The idea of the progressive development of the earth in its greater
features throughout all geologic time by the action of forces resident
in the earth itself preceded the acceptance of the evolution of organic
forms. We have said that the fundamental idea of geology is that of the
evolution of the earth through all time. Now, it was Dana who first
studied geology wholly from this point of view. For him geology was
the development of the earth as a unit. Before him, doubtless, geology
was a kind of history--i. e., a chronicle of thrilling events--but
Dana first made it a philosophic history. Before Dana, geology was an
account of the succession of formations and their fossil contents. Dana
made it an account of the evolution of earth forms and the concomitant
and resulting evolution of organic forms. It is true that first and for
a long time his evolutional conception was incomplete. It is true that
while he attributed the evolution of earth forms to natural causes and
processes, he still shrank from applying similar causes to the changes
in life forms, but this was the almost necessary result of the then
universal belief in the supernatural origin and the unchangeableness
of organic forms. He lived to make his conception of evolution as a
natural process, both of the earth and of organic forms, complete.
_Ocean Basins and Continents._--If we divide geological causes and
processes into two general kinds as to their origin--viz., internal,
or earth-derived, and external, or sun-derived--evidently the former
is the original and fundamental kind. These determine earth forms,
while the other only modify them; these determine the great features,
the other only the lesser features; the former rough-hews the earth
features, the latter shapes them. It is the effects of these interior
earth forces which are the most important to study. And among these
effects the most fundamentally important of all is the formation of
those greatest features--the ocean basins and continental arches.
The most probable view is that they are formed by unequal radial
contraction in the secular cooling of the earth. The earth was
certainly at one time an incandescently hot mass, which gradually
cooled and contracted to its present temperature and size. Now, if it
were perfectly homogeneous both in density and in conductivity in all
parts, then, cooling and contracting equally in every part, it would
retain its symmetric oblate-spheroid form, though diminishing in size.
But if there were any, the least, heterogeneity either in density or
especially in conductivity over large areas, then the more conductive
areas, contracting more rapidly toward the center radially, would form
hollows or basins, and the less conductive areas would stand out as
higher arches. Thus were formed the oceanic basin and the continental
arches of the lithosphere. The same causes which produced would
continue to increase them, and thus the ocean basins would increase in
depth and the continents in height.
The hydrosphere is still to be added. In the beginning of this process
doubtless the lithosphere was hot enough to maintain all the water
in the form of vapor in the atmosphere. But when the surface was
cool enough the water would precipitate and partly or wholly cover
the earth--whether partly or wholly would depend on the amount of
precipitated water and the amount of inequality which had already
taken place. The amount of water, as we know, is sufficient, if the
inequalities were removed, to cover the whole surface two and a half
miles deep. Inasmuch as the forming of the inequalities is progressive
and still going on, it seems improbable that the inequalities had
become sufficiently great, at the time of precipitation, to hold the
waters. If this be so, then the primeval ocean was universal and the
future continents existed only as continental banks in the universal
ocean.
However this may have been, there seems little doubt that the same
cause which produced the inequalities continued to operate to increase
them. The ocean basins, so far as these causes are concerned, must
have become deeper and deeper, and the continents larger and larger.
In spite of many oscillations producing changes mostly on the margins,
but sometimes extending over wide areas in the interior of the
continent, this, on the whole, seems to be in accordance with the
known geological history of the earth. If so, then _the oceanic basins
have always been oceanic basins, and the places of the continents have
always been substantially the same_. This introduces a subject on which
there has been much discussion recently--viz.:
_The Permanency of Ocean Basins._--Closely associated with the
Lyellian uniformitarianism was the doctrine of extreme instability
of earth features, especially the forms and places of sea and land.
Crust movements were irregularly oscillating to such a degree that
in the course of geologic history sea and land frequently and
completely changed places. Abundant evidence of this was supposed
to be found in the unconformities so frequent in the stratified
series. The tendency of that time was toward a belief in up-and-down
movements, back-and-forth changes, without discoverable law, rather
than progressive onward movement. On first thought it might seem
that such lawless movement was rather in keeping with catastrophism
than uniformitarianism. But not so, for the movements are supposed
to be very slow. Again, it might seem on first thought that gradual
progressive change--in a word, evolution--would be peculiarly in accord
with uniformitarian ideas. But again not so, because this doctrine
was, above all, a revulsion from the idea of supernatural purpose or
design or goal contained in catastrophism. Uniformitarianism strongly
inclined toward purposelessness, because of its supposed identity with
naturalism. Thus for a long time, and still with many geologists,
the tendency is toward a belief in irregular movements without
discoverable law, toward instability of even the greatest features of
the earth--viz., sea basins and continental arches. Geology for them is
a chronicle, not a life history.
The contrary movement of thought may be said to have commenced with
Dana. Dana studied the earth as a unit, as in some sense an organism
developing by forces within itself. The history of the earth is a
life history moving progressively toward its completion. The forces
originating oceanic basins and continental arches still continue
to deepen the former and enlarge the latter. From this point of
view, oceanic basins and continental arches must have always been
substantially in the same places. Oscillations there have been at
all times and in all places, but they affect mainly the outlines of
these great features, though sometimes affecting also the interior of
continents and mid-sea bottoms, but not sufficiently to change greatly
their general form, much less to interchange their places.
Such is the doctrine of permanency of oceanic basins. It is undoubtedly
a true doctrine, but must not be held in the rigid form characteristic
of early thought. The forces originating oceanic basins still continue
to deepen them and to increase the size and height of continents, but
other forces are at work, some antagonizing (i. e., cutting down the
continents and filling up the ocean beds), and still others determined
by causes we little understand, by oscillations over wide areas,
greatly modifying and often obscuring the effects of the basin-making
movements. Here, then, we have two kinds of crust movements: the one
fundamental and original, determining the greatest features of the
earth and moving steadily onward in the same direction, ever increasing
the features which it originates; the other apparently lawless,
uncertain, oscillating over very wide areas, modifying and often
obscuring the effects of the former. The old uniformitarians saw only
the effects of the latter, because these are most conspicuous; the
new evolutionists add also the former and show its more fundamental
character, and thus introduce law and order into the previous chaos.
The former is the one movement which runs ever _in the same direction
through all geologic time_. The latter are the most common and
conspicuous now and in all previous geologic time. The former underlies
and conditions and unifies the history; the latter has practically
determined all the details of the drama enacted here on the surface
of the earth. Of the causes of the former we know something, though
yet imperfectly. Of the causes of the latter we yet know absolutely
nothing. We have not even begun to speculate profitably on the subject,
and hence the apparent lawlessness of the phenomena. A fruitful theory
of these must be left to the coming century.
_Mountain Ranges._--If oceanic basins and continental domes constitute
the greatest features of the earth’s face, and are determined by the
most fundamental movements of the crust, surely next in importance
come great mountain ranges. These are the glory of our earth, the
culminating points of scenic beauty and grandeur. But they are so only
because they are also the culminating points, the theaters of greatest
activity, of all geological forces, both igneous and aqueous--igneous
in their formation, and aqueous both in the preparatory sedimentation
and in the final erosive sculpturing into forms of beauty. A theory
of mountain ranges therefore lies at the bases of all theoretical
geology. To the pre-geologic mind mountains are the type of permanence
and stability. We still speak metaphorically of the _everlasting_
hills. But the first lesson taught by geology is that nothing is
permanent; everything is subject to continuous change by a process of
evolution. Mountains are no exception. We know them in embryo in the
womb of the ocean. We know the date of their birth; we trace their
growth, their maturity, their decay, their death; we even find in the
folded structure of the rock, as it were, the fossil bones of extinct
mountains. In a word, we are able now to trace the whole life history
of mountains.
Mountains, therefore, have always been a subject of deepest interest
both to the popular and the scientific mind--an interest intensified
by the splendors of mountain scenery and the perils of mountain
exploration. The study of mountains is therefore coeval with the study
of geology. As early as the beginning of the present century Constant
Prevost observed that most characteristic structure of mountains--viz.,
their folded strata--and inferred their formation by lateral pressure.
All subsequent writers have assumed lateral pressure as somehow
concerned in the formation of mountains. But that the whole height
of mountains is due wholly to this cause was not generally admitted
or even imagined until recently. It was universally supposed that
mountains were lifted by volcanic forces from beneath, that the lifted
strata broke along the top of the arch, and melted matter was forced
through between the parted strata, pushing them back and folding them
on each side. And hence the typical form of mountain ranges is that
of a granite axis along the crest and folded strata on each flank.
But attention has lately been drawn to the fact that some mountains,
as, for example, the Appalachian, the Uintah, etc., consist of folded
strata alone, without any granite axis. In such ranges it is plain that
the whole height is due not to any force acting from below, but to a
lateral pressure crushing and folding the strata, and a corresponding
thickening and bulging of the same along the line of crushing. Then the
idea was applied to _all_ mountain ranges. So soon as the prodigious
amount of erosion suffered by mountains, greater often than all that is
left of them, was fully appreciated, it became evident that the granite
axis so characteristic of mountains was not necessarily pushed up from
beneath and protruded through the parted strata, but was in many cases
only a sub-mountain core of igneous matter slowly cooled into granite
and exposed by subsequent erosion greatest along the crest.
Next, attention was drawn to the enormous thickness of the strata
involved in the folded structure of mountains. From this it became
evident that the places of mountains before they were formed were
marginal sea bottoms off the coasts of continents, and receiving the
whole washings of the continents. Thus the steps of the process of
mountain formation were (1) accumulation of sediments on offshore sea
bottoms until by _pari passu_ subsidence an enormous thickness was
attained. This is the _preparation_. (2) A yielding along these lines
to the increasing lateral pressure with folding and bulging of the
strata along the line of yielding, until the mountain emerges above
the ocean and is added to the land as a coast range. This is mountain
_birth_. (3) As soon as it appears above the water it is attacked by
erosive agents. At first the rising by continuance of the crushing and
bulging is in excess of the erosion, and the mountain grows. This is
mountain _youth_. (4) Then supply and waste balance one another, and we
have mountain _maturity_. (5) Then the erosive waste exceeds the growth
by up-bulging, and mountain _decay_ begins. (6) Finally, the erosive
forces triumph and the mountain is clean swept away, leaving only the
complexly folded rocks of enormous thickness to mark the place of a
former mountain. This is mountain _death_. Such briefly is the life
history of a mountain range.
In all this we have said nothing about causes. In this connection there
are two points of especial importance: (1) Why does the yielding to
lateral pressure take place along lines of thick sediments? (2) What is
the cause of the lateral pressure?
1. _Cause of Yielding to Lateral Pressure along Lines of Thick
Sediments._--The earth was once very hot. It is still very hot within,
and still very slowly cooling. If sediments accumulate upon a sea
bottom the interior heat will tend to rise so as to keep at the same
distance from the surface. If the sediments are very thick, say five to
ten miles, their lower parts will be invaded by a temperature of not
less than 500° to 1,000° F. This temperature, in the presence of water
(the included water of the sediments), would be sufficient to produce
softening or even fusion of the sediments and of the sea floor on which
they rest. This would establish a line of weakness, and therefore a
line of yielding, crushing, folding, bulging, and thus a mountain
range. In the first formation of a range, therefore, there would
necessarily be a sub-mountain mass of fused or semifused matter which
by the lateral crushing might be squeezed into cracks or fissures,
forming dikes. But in any case the sub-mountain mass would cool into
a granite core which by erosion may be exposed along the crest. The
explanation seems to be satisfactory.
2. _Cause of the Lateral Pressure._--No question in geology has been
more discussed than this, and yet none is more difficult and the
solution of which is more uncertain. But the most obvious and as
yet the most probable view is that it is the result of the secular
contraction of the earth which has gone on throughout its whole
history, and is still going on.
It is admitted by all that in an earth cooling from primal
incandescence there must come a time when the surface, having become
substantially cool and receiving heat also from the sun, would no
longer cool or contract, but, the interior being still incandescently
hot, would continue to cool and contract. The interior, therefore,
cooling and contracting faster than the exterior crust, the latter
following down the ever-shrinking nucleus, would be thrust upon itself
by a lateral or tangential pressure which would be simply irresistible.
If the earth crust were a hundred times more rigid than it is, it still
must yield to the enormous pressure. It does yield along its weakest
lines with crushing, folding, bulging, and the formation of mountain
ranges.
This is the barest outline of the so-called “contractional theory of
mountain formation.” Very many objections have been brought against
it, some of them answerable and completely answered, but the complete
answer to others must be left to the next century. Perhaps the greatest
objection of all is the apparent insufficiency of the cause to produce
the enormous amount of folding found not only in existing mountains but
in the folded structure of rocks where mountains no longer exist. But
it will be observed that I have thus far spoken only of contraction by
loss of heat. Now, not only has this cause been greatly underestimated
by objectors, but, as shown by Davison and especially by Van Hise,
there are many other and even greater causes of contraction. It would
be out of place to follow the discussion here. The subject is very
complex, and not yet completely settled.
We have given the barest outline of the history of mountain ranges and
of the theory of their formation as worked out in the last third of the
present century, and, I might add, chiefly by American geologists. So
true is this, that by some it has been called the “American theory.”
_Oscillatory Movements of the Earth’s Crust over Wide Areas._--We
have already spoken of these as modifying the effect of the
ocean-basin-making movements, and therefore now touch them very
lightly. These differ from the movements producing oceanic basins
on the one hand and mountain ranges on the other, by the fact
that they are not continuously progressive in one direction, but
_oscillatory_--now up, now down, in the same place. Again, they do not
involve contraction of the whole earth, but probably are always more or
less local and compensatory--i. e., rising in one place is compensated
by down-sinking in some other place. Nevertheless, they often
affect very wide areas--sometimes, indeed, of more than continental
extent--as, for example, in the crust movements of the Quaternary
period or ice age.
These are by far the most frequent and most conspicuous of all
crust movements--not only now, but also in all geological times. If
ocean-basin-forming movements are the underlying cause and condition
of the evolution of the earth, these wide oscillations, by increasing
and decreasing the size and height of continents and changing greatly
their contours, have determined all the details of the drama enacted
on the surface, and were the determining cause of the varying rates
and directions of the evolution of the organic kingdom. These were the
cause of the unconformities and the corresponding apparent wholesale
changes in species so common in the rocky strata, and which gave rise
to the doctrine of catastrophism of the early geologists. These also
have so greatly modified the contours of the continents and their size
by temporary increase or decrease that they have obscured the general
law of the steady development of these, and therefore their substantial
permanency.
Although the most important of all crust movements in determining the
whole history of the earth, and especially of the organic kingdom, we
shall dwell no further on them, because no progress has yet been made
in their explanation. This, too, must be left to the workers of the
twentieth century.
_The Principle of Isostasy._--The principle of static equilibrium as
applied to earth forms was first brought forward (as so many other
valuable suggestions and anticipations in many departments of science)
by the wonderfully fertile mind of Sir John Herschel, and used by him
in the explanation of the sinking of river deltas under the increasing
weight of accumulating sediments.[C] It was afterward applied to
continental masses by Archbishop Pratt[D] and by the Royal Astronomer
Professor Airy.[E] But for its wide application as a principle in
geology, its clear definition, and its embodiment in an appropriate
name, we are indebted to Major Dutton, United States Army.[F]
[C] Philosophical Magazine, vol. ii, p. 212, 1837; Quarterly
Journal of Geological Society, vol. ii, p. 548, 1837.
[D] Philosophical Magazine, vol. ix, p. 231, and vol. x, p.
240, 1855.
[E] Philosophical Trans., 1855, p. 101.
[F] Philosophical Society of Washington, 1892.
The principle may be briefly stated as follows: A globe so large as the
earth, under the influence of its own gravity, must behave like a very
stiffly viscous body--that is, the general form of the earth and its
greatest inequalities must be in substantial static equilibrium. For
example, the general form of the earth is oblate spheroid, because that
is the only form of equilibrium of a rotating body. Rotation determines
a distribution of gravity with latitude which brings about this form.
With any other form the earth would be in a state of strain to which
it must slowly yield, and finally relieve itself by becoming oblate.
If the rotation stopped, the earth would accommodate itself to the new
distribution of gravity and become spherical.
The same is true of the large inequalities of surface. Oceanic
basins and continental arches must be in static equilibrium or they
could not sustain themselves. In order to be in equilibrium the
sub-oceanic material must be as much more dense than the continental
and sub-continental material as the ocean bottoms are lower than
the continental surfaces. Such static equilibrium, by difference of
density, is completely explained by the mode of formation of oceanic
basins already given.
So also plateaus and great mountain ranges are at least partly
sustained by gravitative equilibrium, but partly also by earth
rigidity. It is only the smaller inequalities, such as ridges, peaks,
valleys, etc., that are sustained by earth rigidity alone.
These conclusions are not reached by physical reasonings alone, but are
also confirmed by experimental investigations. For example, a plumb
line on the plains of India is deflected indeed toward the Himalayas,
as it ought to be, but much less than it would be if the mountain and
sub-mountain mass were not less dense and the sub-oceanic material
more dense than the average.[G] Again, gravitative determinations by
pendulum oscillations, undertaken by the United States along a line
from the Atlantic shore to Salt Lake City, show that the largest
inequalities, such as the Appalachian bulge, the Mississippi-basin
hollow, and the Rocky Mountain bulge, are in gravitative
equilibrium--i. e., the mountain and sub-mountain material is as much
lighter as the mountain region is higher than the Mississippi-basin
region.
[G] Pratt, Philosophical Magazine, vol. ix, p. 231, 1855; vol.
x, p. 340, 1855; vol. xvi, p. 401, 1858.
Now, so sensitive is the earth to changes of gravity that, given time
enough, it responds to increase or decrease of pressure over large
areas by corresponding subsidence or elevation. Hence, all places where
great accumulations of sediment are going on are sinking under the
increased weight, and, contrarily, all places where excessive erosion
is going on, as, for example, on high plateaus and great mountain
ranges, are rising by relief of pressure.
This principle of isostasy is undoubtedly a valuable one, which must be
borne in mind in all our reasonings on crust movements, although its
importance has been exaggerated by some enthusiastic supporters. Its
greatest importance is not as a cause _initiating_ crust movements or
determining the features of the earth, but rather as conditioning and
modifying the results produced by other causes. The idea belongs wholly
to the latter half of the present century. Commencing about 1840, it
has grown in clearness and importance to the present time.
[_To be concluded._]