontinental portions), (6) the Mediterranean, (7) the
Caribbean, and (8) the chain of deep pits between the Philippine ridge
and the Bornean platform, constitute a subordinate group.
Each member of the minor group is an irregular chain of depressed pits
rather than a single continuous deep, unless the Arctic depression, of
which little is now known, proves an exception. They lie between the
greater segments at what may be conceived to be points of _critical
working relations_, and are accompanied by small elevated blocks. The
Caribbean, the Mediterranean, and the Bornean regions are the seats of
the greatest present volcanic and related activities.
In a general view, there are then four great sunken quadrilaterals and
four great elevated triangles, with minor attendants in each class.
Lest fondness for simplicity and symmetry lead too far, we must hasten
to observe that the _dimensions_ are not alike in either class. The
Pacific segment is more than twice the size of any other basin segment,
and four times that of the North Atlantic. The Eurasian triangle is
more than twice the average size of the other land segments, and
nearly three times that of the South American. Nor is there any large
common divisor of approximate accuracy. This is not at all strange if
the earth be regarded as a body of somewhat heterogeneous composition
which naturally shrank in rather irregular segments. On the other hand,
this irregularity is somewhat strange if the earth has evolved from a
very homogeneous and symmetrical, primitive, fluid state. It is also a
serious consideration in any theory that appeals to crystalline form,
or analogy, as in the doctrine of a tetrahedral earth.
Roughly approximated in millions of square miles, the major depressed
segments are as follows: the Pacific, 60, the Indian, 27, the South
Atlantic, 24, and the North Atlantic, 14, leaving 8 for minor
depressions. The elevated segments are Eurasian, 24, African, 12, North
American, 10, and South American, 9, leaving 10 for the minor blocks.
If these segments be regarded as the great integers of body-movement,
two-thirds of them taking precedence in sinking and the other third
in suffering distortion, it is easy to pass to the conception of
sub-segments, moving somewhat differently from the main segments,
so as to aid in their adjustment to one another, and thus to the
conception of plateaus and deeps. It is easy also to pass to the
conception of mutual crowding and crumpling at the edges of these
segments, accompanied by fracture and slipping. These conceptions
perhaps represent the true relations between the massive movements
of the abysmal and continental segments, as well as the less massive
plateau-forming movements and the mountain-forming distortions. The
mountains and plateaus are probably the incidental results of the
great abysmal and continental readjustments.
The great movements are probably to be attributed to stresses that
gradually accumulated until they overcame the rigidity of the thick
massive segments involved, and forced a readjustment. In accumulating
these stresses, some local yielding on weak lines and at special
points was an inevitable incident in distributing more equably the
accumulating stresses. So, also, the first great readjustments probably
left many local strains and unequal stresses which gradually eased
themselves by warpings, minor faultings, etc., so that some minor
movements were a natural sequence of the great movements. But there
were doubtless many local and superficial causes, such as irregular
gains and losses of heat, regional loading and unloading, solution,
hydration, etc., that have caused local or regional movement, and which
have little to do with the great deformations of the earth’s body. As
implied above, the gentle, nearly constant movements probably fall
mainly into a different category from the great periodic movements.
Both will be considered further.
=The differential extent of the movements.=—Between the highest
elevation of the land and the lowest depth of the ocean, there is a
vertical range of nearly twelve miles. There may have been higher
elevations, relatively, in past times, but probably not deeper
depressions; and so, if we assume that the surface was once perfectly
spheroidal, this may be taken as a maximum expression of _differential_
movement, not _absolute_ vertical movement. From the Thibetan plateau,
where a considerable area exceeds three miles in height, to the
Tuscarora deep, where a notable tract exceeds five miles in depth, the
range is eight miles, which may fairly represent the vertical range of
rather massive differential movement. From the average height of the
continents to the average abysmal bottoms of the oceans the range is
nearly three miles, which may be taken as the differential movement of
the great segments. Under certain hypotheses of the origin and early
history of the earth, to be sketched later, the surface is not assumed
to have been perfectly spheroidal originally, and hence the present
irregularities do not necessarily imply so great differential movement.
If the protruding portions of the lithosphere were graded down and the
basins graded up to a common level, this level would lie about 9000
feet below the ocean-surface. This equated level is the best basis
of reference for relative segmental movements. Referred to this datum
plane, the continents, having an area about half as great as that of
the ocean depths, have been squeezed up relatively about two miles,
and the basins have sunk about one mile from the ideal common plane.
The total downward movement, representing the total shrinkage of the
earth, is quite unknown from observation. It is probably very much
greater than the differential movement, as will appear from theoretical
considerations as we go on.
The extent of the _lateral_ movements has a peculiar interest, for
it bears theoretically on the shrinkage of the earth. Every mile of
descent of the crust represents 6 miles (6.28) shortening of the
circumference. If the vertical movements were limited to the _relative_
ones just named, the mile of basin descent would give but little more
than 6 miles of surplus circumference for lateral thrust and crumpling.
How far does this go in explaining the known facts? By measuring the
folds of the Alps, Heim has estimated the shortening represented by
them to be 74 miles.[243] Claypole estimated the shortening for the
Appalachians in Pennsylvania, not including the crystalline belt on
the east, at 46 miles;[244] McConnel placed that of the Laramide range
in British America at 25 miles,[245] and LeConte that of the Coast
range in California at 9 to 12 miles.[246] These estimates must be
corrected for the thickening and thinning of the beds in the process
of folding, for the composite character of the folds, and for the
effects of shearing and faulting. These will in part tend to increase
and in part to decrease the estimates. The first effect of horizontal
thrust is to close up all crevices and compact the beds as much as they
will stand without bending. A part of the unusual thickness which the
beds of folded regions commonly show is probably due to this edgewise
compression. In experiments on artificial strata made to illustrate
foldings (Fig. 449_a_), the thickening of the layers is a very
appreciable part of the process, though probably natural beds do not
thicken in equal proportion. After the beds have been closely folded
and the thrust is athwart them, they are thinned and stretched on the
limbs of the fold. How far this and other causes of extension offset
initial compression is undetermined, and is differently estimated. It
seems highly probable from the nature of the case that the edgewise
compression which resulted from sustaining the full stress before the
beds bent, was much greater than the crosswise compression on the limbs
of the folds, which came into action only after the stress had been
largely satisfied by folding.
[Illustration: +Fig.+ 449_a_.—Illustrations of Willis’ experiments in
the artificial representation of mountain folding. The sections were
formed of layers of wax of different colors, and were mechanically
compressed from the right. The upper section shows the original
state, and the offsets of the succeeding sections at the right
indicate the amount of shortening. (Thirteenth Ann. Rep. U. S. Geol.
Surv.)]
Whatever the correction, and whatever the probable errors of the above
estimates, the amount of shortening involved in folding is large. The
estimates given are merely those for certain periods of folding, and
represent only that portion of the compression of the circumference
which was concentrated in a given mountain range. The whole shortening
of a circumference is to be found by adding together all the transverse
foldings on a given great circle, following it about the globe at
right angles to a given folded tract. In so doing, it will be seen that
the belt does not usually cross more than one or two strongly folded
tracts of the same age, from which it is inferred that the shortening
on each great circle was largely concentrated in a few tracts running
at large angles to each other, to accommodate the shrinkage of the
globe in all directions. If the folding in a main range crossing
any great circle is doubled, it will probably represent roughly the
shortening for that entire circle for that age. If one is disposed
to minimize the amount of folding, the estimate may perhaps be put
roundly at 50 miles, on an entire circumference, for each of the great
mountain-making periods. If, on the other hand, one is disposed to
give the estimates a generous figure so as to put explanations to the
severest test, he may perhaps fairly place the shortening at 100 miles,
or even more. For the whole shortening since Cambrian times, perhaps
twice these amounts might suffice, for while there have been several
mountain-making periods, only three are perhaps entitled to be put
in the first order, that at the close of the Paleozoic, that at the
close of the Mesozoic, and that in the late Tertiary. The shortening
in the Proterozoic period was considerable, but is imperfectly known.
The Archean rocks suffered great compression in their own times, and
probably shared in that of all later periods, and if their shortening
could be estimated closely, it might be taken as covering the whole.
Assuming the circumferential shortening to have been 50 miles during a
given great mountain-folding period, the appropriate radial shrinkage
is 8 miles. For the more generous estimate of 100 miles, it is 16
miles. If these estimates be doubled for the whole of the Paleozoic and
later eras, the radial shortening becomes 16 and 32 miles, respectively.
THE CAUSES OF MOVEMENT.