THE GEOLOGIC FUNCTIONS OF LIFE.
I. +The Distinctive Features of Organic Processes+ 638
_The Chemical Work of Life_ 638
Life material chiefly atmospheric, 638. The non-atmospheric
factors, 639.
(1) _Changes in the composition of the atmosphere_ 639
The consumption and restoration of carbon dioxide, 640.
The freezing and consumption of oxygen, 640. The organic
residue, 640. The meaning of the organic residue, 641. The
more inert factor, 642. Probable fluctuations of atmospheric
composition, 642. The climatic effects of organic action, 643.
(2) _Aid and hindrance to inorganic action_ 644
The promotion of disintegration, 644. Protection against
erosion, 644. The influence of land vegetation on the
character of the sediments, 645.
(3) _Distinctive deposits_ 646
Organic rocks, 646. Inorganic rocks due to life, 646.
_Fossils_ 646
The general order of life succession determined by
stratigraphy, 647. Fossils as means of correlation, 647.
_Special Modes of Aggregation and of Movement_ 648
_The Mental Element_ 649
(1) The material effects of the mental element, 649. Human
modification of the animal and vegetal kingdoms, 650. (2) The
psychological factors as such, 651.
II. +Special Contributions of the Organic Kingdoms+ 652
_Contributions of the Plant Kingdom_ 652
Reference table of the principal groups of plants, 653. The
contribution of the Thallophytes, 653. The contribution of
the Bryophytes, 656. The contribution of the Pteridophytes,
657. The contribution of the Spermatophytes, 657. Plant life
terrestrial rather than marine, 658.
_Contributions of the Animal Kingdom_ 658
Reference table of the principal groups of animals, 659. The
contribution of the Protozoa, 660. The contribution of the
Cœlenterata, 661. The contribution of the Echinodermata, 661.
The contribution of the Vermes, 662. The contribution of the
Molluscoidea, 662. The contribution of the Mollusca, 662. The
contribution of the Arthropoda, 662. The contribution of the
Vertebrata, 663.
III. +The Associations and Ecological Relations of Life+ 663
_The Basis of Floras and Faunas_ 663
_Assemblages Influenced by the Mutual Relations of Organisms_ 664
Food relations, 664. Adaptive relations, 665. Competitive
relations, 665. Offensive and defensive relations, 665.
Implied forms of life, 666.
_Assemblages Influenced by Environment_ 666
Plant societies, 667.
_The Influence of Geographic Conditions on the Evolution of
Floras and Faunas_ 668
The development of provincial and cosmopolitan faunas, 668.
Restrictive and expansional evolution, 672.
PLATES.
+plate+ +face page+
I. Bathymetrical Chart of the Oceans 10
II. Fig. 1. New Jersey. Fig. 2. Kansas. Fig. 3. Indiana. Fig. 4.
Nebraska 39
III. Fig. 1. Kansas. Fig. 2. Nevada 72
IV. Fig. 1. Illinois. Fig. 2. North Dakota 73
V. Fig. 1. Kentucky. Fig. 2. Virginia 85
VI. Parts of Los Angeles and San Bernardino Counties, California. 87
VII. Kansas 90
VIII. About 16 Miles Southwest of St. Louis, Mo. 91
IX. Niagara Falls 100
X. Fig. 1. Yellowstone Park. Fig. 2. Arizona 101
XI. Part of the Catskills, N. Y. 106
XII. Fig. 1. New Mexico. Fig. 2. Virginia, West Virginia, and
Maryland 107
XIII. Fig. 1. Colorado. Fig. 2. Kansas 162
XIV. Fig. 1. Pennsylvania. Fig. 2. California 163
XV. Near Hahnville, Louisiana 188
XVI. Missouri 189
XVII. Fig. 1. Hunterdon County, N. J. Fig. 2. Near Pikeville, Tenn. 232
XVIII. Fig. 1. Washington. Fig. 2. California 233
XIX. Part of the Big Horn Range, Wyoming 286
XX. Section of the California Coast near San Mateo, Cal. 287
XXI. New Jersey 356
XXII. Fig. 1. Portion of South Coast of Martha’s Vineyard, Mass.
Fig. 2. Portion of the California Coast near Tamalpais 357
XXIII. Fig. 1. Massachusetts. Fig. 2. Maine 364
XXIV. Portion of the Coast of Maine 365
TABLES.
Analyses of American River-waters 106
Analyses of American Spring-waters 236
Analyses of the Waters of Inclosed Lakes 392
GEOLOGY
CHAPTER I.
PRELIMINARY OUTLINE.
+Geology+ treats of the structure of the earth, of the various stages
through which it has passed, and of the living beings that have dwelt
upon it, together with the agencies and processes involved in the
changes it has undergone. _Geology is essentially a history of the
earth and its inhabitants._ It is one of the broadest of the sciences,
and brings under consideration certain phases of nearly all the
other sciences, particularly those of astronomy, physics, chemistry,
zoology, and botany. It also embraces the earlier expressions of mental
development and of life-relationships, chiefly as found in the lower
animals.
=Subdivisions.=—Naturally so broad a science has many special aspects
which constitute subdivisions, in a sense, though they are rather
dominant phases than independent sections. That phase which treats
of the outer relations of the earth is _Cosmic_ or _Astronomic
geology_; that which treats of the constituent parts of the earth
and its material is _Geognosy_, of which the most important branch
is _Petrology_, the science of rocks. That branch which investigates
the structural arrangement of the material, or “the architecture of
the earth,” is _Geotectonic_, or _Structural geology_; while that
which deals with the surface changes and topographic forms, that is,
with the face of the earth, is _Physiographic geology_. The study of
the fossils that have been preserved in the rocks, and of the faunas
and floras that these imply, constitutes _Paleontologic geology_,
or _Paleontology_. The treatment of the succession of events forms
_Historical geology_. This is chiefly worked out by the succession
of beds laid down in the progress of the ages, which constitutes
_Stratigraphic geology_. The treatment of causes, agencies, and
processes is the function of _Dynamic_ or _Philosophic geology_.
Besides these there are special applications which give occasion
for other terms, as _Economic geology_, which is concerned with the
industrial applications of geologic knowledge; _Mining geology_, which
is a sub-section of economic geology, relating to the application
of geologic facts and principles to mining operations; _Atmospheric
geology_, _Glacial geology_, and others that define themselves, and are
for the greater part but limited aspects of the broad science.
=Dominant processes.=—Three sets of processes, now in operation
on the surface of the lithosphere, have given rise to most of the
details of its configuration, and even many of its larger features.
These processes have been designated _diastrophism_, _vulcanism_, and
_gradation_. _Diastrophism_ includes all crustal movements, whether
slow or rapid, gentle or violent, slight or extensive. Many parts of
the land, especially along coasts, are known to be slowly sinking
relative to the sea-level, while other parts are known to be rising.
The fact that rocks originally formed beneath the sea now exist at
great elevations, and the further fact that areas which were once land
are now beneath the sea, are sufficient evidence that similar changes
have taken place in the past. _Vulcanism_ includes all processes
connected with the extrusion of lava and other volcanic products,
and with the rise of lava from lower to higher levels, even if not
extruded. Vulcanism and diastrophism may be closely associated, for
local movements at least are often associated with volcanic eruptions,
and more considerable movements may be connected with the movements
of subsurface lavas, even when the connection is not demonstrable.
_Gradation_ includes all those processes which tend to bring the
surface of the lithosphere to a common level. Gradational processes
belong to two categories—those which level down, _degradation_, and
those which level up, _aggradation_. The transportation of material
from the land, whether by rain, rivers, glaciers, waves, or winds,
is _degradation_ and the deposition of material, whether on the land
or in the sea, is _aggradation_. Degradation affects primarily the
protuberances of the lithosphere, while aggradation affects primarily
its depressions.
+Astronomic Geology.+
=The earth as a planet.=—Though supremely important to us, the earth is
but one of the minor planets attendant upon the sun, and is in no very
special way distinguished as a planetary body. Of the eight planets,
four, Jupiter, Saturn, Uranus, and Neptune, are much larger than the
earth, while three, Mars, Venus, and Mercury, are smaller. There are a
host of asteroids, but all together they do not equal the mass of the
smallest planet. The average mass of the eight planets is more than
fifty times that of the earth, while the largest, Jupiter, is more than
three hundred times as massive as the earth. The earth’s position in
the group is in no sense distinguished. It is neither the outer nor the
inner, nor even the middle planet. Even in the minor group to which it
belongs, it is neither the outermost nor the innermost member, though
in this group it is the largest. Its average distance from the sun is
about 92.9 million miles, and this fixes its revolution at 365¼ days,
for its period of revolution is directly dependent on its distance from
the sun, and is necessarily longer than the revolutions of the inner
planets and shorter than those of the outer planets. Its rotation in
twenty-four hours is not far different from that of its neighbor Mars,
but is much slower than the more distant and larger planets, Jupiter
and Saturn, which rotate in about ten hours. Comparison cannot be made
with the innermost and outermost planets, because their rotations are
not yet satisfactorily determined. The plane of the earth’s revolution
lies near the common plane of the whole system, but this is not
peculiar, as all of the planets revolve in nearly the same plane.
Only a few of the small asteroids depart notably from this common
plane. This has an important bearing on theories of the origin of
the system, since this close coincidence of the planes of the orbits
is not consistent with any haphazard aggregation of the material. Of
similar importance is the fact that all of the planets revolve in
the same direction and in ellipses that do not depart widely from
circles. The eccentricity of the earth’s orbit is only about ¹⁄₆₀. This
eccentricity varies somewhat, due to the disturbing influences of the
other planets, and this variation has been regarded by some geologists
as an influential cause of climatic changes, but its adequacy to
produce great effects has been doubted by others. The inclination of
the earth’s axis, now about 23½°, holds an intermediate position, some
of the planets having axes more inclined, as Saturn, 26⅚°, and others
less inclined, as Jupiter, 3°. The inclination of the axis is subject
to trivial variations at present, and in the long periods of the past
has possibly changed more notably. This possible change has also been
thought to be a cause of climatic variation, but its efficiency has not
been demonstrated.
=Its satellite.=—The earth is peculiar in having one unusually large
satellite, which has a mass ¹⁄₈₁ of its own. The great planets have
several satellites whose combined mass exceeds that of the moon, and
perhaps in some few cases the individual satellites may be larger than
the moon, but they do not sustain so large a ratio to their planets,
for Titan, probably the largest, is only ¹⁄₄₆₀₀ of the mass of Saturn.
There is little doubt that the moon has played an important part in
the history of the earth. It is the chief agency in developing oceanic
tides, and it possibly also develops a body tide in the earth itself.
These tides act as a brake on the rotation of the earth and tend to
reduce its rate, and thereby to lengthen the day. While this may have
been counteracted in some measure by the shrinkage of the earth, which
tends to increase its rate of rotation, it has been held by eminent
physicists and geologists that the rotation of the earth has been
greatly lessened during its history, and that a long train of important
consequences has resulted. If the contraction of the earth has been
sufficient to offset this lessening, the tidal brake must be credited
with the prevention of the excessive speed of rotation which would
otherwise have been developed. The tides are efficient agencies in the
shore wear of the oceans, and in the distribution of marine sediments,
and these, it will be seen later, are important elements in the
formation of strata.
=Dependence on the sun.=—By far the most important external relation
of the earth, however, is its dependence on the sun. The earth is a
mere satellite of the sun, less than ¹⁄₃₀₀₀₀₀ of its mass, and hence
under its full gravitative control. The earth is dependent on the sun
for nearly all its heat and light, and, through these, for nearly all
of the activities that have given character to its history. It is
too much to say that all activities on the surface of the earth are
solely dependent on those of the sun, for a certain measure of heat and
light and other energy is derived from other bodies, and a certain not
inconsiderable source of energy is found in the interior of the earth
itself; yet all of these are so far subordinate to that great flood of
energy which comes from the sun that they are quite inconsequential.
The history of the earth in the past has been intimately dependent upon
that of the sun, and its future is locked up with the destiny of that
great luminary. Geology in its broadest phases can therefore scarcely
be separated from the study of the sun, but this falls within the
function of the astronomer rather than the geologist.
=Meteorites.=—There are a multitude of small bodies passing through
space in varying directions and with varying velocities and
occasionally encountering the earth, to which they add their substance.
Some of these meteorites revolve about the sun much as if they were
minute planets, but some of them come from such directions and with
such velocities as to show that they do not belong to the solar family.
Some consist almost wholly of metal, chiefly iron alloyed with a
small percent. of nickel (holosiderites); some consist of metal and
rock intimately mixed (syssiderites and sporadosiderites); and some
consist wholly of rock (asiderites). The rock is usually composed of
the heavier basic minerals, though some meteorites consist largely
of carbonaceous material. Besides meteorites, there is little doubt
that wandering gaseous particles strike the earth, but this is beyond
the reach of present demonstration. The amount of substance added to
the earth by these meteorites and gases in recent times is relatively
slight compared with the whole body of the earth. What contribution may
have come to the earth in earlier times from such sources is a matter
of hypothesis which will be discussed later.
+Geognosy.+
=The constitution of the earth.=—Turning from its external relations to
the earth itself, a natural threefold division is presented: (1) the
atmosphere, (2) the hydrosphere, and (3) the lithosphere.
_I. The Atmosphere._
The atmosphere is an intimate mixture of (1) all those substances
that cannot take a liquid or solid state at the temperatures and
pressures which prevail at the earth’s surface, together with (2) such
transient vapors as the various substances of the earth throw off. The
first class form the permanent gases of the atmosphere, and consist
of nitrogen about 79 parts, oxygen about 21 parts, carbon dioxide
about .03 part, together with small quantities of argon, neon, xenon,
krypton, helium, and other rare constituents. The second class are
the transient and fluctuating constituents of the atmosphere, chief
among which is aqueous vapor, which varies greatly in amount according
to temperature, pressure, and other conditions. To this are to be
added volcanic emanations and a great variety of volatile organic
substances. Theoretically, every substance, however solid, discharges
particles which may transiently become constituents of the atmosphere.
Practically, only a few of these exist in such quantity as to be
appreciable. Dust and other suspended matter are usually regarded as
impurities rather than constituents of the atmosphere, but they play a
not unimportant part by affecting its temperature and luminosity, and
by facilitating the condensation of moisture.
=Mass and extent.=—The total mass of the atmosphere is estimated at
five quadrillion tons, or ¹⁄₁₂₀₀₀₀₀ of the mass of the earth. It is
relatively dense at the surface of the earth and decreases in density
outwards in a manner difficult of absolute determination, so that the
actual height of the appreciable atmosphere is not positively known.
The true conception of the atmosphere is perhaps that of a tenuous
envelope exerting a pressure of about fifteen pounds per square inch
at the sea-level, and thinning gradually upwards until it reaches a
tenuity which is inappreciable, but perhaps not ceasing absolutely
until the sphere of gravitative control of the earth is passed, about
620,000 miles from the lithosphere. In the lower portion, according to
the kinetic theory of gases, the molecules fly to and fro, colliding
with each other with almost inconceivable frequency, and with very
short paths between successive collisions, but in the upper rare
portion some of the molecules bound outwards, and do not strike other
molecules, and hence pursue long elliptical paths until the gravity of
the earth overcomes their momentum, when they return, perhaps to bound
off again or to force other molecules to do so. This fountain-like
nature of the outer part of the atmosphere makes any sharp definition
of its limit impracticable. Some molecules are believed to be shot away
at such speed that they do not return. Beyond about 620,000 miles from
the surface of the lithosphere, the differential attraction of the sun
is greater than that of the earth, and if the attraction of the earth
does not turn the molecules back before reaching this distance, they
are almost certain to be lost to the earth.
The measurement of heights by the aneroid barometer, which is much used
in practical geology, is dependent on the lessening of pressure as the
instrument is carried upward.
=Geologic activity.=—The atmosphere is the most mobile and active
of the three great subdivisions of the earth, and when its indirect
effects through the agency of water, as well as its direct effects,
are considered, it is to be regarded as one of the most effective
agencies of change. It acts chemically upon the rock substance of the
earth, causing induration in some instances, but more often inducing
disintegration and change of composition by means of which rock is
reduced to soil, or soil-like material, and rendered susceptible of
easy removal by winds and waters. When in motion the atmosphere acts
mechanically on the surface of the earth, transporting dust and sand,
and by the friction of these it abrades the surface. It is chiefly
effective, however, in furnishing the conditions for water action.
Partly by its mechanical aid, but chiefly by securing the right
temperature, it is a necessary factor in the action of rains, streams,
glaciers, and the various forms of moving water upon land. So also,
on the ocean, wave action is essentially dependent on the winds. In
the absence of atmospheric propulsion, wave action would be chiefly
confined to the tides and to occasional earthquake impulses, and would
lose nearly all its efficiency. Stream action and wave action, which
are the most declared of the geological agencies, are therefore to be
credited as much to the atmosphere as to the hydrosphere, since the
action is a joint one to which both envelopes are essential.
=A thermal blanket.=—A function of the atmosphere of supreme importance
is the thermal blanketing of the earth. In its absence the heat of
the sun would reach the surface with full intensity, and would be
radiated back from the surface almost as rapidly as received, and only
a transient heating would result. During the night an intensity of cold
would intervene scarcely less severe than the temperature of space. In
penetrating the atmosphere certain portions of the radiant energy of
the sun are absorbed. Of the remainder which reaches the surface of the
earth, a part is transformed into vibrations of lower intensity, which
are then more effectively retained by the atmosphere. The air thus
distributes and equalizes the temperature. The two constituents of the
atmosphere which are most efficient in this work are aqueous vapor and
carbon dioxide, and the climate of the earth is believed to have been
very greatly affected by the varying amounts of these constituents in
the atmosphere, as well as by the total mass of the atmosphere.
The function of the atmosphere in sustaining life and promoting all
that depends on life is too obvious to need comment.
The special geological action of the atmosphere will be discussed in
the next chapter.
_II. The Hydrosphere._
About 1300 quadrillion tons of water lie upon the surface of the solid
earth. This equals about ¹⁄₄₅₄₀ part of the earth’s mass. Were the
surface of the solid earth perfectly spheroidal, this would constitute
a universal ocean somewhat less than two miles deep. Owing to the
inequalities of the rock surface, the water is chiefly gathered into a
series of great basins or troughs occupying about three-fourths (72%)
of the earth’s surface. These basins are all connected with each other
and act as a unit, so that anything which changes the level of the
water in one changes the level of all. This helps to make a common
record of all great movements of the earth’s body, for the level of
the ocean determines where the detritus from the land shall lodge, and
hence where the edge of the marine beds shall be formed. This will
appear more clearly when the formation of marine strata is discussed.
=Oceanic dimensions.=—The surface area of the ocean is estimated
by Murray at 143,259,300 square miles. Of this, somewhat more than
10,000,000 square miles lie on the continental shelf, i.e., lap up on
the borders of the continental platforms. This shows that the great
basins are somewhat more than full. If about 600 feet of the upper
part of the ocean were removed, the true ocean basins would be just
full, and the surfaces of the true continental platforms would be dry
land. The area of the true oceanic basins is about 133,000,000 square
miles, and that of the true continental platforms about 64,000,000
square miles. Under about 20% of the ocean area, the bottom sinks to
depths between 6000 and 12,000 feet; under about 53% it sinks to depths
between 12,000 and 18,000 feet; and under the remaining 4% it ranges
from 18,000 feet down to about 30,000. The last includes those singular
sunken areas known as “deeps,” and sometimes called _anti-plateaus_, as
they extend downward from the general ocean bottom much as the plateaus
protrude upwards from the general land surface.
Besides the ocean, the hydrosphere includes all the water which
constitutes the surface streams and lakes, together with that which
permeates the pores and fissures of the outer part of the solid earth;
but altogether these are small in amount compared with the great ocean
mass.
=Geologic activity.=—Of all geological agencies water is the most
obvious and apparently the greatest, though its efficiency is
conditioned upon the presence of the atmosphere, upon the relief of
the land, and upon the radiant energy of the sun. Through the agency
of rainfall, of surface streams, of underground waters, and of wave
action, the hydrosphere is constantly modifying the surface of the
lithosphere, while at the same time it is bearing into the various
basins the wash of the land and depositing it in stratified beds. It
thereby becomes the great agency for the degradation of the land and
the building up of the basin bottoms. It works upon the land partly
by dissolving soluble portions of the rock substance, and partly by
mechanical action. The solution of the soluble part usually loosens
the insoluble, and renders it an easy prey of the surface waters.
These transport the loosened material to the valleys and at length to
the great basins, meanwhile rolling and grinding it and thus reducing
it to rounder forms and a finer state, until at length it reaches the
still waters or the low gradients of the basins and comes to rest.
The hydrosphere is therefore both destructive and constructive in its
action. As the beds of sediment which it lays down follow one another
in orderly succession, each later one lying above each earlier one,
they form a time record. And as relics of the life of each age become
more or less imbedded in these sediments, they furnish the means of
following the history of life from age to age. The historical record
of geology is therefore very largely dependent upon the fact that the
waters have thus buried in systematic order the successive life of the
ages. Aside from this, the means of determining the order of events of
the earth’s history are limited and more or less uncertain.
The special processes of the hydrosphere in its various phases will
be the subject of discussion hereafter (Chaps. III, IV, VI). Suffice
it here to recognize its great function in the constant degradation
of the land, and in the deposition of the derived material in orderly
succession in the basins.
=Chief horizons of activity.=—The great horizons of geological activity
are (1) the contact zone between the atmosphere and the hydrosphere,
chiefly the surface of the ocean, (2) the contact zone between the
hydrosphere and the lithosphere, chiefly the shore belts, and (3) the
contact zone of the atmosphere and surface waters, with the face of the
continents. It is in these three zones that the greatest external work
is being done and has been done in all the known ages.
_III. The Lithosphere._
The atmosphere and hydrosphere are rather envelopes or shells than true
spheres, though in some degree both penetrate the lithosphere. The
lithosphere, on the other hand, is a nearly perfect oblate spheroid
with a polar diameter of 7899.7 miles, and an equatorial diameter of
about 26.8 miles more. Its equatorial circumference is 24,902 miles,
its meridional circumference 24,860 miles, its surface area 196,940,700
square miles, its volume 260,000,000,000 cubic miles, and its average
specific gravity about 5.57. The oblateness of the spheroid is an
accommodation to the rotation of the earth, the centrifugal force
at the equator being sufficient to cause the specified amount of
bulging there. Computations seem to indicate that the accommodation
is very nearly what would take place if the earth were in a liquid
condition, from which the inference has been drawn that it must have
been in that condition when it assumed this form, and must have
continued essentially liquid until it attained its present rate of
rotation, since, if the earth once rotated at a much higher speed, the
flattening at the poles and the bulging at the equator must have been
correspondingly greater. It is thought by others, however, that the
plasticity of the earth is such that it would at all times assume a
close degree of approximation to the demands of rotation, even if the
interior were in a solid condition. By still others it is thought that
the contraction of the earth has tended to accelerate the rotation
about as much as the tides have tended to retard it, and that it has
undergone little change of form.
=Irregularities.=—It is only in a general view, however, that there is
a close approximation to a perfect spheroidal surface. In detail there
are very notable variations from it. Geodetic surveys seem to have
shown that the equatorial diameters are not all equal, even when the
measurements are reduced to sea-level, but research along this line has
not reached a sufficient stage of completeness to permit satisfactory
discussion. It is, however, highly probable that the ocean surface
as well as the average land surface is warped out of the perfect
spheroidal form to some notable degree. This is very likely due to
inequalities in the density of the earth’s interior. The fact that the
larger portion of the water is gathered on one side of the globe, while
the land chiefly protrudes on the opposite side, is very possibly due
to unequal specific gravity in the interior of the earth.
The most obvious departure from a spheroidal form is found in the
protrusion of the continents and in the sinking away of the earth
surface under the oceans. As these inequalities present themselves
to-day, they are known as continental platforms and ocean basins. These
do not correspond accurately with the present land and water surfaces.
About the continental lands there is a submerged border extending some
distance out from the shore, and constituting a sea-shelf beyond which
the surface descends rapidly to the great depths of the ocean. This
slightly submerged portion, known as the continental shelf, belongs
as properly to the continent as the adjacent low lands which are not
submerged. The submergence of the edge of this shelf at present is
usually about 100 fathoms, so that if the upper 600 feet of the
ocean were removed, the outlines of the land would correspond quite
closely with the border of the true continental platform.
[Illustration: =BATHYMETRICAL CHART OF THE OCEANS=
SHOWING THE “DEEPS” ACCORDING TO SIR JOHN MURRAY]
It is customary to look upon the protrusions of the continents as the
great features of the earth’s surface, but in reality the oceanic
depressions are the master phenomena. In breadth, depth, and capacity
they much exceed the continental protrusions, and if the earth be
regarded as a shrunken body, the settling of the ocean bottoms has
doubtless constituted its greatest surface movement. From the estimates
of Murray, Gilbert has derived the following tables, showing the
relative areas of the lithosphere above, below, and between certain
levels.[1]
From these estimates it appears that if the surface were graded to a
common level by cutting away the continental platforms and dumping the
matter in the abysmal basins, the average plane would lie somewhere
near 9000 feet below the sea-level. The continental platform may
be conceived as rising from this common plane rather than from the
sea-level.
-------------------------------------+----------------+--------------
| Percent. of | Percent. of
Contours. | Surface | Surface
| above. | below.
-------------------------------------+----------------+--------------
Contour 24,000 feet above sea-level | 0.004 | 99.996
„ 18,000 „ „ „ | 0.09 | 99.91
„ 12,000 „ „ „ | 0.7 | 99.3
„ 6,000 „ „ „ | 2.3 | 97.7
Sea level | 27.7 | 72.3
Contour 6,000 feet below sea-level | 42.5 | 57.5
„ 12,000 „ „ „ | 57.3 | 42.7
„ 18,000 „ „ „ | 96.8 | 3.2
„ 24,000 „ „ „ | 99.93 | 0.07
-------------------------------------+----------------+--------------
Percent.
More than 6000 feet above sea-level 2.3
Between sea-level and 6000 feet above 25.5
Between sea-level and 6000 feet below 14.8
Between 6000 and 12,000 feet below sea-level 14.8
Between 12,000 and 18,000 feet below sea-level 39.4
Between 18,000 feet and 24,000 feet 3.1
=Epicontinental seas.=—Those shallow portions of the sea which lie
upon the continental shelf, and those portions which extend into the
interior of the continent with like shallow depths, such as the Baltic
Sea and the Hudson Bay, may be called epicontinental seas, for they
really lie _upon_ the continent, or at least upon the continental
platform; while those other detached bodies of water which occupy deep
depressions in the surface are to be regarded as true abysmal seas,
as, for example, the Mediterranean and Caribbean seas and the Gulf of
Mexico, whose bottoms are as profound as many parts of the true ocean
basin itself.
=Diversities of surface.=—The bottoms of the oceanic basins are
diversified by broad undulations which range through many thousands
of feet, but they are not carved into the diversified forms that give
variety to land surfaces. The ocean bottoms are also diversified by
volcanic peaks, many of which rise to the surface and constitute
isolated islands. Some of them have notable platforms at or near the
surface, cut by the waves or built up by the accumulation of sediment
and of coralline and other growths about them. Aside from these
encircling platforms, the solid surface usually shelves rapidly down to
abysmal depths, so that the islands constitute peaks whose heights and
slopes would seem extraordinary if the ocean were removed.
The surface of the land is diversified in a similar way by broad
undulations and volcanic peaks, and also by narrower wrinklings and
foldings of the crust; but all of these irregularities have been carved
into diversified and picturesque forms by subaërial erosion. In this
respect the surface of the land differs radically from the bed of the
sea. The agencies which have produced the continental platforms and
abysmal basins, and the great undulations and foldings, as well as the
volcanic extrusions that mark them, are yet subjects of debate. Here
lie some of the most difficult problems of geology, but these cannot be
stated with sufficient brevity to find a place here.
=The surface mantle of the lithosphere.=—The surface of the lithosphere
is very generally mantled by a layer of loose material composed of
soil, clay, sand, gravel, and broken rock. This loose material is
sometimes known as _mantle rock_, and sometimes as _rock waste_. On
the land, mantle rock is often composed of the disintegrated products
of underlying rock formations. It represents the results of the recent
action of the atmosphere, of water, of changes of temperature, and of
other physical agencies acting on the outer part of the rock sphere.
The surface of this mantle is being constantly removed by wind and
water, but as constantly renewed by continued decomposition of the rock
below. In some areas, especially in the northern part of North America
and the northwestern part of Europe, the soil graduates down into an
irregular sheet of mixed clay, sand, gravel, and bowlders, known as
_drift_. From this and other evidence it is inferred that at a time
not greatly antedating our own, ice, chiefly in the form of glaciers,
spread extensively over the high latitudes of the northern hemisphere.
In some parts of the earth the surface is still covered by fields of
snow and ice, comparable to those which formed the drift. In still
other places, especially along the flood plains of streams, the mantle
rock consists of deposits made by streams which were unable to carry
their loads of sediment to the sea.
=The crust of the lithosphere.=—Much of the detritus washed down
from the land finds its way to bodies of standing water, and beneath
lakes and seas the mantle of loose material is made up largely of the
gravel, sand, and mud derived from the land. Before deposition these
materials are more or less assorted and arranged in layers by waves
and currents. When consolidated they constitute rock. The weathering
of the rocks of the land, the wearing away of the resulting detritus,
and its deposition beneath standing water, are among the most important
processes of geologic change.
On the land, the mantle of loose material is sometimes absent, and
in such places the surface of solid rock of the crust appears. Bare
surfaces of rock are most commonly seen where the topography is rough,
especially on the slopes of steep-sided valleys and mountains, and on
the slopes of cliffs which face seas or lakes. Solid rock, without
covering of soil or loose material of any sort, is also frequently seen
in the channels of streams, especially where there are falls or rapids.
We have but to note the effects of a vigorous shower on a steep slope,
or of a swift stream on its channel, or of waves on the cliffs which
face lakes and seas, to understand at least one of the reasons why
loose materials are frequently absent from steep slopes. The very
general exposure of solid rock where conditions favor surface erosion
suggests that rock is everywhere present beneath the soil or subsoil.
Fortunately there is an easy way of testing the universality of the
crust beneath the mantle. In all lands inhabited by civilized peoples
there are numerous wells and other excavations ranging from a few feet
to several hundred feet in depth, and occasional wells and mine-shafts
reach depths of several thousand feet. Even in shallow excavations
rock is often encountered, and in most regions excavations as much as
two or three hundred feet deep usually reach rock, and no really deep
boring has ever failed to find it. It may, therefore, be accepted as a
fact that the upper surface of the solid rock is nowhere far below the
surface.
Concerning the thickness of the crust, if there be any true crust
at all, little is known by direct observation. The deepest valleys,
such as the canyon of the Colorado, and the shafts and borings of
the deepest mines and wells, give knowledge of nothing but rock. The
deepest excavations extend rarely more than a mile below the surface.
It is certain that rock of known kinds extends to far greater depths.
=The interior.=—Concerning the great interior of the earth, little
is known except by inference. From the weight of the earth,[2] it is
inferred that its interior is much more dense than its surface. From
its behavior under the attraction of other bodies, it is believed to
be at least as rigid as steel, and its interior cannot, therefore,
be liquid, in the usual sense of that term. From the phenomena of
volcanoes, and from observations on temperature in deep borings, it is
inferred that its interior is very hot. Further inferences concerning
its character are less simply stated, and will be referred to later.
The solid part of the earth is therefore composed of (1) a thin layer
of unconsolidated or earthy material, a few feet to a few hundred feet
in thickness, covering (2) a layer or zone many thousands of feet, and
probably many miles, thick, composed of solid rock comparable to that
exposed at numerous points on the surface, and (3) a central mass, to
which the preceding layers are but a shell, composed of hot, dense, and
rigid rock, the real nature of which is not known by observation.
=Varieties of rock in crust.=—If the mantle of soil, subsoil, and
glacial rubbish were stripped from the land, the surface beneath would
be found to be made up of a great variety of rocks, all of which may be
grouped into two great classes. About four-fifths of the land surface
would be of rock arranged in layers, and the other fifth would be of
crystalline rock, generally without distinct stratification, and often
bearing evidence of the effects of high temperature.
=Stratified rocks.=—The composition of most stratified rocks
corresponds somewhat closely with the composition of sediments now
being carried from the land and being deposited in the sea. Their
arrangement in layers is the same, and the markings on the surfaces
of the layers, such as ripple-marks, rill-marks, wave-marks, etc.,
are identical. Furthermore, the stratified rocks of the land, like
the recent sediments of the sea, frequently contain the shells and
skeletons of animals, and sometimes the impressions of plants. Most of
the relics of life found in the stratified rocks belonged to animals
or plants which lived in salt water. Because of their structure, their
composition, their distinctive markings, and the remains of life which
they contain, it is confidently inferred that most of the stratified
rocks which lie beneath the mantle rock of the land were originally
laid down in beds beneath the sea, and that the familiar processes of
the present time furnish the key to their history.
[Illustration: +Fig.+ 1.—Beds of (Cambrian) sandstone, _a_, are
conformable with one another, but unconformable on beds of (Huronian)
quartzite, _b_, Near Ableman, Wis.]
=Conformability.=—When the stratified rocks exposed by the removal of
the mantle rock are examined, the successive beds are sometimes found
to lie on one another in regular succession, showing that they were
laid down one after another, without change in the attitude of the
surface on which they were deposited. Such rocks are _conformable_
(the beds of series _a_, Fig. 1). In other cases it would be seen that
certain beds overlie the worn surfaces of lower beds, the layers of
which may have a different angle of inclination (series _a_, Fig. 1,
is unconformable on series _b_). Such relations show that the lower
series of beds was disturbed and eroded before the overlying beds were
deposited on them. Such series of rocks are _unconformable_.
=Relative ages.=—The structure and relations of rocks lead to
inferences as to their relative ages. In the case of stratified
rocks it is obvious that overlying beds were deposited later than
those below, and where there is unconformity it is evident that an
interval of time elapsed between the deposition of the unconformable
series. Another and in some respects more important means of telling
their order of formation is found in the remains of life entrapped
in the water-laid sediments. Whatever life existed in the waters
in which the sediments were deposited was liable to burial, and if
it was possessed of hard parts, such as bones, teeth, shells, hard
integuments, etc., these parts, or at least their impressions, were
likely to be preserved in the sediments. Even tracks and imprints of
perishable parts are sometimes preserved. All these relics, which we
call _fossils_, give indications of the kinds of life which existed
when the beds were formed. The fossils of the youngest beds show that
the life which existed when they were deposited was quite like that
of the present time. The fossils of the next older and lower beds
show greater departure from present types. This series of changes
continues downward as lower and lower beds are studied, until beds at
considerable depths contain no relics of existing species but, in lieu
thereof, forms of more primitive types. Some of these earlier types are
clearly the ancestors of more modern forms, while others seem to have
no living descendants. Going still deeper, the fossils indicate life of
more and more primitive types, until they depart very widely from the
living forms, and seem to be but remotely ancestral. So the beds may be
followed downward until the lowest, which contain distinct evidences of
life, are reached.
It should be understood that it is not possible to proceed directly
downward through the whole succession of bedded rocks, but that the
edges of the various beds may be found here and there where they have
been brought to the surface by warpings or tiltings, or exposed by
the wearing away of the beds which once overlay them. The full series
of strata is made out only by putting together the data gathered
throughout all lands, and even when this is done an absolutely complete
series cannot yet be made out or, at least, has not been.
=The crystalline rocks.=—The crystalline rocks which would appear
if the mantle rock were removed are of two types, _igneous_ and
_metamorphic_. Igneous rocks may be loosely defined as hardened lavas.
Metamorphic rocks are those which are greatly changed from their
original condition. Either stratified or igneous rocks may become
metamorphic.
Igneous rocks sustain various relations to the stratified rocks, as
illustrated by Fig. 2. From these relations it is possible to tell
something of the order of their formation. Where the stratified rocks
are broken through by lavas, it is obvious that the stratified rocks
were formed first, and the lavas intruded later. Lava sheets intruded
between beds of stratified rock can be told from those which flowed out
on the surface and were subsequently buried, for in the former case the
sedimentary rocks, both above and below the igneous rock, were affected
by the heat, while in the latter case only those below were so affected.
[Illustration: +Fig.+ 2.—Diagrammatic representation of the relations
of igneous rock to stratified rock. The igneous rocks, represented in
black, have been forced up from beneath.]
More commonly than otherwise the metamorphic rocks (Fig. 3) lie beneath
the sedimentary beds and are often broken through by the igneous rocks.
From their position in many places their great age may be inferred, but
locally, especially where dynamic action has been severe, relatively
young rocks are metamorphic.
[Illustration: +Fig.+ 3. The figure represents a section of the
earth about 1000 miles long. The unequally thick black line at the
top represents on something like its proper scale the depth of the
stratified rock. The area below represents crystalline rock, largely
metamorphic.]
=Four great sedimentary eras.=—The water-laid series represents four
great eras in the history of the earth, as shown by the relics of life
imbedded in them. Beginning with the latest, these are the _Cenozoic_
(recent life), during which the life took on its modern aspect; the
_Mesozoic_ (middle life), during which the life bore a mediæval aspect;
the _Paleozoic_ (ancient life), during which the life belonged to
older types; and the _Proterozoic_ (earlier life), during which it is
inferred that much life prevailed, though its record is very imperfect.
It may safely be assumed to have been more primitive than that of the
Paleozoic, as it was earlier. Each of these great divisions embraced
several lesser periods or epochs, and these again are subdivided more
and more closely according to the degrees of refinement to which
studies are carried. The chief of these subdivisions are given in the
table on page 19, and others will come under consideration in the
historical chapters.
In these four great series of sedimentary rocks there are, here and
there, intrusions of igneous rocks, and in some places the sedimentary
beds have been metamorphosed into crystalline rocks by heat and
pressure. This is particularly true in the lowest of these series, the
Proterozoic, where a large part of the sediment is metamorphosed, and
where there is much igneous rock, but it is still clear that the main
portion of this series was originally water-laid sediment, and so it
belongs to the sedimentary series rather than the Archean, in which the
sediments are the minor rather than the main factor. It has, however,
usually been classed with the Archean, and it is certainly not always
easy to draw the dividing line. In a sense it may be regarded as a
transition series.
=The Archean complex.=—Beneath the dominantly sedimentary but partly
metamorphic and igneous series there is a very complex group of rocks
largely of metamorphosed igneous origin, though containing some
metamorphosed sediments. These extend downwards to unknown depths.
While all the great formations are occasionally bent and broken, these
lowest ones are almost everywhere warped, folded, and contorted, often
in the most intricate way. They have been very generally mashed and
sheared by enormous pressure, so that they have become foliated, and
their original character is much masked. They therefore form a series
of great obscurity and complexity. As they are at the bottom of the
known series, they have been called the “Fundamental gneiss” and the
“Basement complex,” but as the part which we see is not the true base
nor the true foundation, it is safer to call them simply the Archean
(very ancient) complex. As life appears to have been present during
a part at least of the period of its formation is referred to the
Archeozoic era.
[Illustration: +Fig.+ 4.—Diagram to illustrate the relations of the
five great groups of formations. _AR_ = Archean, _Pr_ = Proterozoic,
_P_ = Paleozoic, _M_ = Mesozoic, _C_ = Cenozoic.]
Beyond and below this series, the structure of the earth is a matter
of inference. Vast as are the preceding series, they together form
relatively but a thin shell on the outer surface of the globe.
The foregoing series are diagrammatically expressed in Fig. 4, and
systematically presented to the eye in the following table.
GENERAL TABLE OF GEOLOGIC DIVISIONS.
{ Present.
{ Pleistocene.
{ Pliocene.
+Cenozoic+ { Miocene.
{ Oligocene.
{ Eocene.
: Transition (Arapahoe and Denver).
{ Upper Cretaceous.
+Mesozoic+ { Lower Cretaceous (Comanche or Shastan).
{ Jurassic.
{ Triassic.
{ Permian.
{ Coal Measures or Pennsylvanian.
{ Subcarboniferous, or Mississippian.
+Paleozoic+ { Devonian.
{ Silurian.
{ Ordovician.
{ Cambrian.
Great interval.
{ Keweenawan.
{ Interval.
+Proterozoic+ { Animikean or Penokean.
{ (Upper Huronian of some authors).
{ Interval.
{ Huronian.
Great interval.
{ { Great Granitoid Series.
{ { (Intrusive in the main,
{ { Laurentian.)
+Archeozoic+ { Archean Complex. { Great Schist Series.
{ { (Mona, Kitchi, Lower
{ { Keewatin, Coutchiching,
{ { Lower Huronian of some
{ { authors.)
The purpose of this general survey is to bring the salient features
of the earth’s structure into view preparatory to entering in more
detail into the study of particular processes and special formations
and to lay a foundation for the fuller apprehension of the successive
stages of the history of the earth, which constitutes the chief purpose
of geological study. It is now advisable to turn to the detailed
consideration of individual processes and specific structures. The
complexity of the actions involved in the history of the earth is so
great that such separate consideration at the outset is helpful.
CHAPTER II.
THE ATMOSPHERE AS A GEOLOGICAL AGENT.
While it is convenient to regard the lithosphere as the earth proper,
and the atmosphere as its envelope, the latter is as truly a part of
the planet as the former, and its activities and its history are as
truly subjects of geological study as the formation of the rocks. This
view is in no way vitiated by the fact that the special study of the
atmosphere is set apart under the name _Meteorology_, for in the same
way the special study of rocks is set apart under the name _Petrology_,
that of ancient life under the name _Paleontology_, and that of other
phases of the subject under other names. The atmosphere is one of the
three great formations of the earth, and as a geological factor takes
its place beside the hydrosphere and the lithosphere. It has played
a part in the history of the earth comparable to that of the water,
though its mass is less and its record more elusive. Unsubstantial as
the atmosphere seems when contrasted with the liquid and solid portions
of the earth, its extreme mobility and its chemical activity compensate
for its lightness and tenuity, and give it a function of the first
order of importance.
The atmosphere plays a direct part as (1) a mechanical and (2) a
chemical agent, and at the same time serves an indirect function
in furnishing favorable conditions under which (3) solar radiation
produces temperature effects, and (4) evaporation gives origin
to precipitation and stream effects, and furnishes the necessary
conditions for land plants and animals, and the important influences
that spring from them.
This chapter is devoted to the work of the atmosphere in these and some
less notable phases. The consideration of the origin and history of the
atmosphere will receive attention later.
A. THE ATMOSPHERE AS A DIRECT AGENCY.
_I. Mechanical Work._
The mechanical work of the atmosphere is accomplished chiefly through
its movement. A feeble breeze is competent to move particles of dust,
and winds of moderate velocity to shift sand. Exceptionally strong
winds sometimes move small pebbles, but winds of sufficient force to
move larger pieces of rock are rare. It follows, therefore, that the
impact of the wind has little direct effect except on surfaces covered
with dust and dry sand.
The transportation of material by the wind is limited by the size
of the particles to which it has access. Dust particles expose more
surface to the wind relative to their mass than sand grains. Winds
which are unable to carry sand may still carry dust, and winds which
are able to shift sand no more than a trivial distance may blow dust
great distances.
The common conception of wind as a horizontal movement of some part
of the atmosphere is not altogether accurate. Every obstacle against
which wind blows causes deflections of its currents, and some of these
deflections are upward. Furthermore, there are exceptional winds,
in which the vertical element predominates. Particles of dust are
often involved in these upward currents, and by them carried to great
heights, and in the upper air are transported great distances.
=Transportation and deposition of dust.=[3]—The universality of the
transportation of dust by the wind is well known. No house, no room,
and scarcely a drawer can be so tightly closed but that dust enters it,
and the movements of dust in the open must be much more considerable.
The visible dustiness of the atmosphere in dry regions during
wind-storms is adequate and familiar proof of the efficiency of the
wind as a transporter of dust.
Under special circumstances, opportunity is afforded for rough
determinations of the distance and height to which wind-blown dust is
transported. Snow taken from snow-fields in high mountain regions is
found to contain a small amount of earthy matter. Its particles are
often found to be in part volcanic, even when the place whence the snow
was taken is scores or even hundreds of miles from the nearest volcano.
There is probably no snow-field so high, or so far from volcanoes, but
that volcanic dust reaches it. If this be true of all snow-fields,
it is probably true of all land surfaces. In the great Krakatoa[4]
eruptions of 1883 large quantities of volcanic ash (pulverized lava)
were projected to great heights into the atmosphere. The coarser
particles soon settled; but, caught by the currents of the upper
atmosphere, many of the finer particles were transported incredible
distances. Through all their long journey, the particles of dust were
gradually settling from the atmosphere, but not until the dust had
traveled repeatedly round the earth did its amount become so small as
to cease to make its influence felt in the historic red sunsets which
it occasioned.[5] Some of this dust completed the circuit of the earth
in 15 days.
In various parts of Kansas and Nebraska[6] there are very considerable
beds of volcanic dust, locally as much as 30 feet thick, which must
have been transported from volcanic vents by the wind, though there
are no known centers of volcanic action, past or present, within some
hundreds of miles of some of the localities where the dust occurs.
These beds of volcanic dust, so far from its source, may serve as an
illustration of the importance of atmospheric movements as a geological
force.
Volcanic dust is shot into the atmosphere rather than picked up by
it. Dust picked up by the wind is perhaps transported not less widely
than volcanic dust, but, after settling, its point of origin is less
readily determined. It would perhaps be an exaggeration to say that
every square mile of land surface contains particles of dust brought to
it by the wind from every other square mile, but such a statement would
probably involve much less exaggeration than might at first be supposed.
Examples of extensive deposits of dust other than volcanic are also
known. In China there is an extensive earthy formation, _the loess_,
sometimes reaching 1,000 feet in thickness, which von Richtofen
believes to have been deposited by the wind.[7] This conclusion has,
however, not passed unchallenged.[8] The loess of some other regions
has been referred to the same origin, and some of it is quite certainly
eolian.[9]
The transportation of dust is important wherever strong winds blow over
dry surfaces, free or nearly free of vegetation, and composed of earthy
matter. Its effects may be seen in such regions as the sage-brush
plains of western North America. The roots of the sage-brush hold the
soil immediately about them, but between the clumps of brush, where
there is little other vegetation, the wind has often blown away the
soil to such an extent that each clump of brush stands up several
inches, or even a foot or two, above its surroundings (Fig. 5). Such
mounds are often partly due to the lodgment of dust about the bushes.
[Illustration: +Fig.+ 5.—This figure shows the effect of sage-brush or
other similar vegetation in holding sand or earth, or in causing its
lodgment, in dry regions.]
Where the earthy matter is moist, the cohesion of the particles is
great, and the wind cannot pick them up. Furthermore, if the surface
is generally moist, it is likely to be covered with vegetation which
protects it against the wind. But even where vegetation is prevalent
the wind finds many a vulnerable point. Thus on the edges of plains
or plateaus facing abrupt valleys, the wind attacks the soil from
the side, and in such situations all earthy matter may be stripped
from the underlying rock for considerable distances from the edge of
the cliff (Fig. 6). This may be seen at numerous points on the lava
plateaus of Washington.
[Illustration: +Fig.+ 6.—Diagram to illustrate the way in which
the wind sometimes strips the soil from the edge of a bluff. This
phenomenon is not rare in the basin of the Columbia River in
Washington.]
The presence of dust in the upper atmosphere during a rain-storm is
sometimes the occasion of phenomena which are often misinterpreted. If
there be abundant dust in the atmosphere through which rain-drops or
snowflakes fall, much of it is gathered up by them, and the water is
thereby rendered turbid and the snow discolored. Here is to be found
the explanation of “mud-rain,” “blood-rain” (red dust), etc.
Since dust is carried to a considerable extent in the upper atmosphere,
its movements and its deposition are little affected by obstacles on
the surface of the land. A building or a hedge can only affect the
lodgment of that part of the atmospheric dust which comes in contact
with it or is swept into its lee. Since most obstacles on the surface
of the solid part of the earth reach up but slight distances into the
atmosphere, the dust of the greater part of the air settles without
especial reference to them, and is spread more or less uniformly over
the surface on which it falls.
[Illustration: +Fig.+ 7.—Diagram to illustrate the effect of an
obstacle on the transportation and deposition of sand. The direction of
the wind is indicated by the upper arrow. The lower arrows represent
the direction of eddies in the air occasioned by the obstacle. If the
surface in which the obstacle was set was originally flat (dotted
line), the sand would tend to be piled up on either side at a little
distance from the obstacle, but more to leeward. At the same time,
depressions would be hollowed out near the obstacle itself (see full
line). (After Cornish.)]
Much of the dust transported by the wind is carried out over seas or
lakes and falls into them. By this means, sedimentation is doubtless
going on at the bottom of the whole ocean, and at the bottoms of all
lakes. While means of determining the amount of dust blown into the
sea are not at hand, it is safe to say that, were such determinations
possible, the result, if stated in terms of weight, would be surprising.
=Transportation and deposition of sand.=—In its transportation by
the wind, sand is not commonly lifted far above the surface of the
land, and its movement is therefore more generally interfered with by
surface obstacles than is the movement of dust. A shrub, a tree, a
fence, a building, or even a stone may occasion the lodgment of sand
in considerable quantity, though it has little effect on the lodgment
of dust. The effect of obstacles is illustrated by Fig. 7 (see also
Fig. 5). If the obstacle which occasions the lodgment of sand presents
a surface which the wind cannot penetrate, such as a wall, sand is
dropped abundantly on its windward side as well as on the leeward;
but if it be penetrable, like an open fence, the lodgment takes place
chiefly on its leeward side. In cultivated regions cases are known
where, in a few weeks of dry weather, sand has been drifted into lanes
in the lee of hedges to the depth of two or three feet, making them
nearly impassable to vehicles.
=Formation of dunes.=—In contrast with dust deposited from the
atmosphere, wind-blown sand is commonly aggregated into mounds and
ridges in the process of lodgment. These mounds and ridges are dunes.
Once a dune is started, it occasions the further lodgment of sand, and
is a cause of its own growth. Dunes sometimes reach heights of 200 or
300 feet, but they are much more commonly no more than 10 or 20 feet in
height. On plane surfaces, there is a limit in height above which they
do not rise, though the limit is different under different conditions.
The velocity of wind at the bottom of the air is not so great as that
higher up, and as a dune is built up, a level is presently reached
where the stronger upper winds sweep away as much sand as is brought to
the top. The very even crests of many dune ridges are probably to be
accounted for in this way. Wind-blown or eolian sand, not piled up in
heaps or ridges, is somewhat widespread, but does not constitute dunes.
=Shapes of dunes.=[10]—Dunes may assume the form of ridges or of
hillocks. The ridges may be transverse to the direction of the
prevailing wind or parallel with it. Where dunes assume the form of
hillocks rather than ridges, a group of them may be elongate in a
direction parallel to the dominant wind, or at right angles thereto.
The shape assumed by a dune or a group of dunes depends on the
abundance of the sand, the strength and direction of the wind, and the
shape of the obstacle which occasions the lodgment.
[Illustration: +Fig.+ 8.—Dune ridges parallel to the direction of
the wind. Southwest part of India. Scale about 3 miles to the inch.
(Cornish.)]
[Illustration: +Fig.+ 9.—Dune ridges transverse to the direction of the
wind. Scale about 3 miles to the inch. (Cornish.)]
The incipient stages of dune formation are readily seen in many dry,
sandy regions. The dune is likely to start in the lee of some obstacle,
and to be elongate in the direction of the wind, especially if the wind
be strong relative to the supply of sand. This shape is permanently
preserved if the proper relations between the supply of sand and
strength and direction of wind are preserved. In the dune region of the
Indian desert[11] the prevailing winds are alternately the southwest
and northeast monsoons, the former being the stronger. The supply
of sand comes from the southwest. Near the southwest coast the dune
ridges are parallel to the direction of the wind (Fig. 8); in the
interior, where the winds are less strong, the dunes are transverse to
it (Fig. 9); while between the districts where these two types prevail
intermediate forms occur. The transverse dune ridges (Fig. 9) are said
to be the result of the lateral growth and erosion of longitudinal
dunes.[12] In regions of changeable winds the shape of the dunes
is subject to great variation. Dunes are sometimes crescentic, the
convexity facing the wind (Fig. 10).
[Illustration: +Fig.+ 10.—Crescentic dunes in ground-plan, the
convexities facing the wind. (Bokhara.) (Walther.)]
Along coasts, dune ridges are often transverse to the wind, and groups
of dune hillocks are frequently elongate in the same direction. Here
the source of supply of the sand is itself an elongate belt, often
transverse to the dominant wind, and the resulting dunes often have
great length transverse to the wind. Where the wind has strong mastery
over the sand, the longitudinal tendency is seen, even along coasts.[13]
[Illustration: +Fig.+ 11.—Section of a dune showing, by the dotted
line, the steep leeward (_bc_) and gentler windward (_ab_) slope. By
reversal of the wind the cross-section may be altered to the form shown
by the line _adc_. (Cornish.)]
[Illustration: +Fig.+ 12.—Cross-section of a dune showing the profiles
developed by scour of the wind on both flanks. (Cornish.)]
The shapes of dunes in section, like the shapes in ground-plan, depend
on the relative strength and constancy of the winds and the supply of
sand. With constant winds and abundant drifting sand, dunes are steep
on the lee side (_bc_, Fig. 11), where the angle of slope is the angle
of rest for the sand. It rarely exceeds 23° or 24°.[14] Under the same
conditions the windward slope is relatively gentle (_ab_, Fig. 11).
If the winds be variable so that the windward slope of one period
becomes the leeward slope of another, and vice versa, this form is not
preserved. Thus, by reversal of the wind, the section _abc_, Fig. 11,
may be changed to _adc_. If the winds and the supply of sand be equal,
on the average, from opposite directions, the slopes should, on the
average, be equal, though perhaps unequal after any particular storm.
The steep slopes of new-made dunes are lost after the sand has ceased
to be blown. At some points where the winds erode (scour) more than
they deposit, new profiles are developed (Figs. 12 and 13). The erosion
profiles may be very irregular if the dunes are partially covered with
vegetation. The effect of vegetation in restraining wind erosion is
shown in Fig. 14, where plants have preserved a remnant of a dune.
=The topographic map.=—Since dunes as well as other topographic
features are conveniently represented on contour maps, and since
such maps will be used frequently in the following pages, a general
explanation of them is here introduced.
“The features represented on the topographic map are of three
distinct kinds: (1) inequalities of surface, called _relief_, as
plains, plateaus, valleys, hills, and mountains; (2) distribution
of water, called _drainage_, as streams, lakes, and swamps; (3) the
works of man, called _culture_, as roads, railroads, boundaries,
villages, and cities.
[Illustration: +Fig.+ 13.—Diagram showing the outline of dunes in
process of destruction. Seven Mile Beach, N. J. (N. J. Geol.
Surv.)]
[Illustration: +Fig.+ 14.—Illustrates the protective effect of
vegetation against wind erosion. Dune Park, Ind. (Cowles)]
=Relief.=—All elevations are measured from mean sea-level. The
heights of many points are accurately determined, and those
which are most important are given on the map in figures. It is
desirable, however, to give the elevation of all parts of the area
mapped, to delineate the horizontal outline, or contour, of all
slopes, and to indicate their grade or degree of steepness. This
is done by lines connecting points of equal elevation above mean
sea-level, the lines being drawn at regular vertical intervals.
These lines are called _contours_, and the uniform vertical space
between each two contours is called the _contour interval_. On
the maps of the United States Geological Survey the contours and
elevations are printed in brown (see Plate II).
[Illustration: +Fig.+ 15.—Sketch and map of the same area to
illustrate the representation of topography by means of contour
lines (U. S. Geol. Surv.)]
The manner in which contours express elevation, form, and grade is
shown in the following sketch and corresponding contour map, Fig.
15.
The sketch represents a river valley between two hills. In the
foreground is the sea, with a bay which is partly closed by a
hooked sand bar. On each side of the valley is a terrace. From
the terrace on the right a hill rises gradually, while from that
on the left the ground ascends steeply in a precipice. Contrasted
with this precipice is the gentle descent of the slope at the left.
In the map each of these features is indicated, directly beneath
its position in the sketch, by contours. The following explanation
may make clearer the manner in which contours delineate elevation,
form, and grade:
1. A contour indicates approximately a certain height above
sea-level. In this illustration the contour interval is 50 feet;
therefore the contours are drawn at 50, 100, 150, 200 feet, and
so on, above sea-level. Along the contour at 250 feet lie all
points of the surface 250 feet above sea; and similarly with any
other contour. In the space between any two contours are found
all elevations above the lower and below the higher contour. Thus
the contour at 150 feet falls just below the edge of the terrace,
while that at 200 feet lies above the terrace; therefore all points
on the terrace are shown to be more than 150 but less than 200
feet above sea. The summit of the higher hill is stated to be 670
feet above sea; accordingly the contour at 650 feet surrounds it.
In this illustration nearly all the contours are numbered. Where
this is not possible, certain contours—say every fifth one—are
accentuated and numbered; the heights of others may then be
ascertained by counting up or down from a numbered contour.
2. Contours define the forms of slopes. Since contours are
continuous horizontal lines conforming to the surface of the
ground, they wind smoothly about smooth surfaces, recede into
all reëntrant angles of ravines, and project in passing about
prominences. The relations of contour curves and angles to forms of
the landscape can be traced in the map and sketch.
3. Contours show the approximate grade of any slope. The vertical
space between two contours is the same, whether they lie along a
cliff or on a gentle slope; but to rise a given height on a gentle
slope one must go farther than on a steep slope, and therefore
contours are far apart on gentle slopes and near together on steep
ones.
For a flat or gently undulating country a small contour interval
is used; for a steep or mountainous country a large interval is
necessary. The smallest interval used on the atlas sheets of the
Geological Survey is 5 feet. This is used for regions like the
Mississippi delta and the Dismal Swamp. In mapping great mountain
masses, like those in Colorado, the interval may be 250 feet. For
intermediate relief contour intervals of 10, 20, 25, 50, and 100
feet are used.
=Drainage.=—Watercourses are indicated by blue lines. If the
streams flow the year round the line is drawn unbroken, but if the
channel is dry a part of the year the line is broken or dotted.
Where a stream sinks and reappears at the surface, the supposed
underground course is shown by a broken blue line. Lakes, marshes,
and other bodies of water are also shown in blue, by appropriate
conventional signs.
=Culture.=—The works of man, such as road, railroads, and towns,
together with boundaries of townships, counties and states, and
artificial details, are printed in black.”[15]
=Topography of dune areas.=—From what has been said, it is clear that
the topography of dune regions may vary widely, but it is always
distinctive. Where the dunes take the form of ridges (Fig. 1, Pl.
II), the ridges are often of essentially uniform height and width for
considerable distances. If there are parallel ridges, they are often
separated by trough-like depressions. Where dunes assume the form of
hillocks (Figs. 2 and 3, Pl. II), rather than ridges, the topography
is even more distinctive. In some regions, depressions (basins) are
associated with the dune hillocks. Occasionally they are hardly less
notable than the dunes themselves. A somewhat similar association of
hillocks and basins is locally developed by other means, but dunes are
made up of sand and usually of sand only, while the composition of
similarly shaped hillocks and depressions shaped by other agencies is
notably different.
In Fig. 1, Plate II (Five Mile Beach, 8 miles northeast of Cape
May, N. J.), the contour interval is 10 feet. There is here but one
contour line (the 10-foot contour), though this appears in several
places. Since this line connects places 10 feet above sea-level,
all places between it and the sea (or marsh) are less than 10
feet above the water, while all places within the lines have an
elevation of more than 10 feet. None of them reaches an elevation
of 20 feet, since a 20-foot contour does not appear. It will be
seen that some of the elevations in Fig. 1 are elongate, while
others have the form of mounds.
Fig. 2 (Pl. II) shows dune topography along the Arkansas River in
Kansas (near Larned); Fig. 4, dune topography in Nebraska (Lat.
42°, Long. 103°), not in immediate association with a valley or
shore; and Fig. 3 shows irregular ridge-like dunes at the head of
Lake Michigan. In Fig. 2 the contour interval is 20 feet. All the
small hillocks southeast of the river are dunes. Some of them are
represented by one contour and some by two. The altitude of the
region is considerable, the heavy contour representing an elevation
of 2100 feet; but the dunes themselves are rarely more than 20 feet
above their surroundings. In Fig. 4, where the contour interval is
also 20 feet, there are, besides the numerous hillocks, several
depressions (basins). These are represented by hachures inside the
contour lines. In some cases there are intermittent lakes (blue) in
the depressions. The heavy contour at Spring Lakes in this figure
is the contour of 4300 feet. There are two depression contours
(4280 and 4260) below it. The bottom of the depression is therefore
lower than 4260, but not so low as 4240. In Fig. 3 the contour
interval is 10 feet, and the dune ridges north of Miller are more
than 50 feet high. The dune ridges here have helped to determine
the position of this branch of the Calumet River, and have blocked
its former outlet. The present drainage is to the westward.
=Migration of dunes.=—By the continual transfer of sand from its
windward to its leeward side, a dune may be moved from one place to
another, though continuing to be made up, in large part, of the same
sand. In their migration dunes sometimes invade fertile lands, causing
so great loss that means are devised for stopping them. The simplest
method (employed in France and Holland) is to help vegetation to get
a foothold in the sand. The effect of the vegetation is to pin the
sand down. As a dune ridge along a coast travels inland, another may be
formed behind it. Successions of dune ridges are thus sometimes formed.
[Illustration: +Fig.+ 16.—Diagram illustrating the migration of dunes
on the Kurische Nehrung. (Credner.)]
A remarkable instance of the migration of a sand dune is recorded
on the Kurische Nehrung on the north coast of Germany. The Nehrung
consists of a long narrow neck of land composed of sand, lying
off the main coast. At the beginning of this century there was a
notable dune ridge on one side. Since that time it has migrated a
considerable distance, and in its migration it has been brought into
the relationships illustrated in the accompanying diagrams (Fig. 16).
In 1800 the dune ridge was on one side of a church, which was then in
use. In 1839 the ridge had been so far shifted to the leeward as to
completely bury the church, and in 1869, its migration had progressed
so far as to again discover the building.[16]
[Illustration: +Fig.+ 17.—Migration of dunes into a timbered region.
Dune Park, Ind. Head of Lake Michigan. (Meyers.)]
When dunes migrate into a timbered region they bury and kill the trees
(Fig. 17). In one instance on the coast of Prussia a tall pine forest,
covering hundreds of acres, was destroyed during the brief period
between 1804 and 1827.[17] At some points in New Jersey orchards have
been so far buried within the lifetime of their owners that only the
tops of the highest trees are exposed. Trees and other objects once
buried may be again discovered by farther migration of the sand (Figs.
18 and 19).[18]
[Illustration: +Fig.+ 18.—A resurrected forest. The dune sand after
burying and killing the timber has been shifted beyond it. Dune Park.
Ind. (Meyers.)]
Eolian sand, not aggregated into distinct dunes, is often destructive.
Even valleys and cities are sometimes buried by it. Drifting sands had
so completely buried Nineveh two centuries after its destruction that
its site was unknown.
=Distribution of dunes.=—Dunes are likely to be developed wherever dry
sand is exposed to the wind. Their favorite situations are the dry and
sandy shores of lakes and seas, sandy valleys, and arid sandy plains.
Along coasts, dunes are likely to be extensively developed only where
the prevailing winds are on shore. Thus about Lake Michigan, where the
prevailing winds are from the west, dunes are abundant and large on the
east shore, and but few and small on the west. In shallow water, shore
currents and storm waves often build up a reef of sand a little above
the normal level of the water. When the waves subside, the sand dries
and the wind heaps it up into dunes. This sequence of events is in
progress at many points on the Atlantic Coast. Sandy Hook, New Jersey,
and the “beaches” farther south started as barrier ridges. When the
waves had built them above normal water-level, the wind re-worked the
sand, piling it up into mounds and hillocks (Fig. 1, Pl. II). Such dune
belts a little off shore are sometimes turned to good account. They are
usually separated from the mainland by a shallow lagoon. Where land
is valuable, the lagoon is sometimes filled in, making new land, thus
anticipating the result which nature would achieve more slowly. This
has been done at some points on the western coast of Europe.
[Illustration: +Fig.+ 19.—Migration of dune sand exposing bones in a
cemetery. Hatteras Island, N. C. (Collier Cobb.)]
Dunes are likely to occur along stream valleys (Fig. 2, Pl. II), if
their bottoms or slopes are of sand, and not covered by vegetation.
Dunes along valleys are usually on the side toward which the
prevailing winds blow. Thus they are more common on the east side of
the Mississippi than on the west. Dunes may be formed in the valley
bottoms, but the sand is often blown up out of the valley and lodged on
the bluffs above.
Apart from these special classes of situations, any sandy region the
surface of which is dry is likely to have its surface material shifted
by the wind and piled up into dune ridges or hillocks (Fig. 4, Pl.
II). Dunes probably reach their greatest development in the Sahara,
where some of them take the form of hillocks, and some the form of
ridges. Travelers in that region report that dune ridges are sometimes
encountered the faces of which are so high and steep as to be difficult
of ascent, and that parties have been obliged to travel miles along
their bases before finding a break where crossing was practicable.
[Illustration: +Fig.+ 20.—Wind-ripples. (Cross, U. S. Geol. Surv.)]
=Wind-ripples.=—The surface of the dry sand over which the wind has
blown for a few hours is likely to be marked with ripples (Fig.
20) similar to those made on a sandy bottom beneath shallow water,
under the influence of waves. Like ripple marks made by the water,
wind-ripples have one side (the lee) steeper than the other. While the
ripples are, as a rule, but a fraction of an inch high, they throw
much light on the origin of the great dune ridges. If the ripples
be watched closely during the progress of a wind-storm, they are
found gradually to shift their position. Sand is blown up the gentler
windward slope to the crest of the ridge and falls down on the other
side. The moment it falls below the crest of the ridge to leeward, it
is protected against the wind, and is likely to lodge. Wear on the
windward side is about equal to deposition on the leeward, and the
result is the orderly progression of the ripples in the direction in
which the wind is blowing, just as in the case of dune ridges.
=Abrasion by the wind.=—While the effect of the wind on sandy and dusty
surfaces may be considerable, its effect on solid rock is relatively
slight and accomplished, not by its own impact, but by that of the
material it carries. The effect of blown sand on rock surfaces over and
against which it is driven is perhaps best understood by recalling the
effects of artificial sand-blasts, by means of which glass is etched.
In a region where sand is blowing, exposed surfaces of rock suffer
from a multitude of blows struck by the sand grains in transit. The
result is that such rock surfaces are worn, and worn in a way peculiar
to the agency accomplishing the work. If the rock be made up of laminæ
which are of unequal hardness, the blown sand digs out the softer ones,
leaving the harder projecting as ridges between them. Adjacent masses
of harder and softer rock of whatever thickness are similarly affected.
The sculpturing thus effected on projecting masses of rock is often
picturesque and striking (Figs. 21 and 22), and is most common in arid
regions. Details of wind-carving are shown in Fig. 23.
[Illustration: +Fig.+ 21.—Wind-carved rock. (Green.)]
[Illustration: PLATE II.
Fig. 1. NEW JERSEY.
U. S. Geol. Surv.
Fig. 2. KANSAS.
U. S. Geol. Surv.
Fig. 3. INDIANA.
U. S. Geol. Surv.
Fig. 4. NEBRASKA.
U. S. Geol. Surv.]
Sand drifted over loose stones lying on the surface often develops
flat or flattish faces or facets on them. These facets are likely to
be three in number, and the exposed portion of the stone is likely to
develop a sort of pyramidal shape, the three flattish surfaces being
mutually limited by tolerably well-defined lines (Fig. 24). Thus arise
the three-faceted stones (_Dreikanter_ of the Germans) commonly seen
where sands have been long in movement.
[Illustration: +Fig.+ 22.—Wind-carved hillock of cross-bedded
sandstone. Missouri River, Montana. (Calhoun, U. S. G. S.)]
Not only does the drifting sand wear the surface over which it passes
and against which it strikes, but the grains themselves are worn in the
process. They are liable to be broken as they strike rock surfaces, and
they are likely to strike one another in the atmosphere. In both cases
they are subject to wear, and so to reduction to a finer and finer
state.
The erosion accomplished by the wind is therefore of various sorts. The
impact of the wind itself picks up the fine materials which are already
loosened, thus wearing down the surface from which they are removed;
the materials picked up wear the rock surfaces against which they are
blown, and the transported materials themselves suffer reduction in
transit.
=Effects of wind on plants.=—Another effect of wind work is seen in the
uprooting of trees (Fig. 25). The uprooting disturbs the surface in
such a way as to make loose earth more readily accessible to wind and
water. The uprooting of trees on steep slopes often causes the descent
of considerable quantities of loose rock and soil. Again, organisms
of various sorts (certain types of seeds, germs, etc.), as well as
dust and sand, are extensively transported by the wind. While this is
important biologically its geological effects are remote.
[Illustration: +Fig.+ 23.—Figure showing details of wind-carving on
rock surface (rhyolite). Mono Valley, California.]
[Illustration: +Fig.+ 24.—Wind-worn stones (_Dreikanter_).]
[Illustration: +Fig.+ 25.—Shows the disturbance of surface earth and
rocks by upturning of trees. (Darton, U. S. Geol. Surv.)]
=Indirect effects of the wind.=—Other dynamic processes are called into
being or stimulated by the atmosphere. Winds generate both waves and
currents, and both are effective agents in geological work. The results
of their activities are discussed elsewhere.
_II. The Chemical Work of the Atmosphere._
The chemical work of the atmosphere (including solution and
precipitation from solution) is principally accomplished in connection
with water, a dry atmosphere having relatively little direct chemical
effect on rock or soils.
=Precipitation from solution.=—The water in the soil is constantly
evaporating. Such substances as it contains in solution are deposited
where the water evaporates, and where evaporation is long continued
without re-solution of the substances deposited, the surface becomes
coated with an efflorescence of mineral matter. Conspicuous examples
are found in the alkali plains of certain areas in the western part of
the United States. Since the alkaline efflorescence is the result of
evaporation it is connected with the atmosphere, but the material of
the efflorescence was brought to its present position by water. The
principle involved is illustrated by the white efflorescence which
frequently appears on brick walls during the dry days which follow a
drenching rain. The water penetrates the brick and mortar and dissolves
something of their substance, and when it is evaporated from the
surface the material in solution is left behind.
In arid regions the deposition of substances other than alkali is
common. The percolating waters dissolve whatever is soluble, and when
they evaporate their mineral content is left. The pebbles and stones of
the arid plains have in many places become heavily coated with mineral
matter deposited in this way, and not infrequently cemented into
conglomerate. One of the commonest mineral substances found in such
situations is lime carbonate. In some cases it was doubtless derived
by solution from limestone beds beneath the surface, but this is not
always the case. It often encrusts the bits of lava on lava plains
where it can hardly have been derived from limestone. The faces of
cliffs of granite or gneiss, hundreds and even thousands of feet above
all other sorts of rock,[19] are sometimes spotted with patches of lime
carbonate. In the first case the lime carbonate was derived by chemical
change from the lava, and in the second, from the granite or gneiss
(see _Carbonation_ below), but its present position is the result of
evaporation.
=Oxidation.=—In the presence of moisture the oxygen of the air enters
into combination with various elements of the soil and rocks. This is
_oxidation_. No other common mineral substance shows the results of
oxidation so quickly and so distinctly as iron. The oxidized portion
is loose and friable, and a mass of iron exposed to a moist atmosphere
will ultimately crumble away. This change is comparable to other less
obvious changes taking place in many minerals at and below the surface.
Oxidation generally involves the disintegration of the rock concerned.
Its effects in this direction will be referred to in other connections.
=Carbonation.=—The production of lime carbonate from rock containing
calcium compounds, but not in the form of carbonates, is known as
_carbonation_, and is one of the important chemical changes effected
by the carbon dioxide of the atmosphere in coöperation with water. In
the process of carbonation the original minerals of complex composition
are decomposed and simpler ones usually formed. Volumetric changes are
involved, which often lead to the disruption of the rock (see _Ground
water_). Furthermore, carbonates are among the more soluble minerals,
and their production therefore brings some of the rock materials into
a soluble condition, and their extraction through solution tends still
further to disintegrate the rock. The carbonation of crystalline rocks
is therefore a disintegrating process, and will be considered further
in its many concrete applications.
=Other chemical changes.=—A third chemical process which often
accompanies oxidation and carbonation is _hydration_. This is effected
by water rather than by air, and will be considered in that connection.
In general it leads to the disintegration of the minerals and rocks
affected. The chemical effects of nitric acid, etc., developed through
the agency of atmospheric electricity, and the corresponding effects
of the gases and vapors which issue from volcanoes, many of them
chemically active, are to be mentioned in this connection.
=Conditions favorable for chemical changes.=—Conditions are not
everywhere equally favorable for the chemical work of the atmosphere.
In general, high temperatures facilitate chemical action, and, other
things being equal, rocks are more readily decomposed by atmospheric
action in warm than in cold regions. Chemical activity is probably
greater where the climate is continuously warm than where there are
great changes of temperature. Changes of temperature, on the other
hand, tend to disrupt rock, and thus increase the amount of surface
exposed to chemical change. Since nearly all the chemical changes
worked by the atmosphere on the rocks are increased by the presence of
moisture, the chemical activity of the atmosphere is greater in moist
than in dry regions.
B. THE ATMOSPHERE AS A CONDITIONING AGENCY.
The most obvious mechanical work of the atmosphere is effected by the
wind, but mechanical results of great importance, conditioned by the
atmosphere, are also effected when the air is still.
_I. Temperature Effects._
When the sun shines on bare rock its surface is heated and expanded,
and the expanded particles crowd one another with great force. Since
rock is a poor conductor of heat its surface is heated and expanded
notably more than parts beneath the surface. It follows that strains
are set up between the expanded outer portion and the cooler and less
expanded parts within. In the cooling of the same rock mass it is the
outermost portion which cools first and fastest, and, contracting as
it cools, strains are again set up between the outer part, which is
cooled more, and the inner part, which is cooled less. The result may
be illustrated by the effect of cold water on hot glass, or of hot
water on cold glass. In either case the fracture is the result of the
sudden and considerable differential expansion or contraction. Since
the heating and cooling of rock are much slower than the heating and
cooling of glass under the conditions mentioned, the rupturing effects
are less conspicuous, but none the less real. The actual effects
of temperature changes are illustrated by familiar phenomena. The
surface portions of bowlders exposed to the sun are frequently seen
to be shelling off (Fig. 26). The loosened concentric shells may be a
fraction of an inch, or sometimes even several inches in thickness.
This process of _exfoliation_ affects not only bowlders, but bare
rock surfaces wherever exposed to the sun (Figs. 27, 28). It is often
conspicuous on the faces of cliffs.
[Illustration: +Fig.+ 26.—Exfoliation. A bowlder of weathering, the
rock being granite. Wichita Mountains, Oklahoma.]
[Illustration: +Fig.+ 27.—A weathered summit of granite in the Wichita
Mountains. Oklahoma. (Willis, U. S. Geol. Surv.)]
Several conditions, some of which are connected with the atmosphere and
some with the rock, determine the efficiency of this process. Since
the breaking of the rock results from the expansion and contraction
due to its changes of temperature, it follows that, other things being
equal, the greater the change, the greater the breaking; but the
suddenness of the temperature change is even more important than its
amount. It follows that great daily, rather than great annual, changes
of temperature[20] favor rock-breaking, though with changes of a given
frequency their effectiveness is greater the greater their range. A
partial exception to this generalization should be noted. If abundant
moisture is present in the pores and cracks of the rock a change of
temperature from 45° to 35° (Fahr.) might be far less effective in
breaking the rock than a change from 35° to 25° in the same time, for
in the latter case the sudden and very considerable expansion (about
one-tenth) which water undergoes on freezing is brought into play. This
may be called the _wedge-work of ice_. The daily range of temperature
is influenced especially by latitude, altitude, and humidity. Other
things being equal, the greatest daily ranges of temperature occur
in high-temperate latitudes, though to this general statement there
are local exceptions, depending on other conditions. High altitudes
favor great daily ranges of temperature, so far as the rock surface
is concerned (see Figs. 29, 30), for though the rock becomes heated
during the sunny day, the thinness and dryness of the atmosphere allow
its heat to radiate rapidly at night. Here, too, the daily range of
temperature is likely to bring the wedge-work of ice into play. Since
the south side of a mountain (in the northern hemisphere) is heated
more than the north, it is subject to the greater daily range of
temperature, and the rock on this side suffers the greater disruption.
Similarly, rock surfaces on which the sun shines daily are subject to
greater disruption than those much shielded by clouds. Isolated peaks,
because of their greater exposure, are subject to rather greater daily
ranges of temperature than plateaus of the same elevation.
[Illustration: +Fig.+ 28.—Exfoliation on a mountain slope. Mt.
Starr-King (Cal.) from the north.]
The daily range of temperature is also influenced by humidity. Because
of the effect of water vapor in the atmosphere on insolation and
radiation, a rock surface becomes hotter in the day and cooler at night
beneath a dry atmosphere than beneath a moist one. Aridity therefore
favors the disruption of rock by changing temperatures.
Turning from the conditions of the atmosphere which affect the
disruption of rock to the conditions of the rock which influence the
same process, several points are to be noted. In the first place, the
disrupting effects of changes of temperature are slight or nil where
the solid rock is protected by soil, clay, sand, gravel, snow, or other
incoherent material. If the constituent parts of the loose material are
coarse, like bowlders, their surfaces are affected like those of larger
bodies of rock. The color of rock, its texture and its composition,
also influence its range of daily temperature by influencing absorption
and conduction. Dark-colored rocks absorb more heat than light-colored
ones, and compact rocks are better conductors than porous ones. Great
absorption and slow conduction favor disruption. A given range of
temperature is unequally effective on rocks of different mineral
composition. In general crystalline rocks (igneous and metamorphic) are
more subject to disruption by this means than sedimentary rocks, partly
because they are more compact, but especially because they are made up
of aggregates of crystals of different minerals which, under changes of
temperature, expand and contract at different rates, while the common
sedimentary rocks are made up largely of numerous particles of one
mineral.
[Illustration: +Fig.+ 29.—Top of Notch Peak, Bighorn Mountains, Wyo.
Shows the thoroughly broken character of the rock on the summit, the
absence of soil, vegetation, etc. (Kümmel.)]
[Illustration: +Fig.+ 30.—A detail from Fig. 29 showing the size of the
rock blocks. (Kümmel.)]
[Illustration: +Fig.+ 31.—Peak north of Kearsarge Pass, the Sierras.
Shows the way in which serrate peaks break up into angular blocks.]
The freezing of water in the pores of rock is effective in disrupting
them only when the pores are essentially full at the time of freezing.
Otherwise there is room for the expansion attending the freezing. If
the pores of the rock are large, the expansion on freezing may force
out sufficient water to balance the increase of volume, even though the
rock was completely saturated. If the pores be very small the water
passes out less readily, and if the rock is saturated, freezing is more
likely to be attended with disruption.[21]
In view of these considerations the breaking of rock by changes
of temperature should be greatest on the bare slopes of isolated
elevations of crystalline rock, where the temperature conditions of
temperate latitudes prevail, and where the atmosphere is relatively
free from moisture. All these conditions are not often found in one
place, but the disrupting effects of changing temperatures are best
seen where several of them are associated (Figs. 29, 30, and 31).
The importance of this method of rock-breaking has rarely been
appreciated except by those who have worked in high and dry regions.
Climbers of high mountains know that almost every high peak is covered
with broken rock to such an extent as to make its ascent dangerous to
the uninitiated. High serrate peaks, especially of crystalline rock,
are, as a rule, literally crumbling to pieces (Fig. 31). The piles
of talus which lie at the bases of steep mountain slopes are often
hundreds of feet in height, and their materials are often in large part
the result of the process here under discussion. In mountain regions
where atmospheric conditions favor sudden changes of temperature, the
sharp reports of the disruption of rock masses are often heard. Masses
of rock, scores and even hundreds of pounds in weight, are frequently
thus detached and started on their downward course.[22] Small pieces
of rock are of course much more commonly broken off than large ones.
The disruption of rock by changes of temperature is not usually the
result of a single change of temperature, but rather of many successive
expansions and contractions.
The sharp needle-like peaks which mark the summits of most high
mountain ranges (Fig. 32) are largely developed by the process here
outlined. The altitude at which the serrate topography appears varies
with the latitude, being, as a rule, higher in low latitudes and lower
in high. But even in the same latitude it varies notably with the
isolation of the mountains and with the aridity of the climate. Thus
within the United States the sharply serrate summits appear in some
places, as in Washington and Oregon, at altitudes of 6000 to 10,000
feet, while in the isolated Wichita range of Oklahoma, much farther
south, but in a much drier climate, the same sort of topography is
developed at altitudes of 2500 to 3000 feet.
Even in low latitudes and moist climates the effects of temperature
changes are often seen. Thin beds of limestone at the bottom of
quarries are sometimes so expanded by the heat of the sun as to arch up
and break.[23] In desert and arid regions,[24] whatever the altitude,
the effects of temperature changes are often striking.
[Illustration: +Fig.+ 32.—Serrate peaks of granitic rock in Black
Hills. (Darton, U. S. Geol. Surv.)]
The disruption of rock by changes of temperature is one phase of
weathering. It tends to the formation of a mantle of rock waste, which,
were it not removed, would soon completely cover the solid rock beneath
and protect it from further disruption by heating and cooling; but the
loose material thus produced becomes an easy prey to running water,
so that the work of the atmosphere prepares the way for that of other
eroding agencies.
_II. Evaporation and Precipitation._
Perhaps the most important work of the atmosphere as a dynamic
agent lies in its function as the medium for the circulation and
distribution of water. Atmospheric temperature is the primary
factor governing evaporation, an important factor in the circulation
of the vapor after it is formed, and controls its condensation and
precipitation.
The average amount of annual precipitation on the land is variously
estimated at from forty to sixty inches, the lesser figure being
probably more nearly correct. Since much of this water falls at high
altitudes, the work which it accomplishes in getting back to the sea
is great. The water which falls on the land, if withdrawn wholly from
the ocean, would exhaust that body of water in 10,000 to 15,000 years
if none of it returned. The work of evaporation is of course not done
by the atmosphere, though the atmosphere determines the effect of the
solar energy which does the work.[25]
The precipitation is distributed with great inequality, and this
inequality affects both the rain and the snow. Some regions have heavy
precipitation and some light; some regions have much rain and little
snow; others have much snow and little rain; others have rain and no
snow, and still others have snow and little or no rain. The amount and
distribution of rain and snow determine the size and distribution of
streams and glaciers, and streams and glaciers are the most important
agencies modifying the surface of the land.
It is impossible to separate sharply the geologic work of the water of
the atmosphere from that of other waters; but so long as moisture is in
the atmosphere (including the time of its precipitation) its effects
are best considered in connection with the atmosphere.
=The mechanical work of the rain.=—In falling the rain washes the
atmosphere, taking from it much of the dust, spores, etc., which the
winds have lifted from the surface of the dry land. Not only this,
but in passing through the atmosphere the water dissolves some of its
gases, and perhaps particles of soluble solid matter. When therefore
the falling water reaches the surface of the land it is no longer pure,
and some of the gases it has taken up in its descent enable it to
dissolve various mineral matters on which pure water has little effect.
As it falls on the surface of the land the rain produces various
effects of a mechanical nature. In the first place, it leaves on the
surface the solid matter taken from the air. The amount of material,
thus added to any given region in any particular shower is trivial,
but in the course of long periods of time the total amount of material
washed out of the air must be very great.
Every rain-drop strikes a blow. If the drops fall on vegetation,
they have little effect, but if they fall on sand or unprotected
earthy matter they cause movements of the particles on one another,
and this movement involves friction and wear. While the results thus
effected are inconsiderable in any brief period of time, they are not
so insignificant when the long periods of the earth’s history are
considered.
Clayey soils contract and often crack on drying. Falling on such a soil
when it is dry the rain causes it to expand, and the cracks are healed
by lateral swelling. The same soils are baked under the influence of
the sun, and when in this condition are softened and made more mobile
by the falling of rain. Under the influence of the expansion and
contraction occasioned by wetting and drying, the soils and earths on
slopes creep slowly downward. When rain falls on dry sand or dust the
cohesion is at once increased, and shifting by the wind is temporarily
stopped.
After the water has fallen on the land its further work cannot be
looked upon as a part of the work of the atmosphere; but any conception
of the geological work of the atmosphere which did not recognize the
fact that the waters of the land have come through the atmosphere would
be inadequate. The work of the water after it has been precipitated
from the atmosphere must be considered in another chapter.
_III. Effects of Electricity._
Another dynamic effect conditioned by the atmosphere is that produced
by lightning. In the aggregate this result is inconsequential; yet
instances are known where large bodies of rock have been fractured by
a stroke of lightning, and masses many tons in weight have sometimes
been moved appreciable distances. Incipient fusion in very limited
spots is also known to have been induced by lightning. Where it strikes
sand it often fuses the sand for a short distance, and, on cooling,
the partially fused material is consolidated, forming a little tube or
irregular rod (a _fulgurite_) of partially glassy matter. Fulgurites
are usually only a few inches in length, and more commonly than
otherwise a fraction of an inch in diameter. Strictly speaking these
results are the effect of the electricity of the atmosphere rather
than of the atmosphere itself, but they are best mentioned in this
connection.
Allusion has already been made to the chemical changes in the
atmosphere occasioned by electric discharges.
[Illustration: +Fig.+ 33.—Stratified jointed rock in process of
weathering. (Cross, U. S. Geol. Surv.)]
[Illustration: +Fig.+ 34.—Represents a later stage of the processes
illustrated by Fig. 33. (Darton, U. S. Geol. Surv.)]
SUMMARY.
=Weathering.=—The result of all atmospheric processes, whether physical
or chemical, by which surface rock is disrupted, decomposed or in any
way loosened, is _weathering_. This convenient term also includes
similar results effected by ground water, plants, etc. The tendency
of weathering is to produce a mantle of residuary earth over solid
rock. Weathering by mechanical means tends to produce material which,
though in a finer state of division, is still like the original rock
in chemical composition. Weathering by chemical means tends to produce
a mantle made up chiefly of the less soluble parts of the rock from
which it was derived. All processes of weathering prepare material for
transportation by wind and water.
[Illustration: +Fig 35.+—Details of a weathered rock surface, due
partly to wind work and partly to solution. The particular phase
of weathering illustrated by this figure is known as “honeycomb”
weathering. (Gilbert, U. S. Geol. Surv.)]
Many considerations determine the thickness which the mantle of
weathered rock (mantle rock) attains. Some of these considerations have
to do with the atmosphere, and some with drainage. Since the latter
are, on the whole, more important, this matter will be discussed in
connection with the work of water (Chapters III and IV).
CHAPTER III.
THE WORK OF RUNNING WATER.
Familiar phenomena, both of land and sea, reveal the constant activity
and importance of water as a geologic agent. Even when there is no
precipitation the moisture in the air influences its activity in
certain ways. Just as iron “rusts” more readily in moist air than
in dry, so changes in other mineral substances are influenced by
atmospheric humidity. Where precipitation takes place the results are
more obvious. The passing shower works changes in the surface of the
land, striking in proportion to the rate and amount of precipitation.
The rains feed the streams, and every stream is modifying its bed, and
with increasing rapidity as its current is swollen. Even the moisture
which is precipitated as snow works its appropriate results. Before it
melts it protects the surface against other agents of change; but if
it accumulates in sufficient quantity in appropriate situations, it
may give rise to avalanches and glaciers, which, like running water,
degrade the surface over which they pass.
A part of the water which falls as rain, and a part of that which
results from the melting of snow and ice, sinks into the soil and into
the rock below, becoming _ground water_. It is this ground water which
especially justifies the name _hydrosphere_, often applied to the
waters of the earth, for it literally forms a spherical layer in the
outer portion of the solid part of the earth. During the stay of the
water beneath the surface it effects changes in the rocks through which
it passes, dissolving mineral matter here and depositing it there,
substituting one substance for another in this place, and effecting new
chemical combinations in that. Slow as these processes are, they have
worked wondrous changes in the course of the earth’s history.
When the waters are gathered together in ponds, lakes, and oceans, they
are still active, and the results of their activity are seen along the
shores, where winds and waves produce their chiefest effects. Even the
ocean currents, far from land, and the processes of the deep sea, are
not without their effect on the course of geological history.
The work of the _surface waters_, _ground_ (_underground_) _waters_,
_standing waters_, and _ice_ will be considered in order.
RAIN AND RIVER EROSION.
Rain and river erosion began when the first rains fell on land
surfaces. Neither the location nor the nature of the first land surface
is known. There is little reason to believe that the ocean was ever
universal, but there is reason to believe that most land areas have at
some time or other been covered by the sea. The prevalent conception
that land areas which were once submerged came into existence by being
elevated above sea-level, should be supplemented by the alternative
conception that submerged areas may have become land by the depression
of the ocean basins, thus drawing off the water from the areas where it
was shallow. Thus in Fig. 36 the sinking of the sea-bottom from _a_ to
_b_ would lower the surface of the water from _cc′_, to _dd′_, and draw
off the water from the surfaces _cd_ and _c′d′_.
[Illustration: +Fig.+ 36.—Diagram to illustrate the origin of lands by
the lowering of the sea-level due to depression of the sea bottom. If
the bottom is depressed from _a_ to _b_ the surface will be drawn
down from _cc′_ to _dd′_, and the surfaces _cd_ and _c′d′_ will become
land.]
Without attempting to picture the character of the original land our
study of subaërial erosion may begin with an area which has just been
changed from sea bottom to land. What is the nature of such a land
surface? Of what material is it composed, and what is the character of
its topography? Concerning its constitution something may be inferred
from the nature of the deposits now found at the bottom of the sea.
Near the shore and in shallow water they often consist of gravel and
sand, though other materials are not wanting. Far from shore and in
deep water they consist for the most part of fine sediments, some of
which were washed or blown from the land, some of which came from the
shells and other secretions of marine animals, some from volcanoes, and
some from various other sources. The topography of the newly emerged
land may have had some likeness to the topography of the sea bottom.
The numerous soundings which have been made over large areas of the
sea have shown that its bottom is, as a rule, free from the numerous
small irregularities which affect the surface of the land. They seem
to show that a large part of the ocean bottom is so nearly flat that,
if the water were removed, the eye would hardly detect irregularities
in the surface. This statement does not lose sight of the fact that
the ocean bottom is, in certain places, markedly irregular. Volcanic
peaks and striking irregularities of other sorts abound in some places.
Nevertheless if the bottom of the sea could be seen as the land is, its
most striking feature, taken as a whole, would be its apparent flatness.
With the topography of the sea bottom the topography of the land is,
in its details, in sharp contrast. In order to get at the history of
the latter, we may study the sequence of events which would follow the
emergence of a portion of the former.
_Subaërial Erosion without Valleys._
For the sake of emphasizing the fundamental principles involved in
the work of running water, a hypothetical case will first be studied
in some detail, even at the risk of elaborating processes already
understood. The principles themselves will find application later in
relations which are much less simple.
Let it be assumed that the area of newly emerged land is a circular
dome-shaped island. The simplest possible condition is represented by
assuming its slope to be the same in all directions from the center,
and its materials to be absolutely homogeneous. Such an island would
be subject to all the forces ordinarily operating on land surfaces.
The chief agency tending to modify land surfaces is atmospheric
precipitation. It will be assumed that the rain falls on the surface
of the island with absolute equality at all points, and that all other
forces which affect it operate equally everywhere.
The rain falling on a land area disappears in various ways; part of it
evaporates, part of it sinks, and part of it runs off over the surface.
If the island be composed of fine and unconsolidated materials, such
as clay, the water which runs off over the surface will carry sediment
down to the sea. If the island be composed of solid rock instead,
exposure to the air will cause it to decay, and the products of decay,
such as sand and mud, will suffer a like fate.
For the sake of a clear understanding of the processes involved, two
cases may be postulated; one in which the waters of the sea remove the
sediment washed down from the hypothetical island as fast as it reaches
the shore, and one in which they allow it to accumulate without let or
hindrance. In both cases the wear of the waves will be neglected.
1. In the first case the water flowing off over the surface (the
_run-off_) will descend equally in all directions. It will constitute a
continuous sheet of surface-water, and both its volume and its velocity
will be the same at all points equally distant from the summit. Erosion
accomplished by sheets of running water, as distinct from streams, is
_sheet_ (or _sheet-flood_) _erosion_.[26] Since the material of the
surface is homogeneous, the wear effected by the water will be equal
at all points where its velocity and volume are equal. For obvious
reasons the depth of the run-off will increase from summit to base.
The gradient (slope) also increases in the same direction, and the
increase of volume and of gradient conspire to augment the velocity
of the water, and therefore of the wear effected by it. If the thin
sheet of water starting from the top of the island with relatively
low velocity be able to wash off even a little fine material from the
surface, the thicker sheet farther down the slope, moving with greater
velocity, will be able to carry away more of the same sort of material,
and the increase will be progressive from summit to base. It follows,
therefore, that the surface will be worn equally at points equally
distant from the summit, but unequally at points unequally distant
from it. The first shower which falls on the island may be conceived
to wash off from its surface a very thin sheet of material, but a
sheet which increases in thickness from top to bottom. The run-off
will not be stopped immediately on reaching the sea, but will displace
the sea-water to some slight depth, and wear the surface some trivial
distance below the normal level of the sea. The result of successive
showers working in the same way through a long period of time will
be to diminish the area of the island and to steepen its slopes. The
results of a considerable period of erosion under these conditions
are shown diagrammatically in Fig. 37, which illustrates both the
diminution in area which the island has suffered, and the increase in
the angle of its slopes. Immediately about it, at the stage represented
by _aa_, Fig. 37, there is a narrow marginal platform, or submerged
terrace, in place of the land area which has been worn away at or just
below the level of the sea. Long successions of rains working in the
same way will give the island steeper slopes, a smaller area, and a
wider marginal terrace. Successive stages are shown by the lines _bb_
and _cc_, Fig. 37.
[Illustration: +Fig.+ 37.—Diagram to illustrate the effect of rain
erosion on an island where there is no deposition or wave erosion
about its borders. The uppermost curve represents the original
surface, while _aa_, _bb_, and _cc_ represent successive surfaces
developed by sheet erosion, on the supposition that no material is
deposited along the shores.]
If rain falls on such an island until it completes the work which
it is possible for running water to do, the island will be reduced
essentially to the level of the sea, and in its place there will be a
plain, the area of which will be equal to that of the original island.
Its central point will be at the level of the sea, and its borders a
trifling distance below it (Fig. 38). The island is gone, and in its
place there is a plain as low as running water can wear it. Other
agencies might come in to defeat the result just outlined, but if the
island did not rise or sink after its formation, rain falling upon
it would, _under the conditions specified_, finally bring about the
result which has been sketched. The plain (Fig. 38) which succeeds the
island is a _base-level of erosion_, though this term is also used in
other ways. Under these conditions the slope of the land would remain
convex at all stages, but the convex erosion profile of the land
would meet a nearly straight line just below sea-level. The relative
lengths of these two elements of the profile, the curve above and the
straight line below, vary as erosion progresses, the convex portion
becoming shorter and the other longer, The two parts of the profile
taken together are concave upward at the lower end all the time, and
for a greater distance from its lower end in all the advanced stages of
erosion (Fig. 37).
[Illustration: +Fig.+ 38.—Diagram to illustrate the final effect of
rain erosion under the conditions specified in the text. The diagram
expresses the final result of the processes suggested by Fig. 37.]
In the destruction of the land under these conditions _neither valleys
nor hills would be developed_, nor would the topography of the land be
fashioned to correspond with the surfaces with which we are familiar.
It is to be distinctly borne in mind that the foregoing is a
hypothetical case; it is not probable that such an island ever existed,
or ever will; but that does not diminish the value of the illustration,
since the principles involved are operating on every land mass, though
in less simple relations.
2. The second case differs from the first in that the sediment washed
down from the land is deposited about its borders. This results in
the building up of a marginal platform, as shown in Figs. 39–41. As
erosion goes on more sediment is washed down and deposited, partly on
the narrow marginal shelf which has already been developed, and partly
on its outer slope, as shown in the figures. The marginal flat is thus
extended beyond the original shores of the island on the one hand, and
toward its center on the other. As it develops, its inner portion,
and indeed all except its outer edge (_ab_, Figs. 40 and 41), will be
gradually built up above the level of the water. This marginal lowland
is developed at a level as low as running water, _under the conditions
then and there present_, can reduce the land. Such a surface may be
said to be at _grade_, since running water neither wears it down nor
builds it up. Its angle of slope is a function of (1) the volume of the
water running over it, and (2) of the load which the water carries.
[Illustration: +Figs.+ 39–41.—Diagrams to illustrate the effect of
rain erosion on an island when all the eroded material is deposited
about the shore. The black portions represent deposition. The dotted
lines represent the original surface. The several diagrams represent
successive stages in the process.]
Since the marginal plain of the above illustration extends beyond the
original shore of the island, the area of land is increased, though
both its average elevation and its mass (above water) are reduced. In
case destructive processes did not operate on the marginal graded
plain the spreading and lowering suggested by Figs. 39 and 40 would go
on until the central mass of the island was brought down to a gradient
in harmony with that of the gently sloping border, as shown in Fig.
41. When this had been accomplished there would be a relatively large
land area with low slopes (Fig. 41) in place of the smaller area with
steeper ones (compare Figs. 39 and 40). The basal part of the larger
island from the center to the original margin would be made up of the
original material in its original position (unshaded part of Fig. 41).
Its surface would be covered, least deeply near its center and most
deeply near the original margin, with débris gradually shifted from
higher levels, as shown in Fig. 41.
Were such an island as that shown in Fig. 41 once formed, the rain
falling on it, and flowing off over its surface, would carry off its
surface soil and spread it about the shores. Though the surface of the
marginal flat of Fig. 40 was as low as running water could bring it at
the time it was developed, the conditions of erosion have changed by
the time the land reaches the conditions shown in Fig. 41, and the same
amount of rainfall may now be effective in erosion. In the first case
(Fig. 40) the water descending from the higher part of the land brought
down sediment and started across the flat with a load. Its energy was
consumed in transporting what it had, not in getting new material. In
the second case (Fig. 41) the water flowing over the gently sloping
surface has no initial load, and its energy is therefore available for
erosion. Under continued rainfall, the area of the land shown in Fig.
41 would be increased as before by successive marginal deposits (see
Fig. 42), and at the same time its average height would be reduced. The
lowering and enlarging of the island would continue until the whole
surface was brought so nearly to the level of the sea that water would
cease to run over it with sufficient velocity to carry away even the
fine material of its surface. Such a surface, brought down as low as
running water can degrade it, is also (see p. 57) a _base-level_. It
will be seen from the foregoing illustrations that a _graded surface_
may pass into a _base-level_, with no sharper line of demarkation than
that which separates a mature man from an old one. In this case, as in
the preceding, the island has been base-leveled, but still without the
formation of valleys or hills.
[Illustration: +Fig.+ 42.—Diagram to illustrate the result of the
continuation of the processes shown in Figs. 39–41.]
Both the preceding hypothetical cases make it clear that, from the
point of view of erosion, every drop of water which runs off over
the surface of the land has for its mission the getting of the land
into the sea. Under ordinary conditions surface drainage must fail to
bring a land area altogether to sea-level, the absolute base-level of
subaërial forces; but it is not simply the water which runs off over
the surface which degrades the land. That which sinks beneath the
surface contributes to the same end by slowly dissolving mineral matter
below the surface, and finally carrying it to the sea. In this way the
reduction of land areas to sea-level may be completed.
The rain-water which evaporates from the surface without sinking
beneath it does not effect much wear; but the water thus evaporated is
subject to reprecipitation, so that, in the long run, it may assist
in the work which has been sketched. Thus it is not simply the waters
which run off over the surface of the land, but all which fall upon it,
which unite to compass its destruction.