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OCR for page 201
CHAPTER XIII
ON SOME OF THE GREATER PROBIIEMS OF PHYSICAL
GEOLOGY *
CLARENCE E. DUTTON (b. 1841, d. 1912)
The greatest problems of physical geology I esteem to be: 1st, What
is the potential cause of volcanic action? Id, What is the cause of the
elevation and subsidence of restricted areas of the earth's surface? 3d,
What is the cause of the foldings, distortions, and fractures of the strata ?
The volcanic problem is at present unsolved. Every theory or hypoth-
esis thus. far offered to explain it goes to pieces at the touch of criti-
cism. For elevations and subsidences we are also without any satis.factor~
explanation. But the third problem, the cause of distortions and frac-
tures in the strata, looks much more hopeful, and it is my intention to
propose this evening a solution of it, not a new one, let me stay, but an
old one remodeled. Before proceeding to discuss it, it is proper to advert
to a hypothesis which has ion;, been in favor, and which is looked upon
by some authorities as affording, an explanation. This is. sometimes c.allecl
the contractional hypothesis.
The earth is regarded as being hot within and undergoing secular cool-
i:n~ by conduction of heat through its external shell and its radiation into
space. This loss of interior heat is presumed to be accompanied by a cor-
responding loss of interior volume, thus. depriving, the cold. exterior shell
o-i' a part. of its support. In a body so large as the earth the tangential
strain set up by this. loss of interior support is demonstrably so! great that
the outer shell or crust, as it is usually called, must be crushed or bucklecl
by it and collapse upon the shrinking nucleus. The objection to this ex-
planation is twofold: Ifs the first place, we cannot, without resorting to
violent assumptions, find in this process a sufficient amount of either
linear or volume contraction to account for the effects attributed to it.
:[n the second place, the distortions of the strata are not of the kind which
could be produced by such a process. As regards the first objection I will
confine myself here to a mere reference to the very able analysis of the
* This is an address read before the Philosophical Society of Washington!
April 27, 1889, and published in the Bulletin of that Society, Vol. XI, pages 51-64,
printed in 1892. This paper was reprinted in the Journal of the Washington
Academy of Sciences, September 19,1925, Vol. 15, No. 15, and also in the Bulletin
Geodesique, of the Section of Geodesy of the International Geodetic and Geo-
physica.l Union, No. 9, 1926. In this paper the word " isostasy " was first used.
201
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202
FIGURE OF THE EARTH
problem by Rev. Os,mond Fisher. I see no satisfactory reply to his a,r~,u-
-ment. As regards the second objection, which, if' possible, is more cogent
still, it may be remarked that the most striking features in the facts to be
explained are the long, narrow tracts occupied by belts of plicated strata
and the approximate parallelism of the axes of their folds. These call for
the action of some great horizontal force thrusting in one direction. Take,
for example, the Appalachian system, stretching from lIaine to Georgia.
Here is a great belt of parallel synclinals and a,nticlinals with a persistent
trencl, and no rational inquirer can doubt that, they have been puckered
up by some vast force acting horizontally in a northwest and southeast
direction. Doubtless it is the most wonderful example of systematic pli-
cation in the world. :E3ut there are many others which indicate the opera-
tion of the same forces with the same broad characteristics. The par-
ticular characteristic with which we are here concerned is that in each
of these folded belts, the horizontal force has acted wholly or almost
wholly in one direction. But the forces which would arise from a col-
lapsing crust would act in every direction equally. There would be no
determinate direction. In short, the process could not form long, narrow
belts of parallel folds. As I have no time to discuss the hypothesis further
I dismiss it with the remark that it is quantitatively insufficient and
qualitatively inapplicable. It is an explanation which explains nothings,
hich we want to explain.
In proposing another view of the problem we may first turn our atten-
tion to those obvious and universally conceded forces which determine
the figure of' the earth. That figure we know to be one which a liquid or
v
I ~ 1
viscous body of large size will take when subject only to the forces arts-
in~, from rotation arouncl~ an axis and to the mutual gravitation of its
own parts. This form is an oblate spheroid.
The spherical form, however, is only approximate.
We find large por-
tions of its surface protruding into continents and islands, while others
are sunken to form oceanic basins. How did these inequalities arise?
If' the I'orm of the earth is nearly spheroidal why is it not exactly so?
It has always been supposed that this nearly spheroidal form implies
that the earth, if not liquid, is certainly not rigid enough to maintain
~,. . · .
near stem Term ~.~ninst the Torces of its own gravitation. Even if' the
~ 1 I By ~ ~11 I_ 1 1 V ~ 44~ ~ ~ ~ e ~ U V^ & ~ ~ ~ ^ ~ ~ \~ ~ ~ vie ~ ~ tt) ~
. . r -I ~ ~ ~ ~ ~1 ~ to I 1 · _ 1~
earth were a mass of unbroken steel no great, ctepa,rture from ants s~l,~e
could be maintainecl for a n~on~ent. It would straightway collapse and
flow into a spheroidal form. But if' gravitation compels it to take a
nearly spheroidal shape why should it stop short of making it perfectly
so ? Perhaps it will be said that while the rigidity of rocks may be insuffi
cient to permit a great deformation of' the normal spheroid it may be
suffieicnt to permit a small one. Before discussing this point it will be
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SOME PROBLEMS OF PHYSICAL GEOLOGY 203
necessary to intro~lu.ce a co~si.deration which has seldom been. touched
upon by geographers or geologists.
If the earth were composed of homogeneous matter its normal figure
of equilibrium without strain would be a true spheroid of revolution;
but if hetero~eneo;us, if some parts. were denser or lighter than others,
its normal figure would no longer be spheroidal Where the lighter mat-
ter was accumulated there would lie a tendency to bulge, and where the
denser matter existed there would be a tendency to flatten or depress the
surface. For this condition of equilibrium of figure, to which gravitation
tends to reduce a planetary body, irrespective of whether it be homo-
geneous or not, I propose the name isostasy. I would have preferred the
word isobar::, but it is preoccupied. MTe may also use the corresponding
adjective, isostatic. An isostatic. earth, composed of homogeneous matter
and without rotation, would be truly spherical. If slowly rotating, it
would be a spheroid of two axes. If rotating rapidly within. a certain
limit, it might be a spheroid o:l three axes.
But if the earth be not homogeneous. if some portions near the sur-
face be lighter than others then the isos.ta.tic figure is no longer a sphere
or spheroid of revolution, but a deformed figure bulged where the matter
is light and depressed where it is heavy. The question which I propose is:
How nearly does the earth's figure approach to isostasy?
Mathematical statics alone will not enable us to answer this question
with a sufficient degree of approximation. It does, indeed, enable us to
]x certain limits to the clepa.rture from isostasy which cannot be exceeded.
This very problem has been treated with great skill by Prof. George
Darwin.
But this problem may be approached from another direction with more
satisfactory results. Geology furnishes us with certain facts which enable
us to draw a much narrower conclusion. There are several categories of
fact to which we may turn. One of the most remarkable is the general
fact that where Breast bodies of strata are deposited they progressively
settle down or sink seemingly by reason of their t,ross mechanical. weight,
just as a railway embankment across a bog sinless into it.. The attention
~ . . .
Of the ea.rller Appalachian geologists was called, as soon as they had a.c-
quired a fair knowledge of their field, to the surprising fact that the
Paleozoic strata. in that wonderful belt, though tens of thousands of feet
in thickness, were all deposited in comparatively shallow water. The
Paleozoic beds of the Appalachian region have a thickness ranting from
10,000 to over 30,000 feet, yet they abound in proofs that when they
revere deposited their surfaces were the bottom o:l a shallow sea. whose
depth could not probably have exceeded a. few hundred feet. No conclu-
sion is left us but that sinking went on part passe with the accumulation
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204
of the strata. When the geology of
t
FI G URE OF THE EAR TH
the Pacific coast was sufficiently dis
closed, the same fact Gonfront.ed us there. As investigation. went on. the
same fact presented itself over the western mountain region of the United
States. One of the most striking cases is the Plateau Country. This
great region, nearly 100,000 square miles in area, lying in the adjacent
parts of Colorado, fJtah, New Mexico, and Arizona, discloses from 8,000
to 12,000 feet of mesozoic and Cenozoic strata. Here the proof is a.bun-
dant that the surface of the strata was throughout that vast stretch of
time never snore than a few feet from sea level. Again and again it
emerged from the water a little way, only to be submerged. At, many
horizons grew forests. which are now represented by those abundant and
beautiful fossil woods which of late have become celebrated. In the cre-
taceous we kind many seams and seamless of coal or carbonaceous shale;
but they are inclucled. between sandstones which. are cross-bedded and
ripple-marked, or between shales and limestones which abound in the
remains of' marine mo;llusca. Here the evidence seems conclusive that
the whole subsidence went on at about the sane rate as the surface was
built up by deposition. In short, it may be laid down as a, general rule
that where great bodies of sediment have been deposited over extensive
areas their deposition hats been accompanied. by a subsidence of the whole
mass.
The second class ol' facts is even more instructive, and stands in a
reciprocal relation to those just mentioned. Wherever broa.cl ~:noulltain
plat:t'orms occur and have been subjected to great erosion the loss of' alti-
tucle by degradati.o~-~ is made good by a' rise o:t' the platform. Ill. the west-
ern portion o-l' the United States there occur mountain ranges situated
upon broad and lofty platforms from 20 to 60 miles wide and from 50
to 200 miles in length. Some of these platforms contain several n~oun-
tain ridges. All of' them have been enormously eroded, and if the matter
removed from them could be replaced it would suffice to build them to
heights of eight or ten miles; yet it is incredible that these mountains
were ever much loftier than now, and may never have been so lofty. The
flanks of these platforms, with the upturned edges of the strata reposing
against them or with gigantic faults measuring their immense uplifts,
plainly declare to us. that they have been slowly pushed upwards as fast
as they were degraded by secular erosion.
It seems little doubtful that these subsidences of accu~nu.lation (1e,-
posits and these progressive upward movements of' eroded mountain
platforms are, in the main, results of gravitation restoring the isosta,sy
which has been disturbed by denudation on the one hand and by sed.i-
mentation~ on the other. The magnitudes of' the masses which thus show
the isostatic tendency are in some cases no greater than a single moun
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SOME PROBLEMS OF PHYSICAL GEOLOGY 205
fain platform, less than 100 miles in length, from 20 to 40 miles wide and
from 2,500 to 3,500 feet mean altitude above the surrounding lowlands.
Prom this we may directly infer that in those regions the effective rigidity
of the earth is insufficient to uphold a mass so great as one of those plat-
I'orms if that mass constituted a real deformation of isosta.sy; and if an
equal mass were to be suddenly removed the earth would flow upward
I'rom below to fill the hiatus; hence we must look to considerably smaller
masses to find a de-f'ect of isosta.sy. It is extremely probable that small or
narrow ridges are not isosta.tic with. respect to the country round about.
them. Some volcanic mountains may be expected to be non-isostatic,
especially isolated volcanic piles.
Thus the geologic changes which have taken place may be regarded
as experiments conducted by Sat.ure herself on a Vast scale, and from
her experiments w ~ may by suitable working hypotheses draw provisional
conclusions, both as to the degree in which the earth approximates to
isost.asy and also as to the mean effective rigidity of large portions of the
subterranean mass. The approach to isostasy is thereby inferred to be
very near, while the mean rigidity of the subterranean masses is. also
inferred to be far less than that of ordinary surface rocks,, and. even ap-
proaching more nearly the rigidity of lead than to that of copper. Pure
physics alone would not have enabled us to reach. such a conclusion, for
the equations employ constants of unknown value. But geologic inquiry
may, and I believe does, furnish us with narrow limits within which. those
values must be talked Thus the two sciences must work cooperatively and
supplement each. other.
There is, however, one other branch of physical inquiry which bears
directly on the fore~oin,~, questions. This is the investi~a,tion oft ter-
restria,1 gravitation by means of' the pendulum. I regret that I have never
had time or opportunity to acquaint myself thoroughly with the results
thus far reached by this branch of investigation, and can only speak from
general knowledge. Pendulum observations are far too few for the wants
of t,eo~raphic or `,eolot,ic science. So far as they ,t,o~ they are highly su~,-
~,estive in the present connection. The pendulum., as. a rule, does, not
show any appreciable variation of gravity, such as would be expected if
the mean density of all flee outer parts of' the earth were uniform. It ind;-
cates rather that the elevated regions and continents, are composed of'
lighter n~at.ter and the depressed regions a.ncl ocean basins of denser ~at-
ter. The exceptions are of' a character which prove the general rule, and
occur where we should fool; for them. The results obtained by the In.d;a
survey upon the R;malayan mass were re~arcled by Archdeacon Pratt as
indicating, that the plateau was composed of' lighter matter than the low-
lands to the southwards A similar result has been obtained in the threat
~. ~. ~,
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206
FIGURE OF THE EARTH
bulge which forms the western half of the Unitecl States. In other words,
the pendulum indicates that those elevated regions are nearly if not quite
isostatic.
()n the other hancl, the observations of Mend.enh.all on Fujiva~na., in
Japan, indicated a slight excess of mass, and a similar result would seem
to follow from Mr. Preston's world in the Hawaiian Islands. From the
rrature of the process by which volcanoes are built these results are to be
expected.
It would also seem natural to expect that the plun~b-line would give
some incli.cations upon this subject; but experience has shown. that most
of the observed deflections of the plumb-line are inexplicable. They oc-
cur where we would least expect them upon broad and. level plains,
Here there is nothing to indicate any cause of cleflection. The: are found
on the tundras of Siberia and the monotonous expanse of British North
America, where the surface of the earth is but feebly diversified. Ire
mou:rltain regions they are often conflicting and unintelligible, but along
the sea coast. the indications are more systematic. On both the Atlantic
and Pacific shores the deflection of the plummet is almost invariably
towards the ocean, and is often of considerable amount; but it is along
the shore that the isostatic theory would lead us to look for just this de-
flection, for it is along the margins of the continents that great bodies
of sediment accumulate; and so long as the earth possesses any note-
worthy degree of rigidity, enabling it to sustain in part the resulting
(reformation of isostas.y, so long must we expect to kind these sediments
constituting- an excess of masts whose attraction will make itself felt upon
the plummet.
The theory of isostasy thus briefly sketched out is essentially the
theory of Babbage and Herschel, propounded nearly a century ago. It is,
however, presented in a modified form, in a new dress, and in greater
detail. We may now proceed to deduce some important consequences.
A little reflection must satisfy us that the secular erosion of the land
and the deposit of sediment along the shore lines constitute a continuous
disturbance of isostasy. The land is ever impoverished of material is
continuously unloaded; the littoral is as continuously loaded up. The
resultant forces of gravitation tend to elevate the eroded land and to de-
press the littoral to their respective i.sostatic levels. Whether these forces
shall become kinetic and produce actual movement or flow will depend.,
-
first, upon their intensity; secon.d, upon the ri~idity of the earth by which
such movement its resisted. Let us consider, then, the intensity of the
forces.
The littoral belts upon which sediments are throw n down are co-
extensive in length with shores. Their widths are no doubt variable, but
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SOME PROBLEMS OF PHYSICAL GEOLOGY 207
must often reach a hundred miles. or more with considerable thickness,
and are not wholly unimportant at much greater distances. The thick-
ness of the deposits may vary much, but may be proportional to the time
of accumulation, and here time is measured by the geologic standarcl.
The gross weight of such masses of sediment must be vast indeed.. If
there is any viscous yielding, at all the problem becomes essentially that
of the flowing solid, which is in a large measure governed by hydrostatic
laws. The intensity of the :force must have a maximum value propor-
tional to the thickness which lies above the isostatic level and also pro-
portional to its specific gravity. The area covered by the deposit enters
as a quantity factor, but not as an intensity factor. The greater the area,
the greater is the total potential energy of movement without any neees-
sary increase of the intensity of the force. This intensity, being propor-
tional to the thickness of the sediments, may become almost indefinitely
great or it may be small. Indeecl, it may, and in fact does, become nega-
tive when we apply the same statical theory to the movement or stress. of
the denuded land areas
But whether these forces are sufficient to produce actual flow is equally
dependent upon else rigidity, or, as we may here term it, the viscosity of
the masses involved. We have already seen reason to infer that the mean
viscosity is not great, being far less than that of the surface rocks alone.
Beyond this rather vague statement I perceive no way of assigning, a value
to the resistance to be overcome.
It remains to inquire what is the resulting direction of motion. The
general answer is, towards the direction of least resistance. The specific
answer, which must express the direction of least resistance, will, of
course, turn upon the configuration of the deposition on the one ha.ncl,
and of denudation on the other, and also upon the manner in which the
rigidity or viscosity varies from place to place. Taking, then, the case of
a land area undergoing denudation, its detritus carried to the sea and
deposited in a heavy littoral belt, we may re~a.rd the weight of each ele-
menta.ry part of the deposited mass as a statical force acting upon a vis-
cous support below. Assuming that we could find a deferential expres-
sion appliea.ble to each and every element of the mass and a. c.orrespond.illg
one for the resistance offered by the viscosity, the integration for the en-
tire mass might give us a series of equipotential surfaces within the mass.
The resultant force at any point of any equipotential surface would be
normal to' that surface. A similar construction may be applied to the a.d-
joining denuded area., in which the defect of isosta.sy may be treated as
so much mass with a negative algebraic sign. The resultants normal to
the equipotentia.l surfaces would in this case, also have the negative sign.
The effective force tending to produce movement would be the a.rith
14
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208
FI CURE OF TlIE EAR TH
metical sum of the normals or of a single resultant compounded of the
two normals. From this construction we may derive a force which tends
to push the loaded sea. bottoms inward upon the unloaded land hori-
zontally.
This gives us a force of the precise kind that is wanted to explain. the
origin of systematic plications. Long reflection and considerable analysis
have satisfied me that it is sufficient both in intensity and in amount
unless. we assume for the mean Viscosity of the superficial and subter-
ranean masses involved in the movement a much greater value than I
am disposed to concede. The result is ~ true viscous flow of the loa.decl
littoral inward upon the unloaded continent.
There may be in this proposition some degree of violence to a certain
mental prejudice against the idea that the rock-ribbed earth, to which
all our notions of stability and immovableness are attached, earl be made
to flow. It may assist our efforts if we reflect upon the motion of the great
ice sheet which covers Greenland. Ilere the masses involved are no greater
than some masses of sediment. The specific gravity of ice is only about
one-third that of the rock masses. The forces called into play to carry
the glacier along horizontally do not seem to diner greatly in intensity
or amount from the described forces, and the rigidity of the ice itself may
not exceed the mean rigidity of the rock masses beneath the littoral.
We may now proceed to inquire how this theory adjusts itself to the
actual facts. And, firstly, where do systematic pliGa.tions occur ?
1) It is a remarkable fact that they occur among sedimentary beds of
great and variable thickness., which were rather rapidly accumulated.
They seldom, and., so far as I now recall, never occur among strata which
are of small thickness, slowly accumulated with uniformity over large
areas; and the theory requires that they should occur in the heavy de-
posits or along their margins., and should have their greatest develop-
ment there, for the forces called into play must be proportional to the
masses involved.
2) They occur in their systematic form along the ancient shore lines.
This is but another way o:t statical the preceding proposition. It hats its
uses, however, for in so far as the continents have preserved approximately
their old shore lines since the ages in which the plications were formed
there is a conspicuous. parallelism of the axes of plica.tion to the neigh-
boring coast. This is true of the Pacific. coast of the United States. As
re~a.rds the Appalachian plications, we have the remarkable fact that in
Paleozoic time the ocean lay to' the west of those vast bodies of folded
strata instead of to the east of them, as now. We must fool: to a Paleozoic
Atlantis for the origin of a Breast portion of those sediments. The flow
of the earth was from west northwest to east southeast.
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SOME PROBLEMS OF PHYSICAL GEOLOGY 209
3
~ The parallelism of the folds and their occurrence in long, narrow
belts formed by horizontal forces acting in one direction become a conse-
quence so obvious as to need no' comment. It is in strong contrast with
the contractional theory, which gives a force without any determinate
clirection.
4) Another important fact is that these systematic flexures were
mainly formed at the times the sediments were deposited. This is a fact
of' geologic observation. The contractional hypothesis gives no determi-
nate time for the formation of these flexures. It holds up to us a. process
continuous through all geological time, proceeding at a rate which dimin-
ishes but slowly as the ages roll by. These plica.tions, according to the
isostatic theory, are the results of the disturbance of isos.tasy, and follow
immediately upon that disturbance or after it has reached a sufficient
amount, and cease with it. These folds, however, hate been subject since
their first formation to great erosion, which is. also a disturbance of
i.so~s.tasy, and thus the original plication may have been increased or modi-
fied thereby.
The theory may also be applied in a most satisfactory manner to the
explanation of subordinate: features associated with plication.
S) One of the features of plica.tion which has attracted great atten-
tion and occasioned great. perplexity to geologists is the so-called fan-
structure. This is very striking in the Alps, and has its counterpart in
the inclined folds of the Appalachians of Pennsylvania, where the north-
western branches of the anticlines. are steeper than the southeastern
hra.nches. If we assume that as the rocks lie deeper in the earth they are
softened somewhat by the increas.in~, heat, it follows that in the flow of
the mass the movement would be easier and more rapid below than above.
Thus a horizontal force arising from this differential movement acts upon
the inverted arches of the synclines and carries their lower vertices for-
ward in the direction of motion.
Thus the general theory here proposed gives an explanation of the
origin of plications. It gives us a. force acting in the direction required
in the manner required, at the times. and places required, and one which
has the intensity and amount required and no more. The contractional
theory gives us a force having neither direction nor determinate mode
of action, nor definite epoch of action. It gives us a force acting with a
far greater intensity than we require, but with far less quantity. To pro-
vide a place -for its action. it must have recourse to an arbitrary postulate
a.ssumin`, for no independent reason the existence of areas of weakness
in a supposed crust which would have no raisorr d'etre except that they
are necessary for the salvation of the hypothesis.
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210
FI CURE OF THE EAR TH
Before closing this discussion it will be necessary to advert to another
one of the great problems of physical geology, viz., the cause of general
eleva.tio~ns and subsidences. I do so, not with the idea of throwing light
upon it, but to guard against a misapprehension which would otherwise
be sure to occur.
Geologic history discloses the fact that some great areas of the earth's
surface which were in former ages below sea level are now thousands of
feet above it. It also gives us reason to believe that other areas now sub-
merged were in other ages terra 1?rrna. Our western mountain region al;
the beginning, of Cenozoic time was at sea level. It is now, on an average,
6,000 feet above it. The great Himalayan. plateau contains. early Cenozoic
beds full of marine fossils which now lie at altitudes of 14,000 feet or
more. The whole North American Continent has, since the close of the
Paleozoic, gained in altitude. Now, it is sufficiently obvious that the
theory of isosta.sy offers no explanation of these permanent changes of
level. On. the contrary, the very idea of isostasy means the conservation
of profiles against lowering by denudation on the land and by deposition
on the sea bottom, provided no other cause intervenes to change those
levels. If, then, that. theory be true, we must look for some independent
principle of causation which can gradually and permanently change
the profiles of the land and sea bottom. And I hold this cause to' be an
independent one. It has been much the habit for geologists to attempt
to explain the progressive elevation of plateaus and mountain platforms,
and also the foldings of the strata by one and the same process. I hold
the two processes to be distinct and as having no necessary relation to
each other. There are plicated regions which are little or not at all ele-
`-ated, and there are elevated regions. which are not plicated. Plication
may go on with little or no elevation in one geologic age and the same
. ~ . , ~ . ,. , . ~ ~ ... . .. .. .
.
region may be elevated without much additional pl~cat~o~n In a subse-
quent age. This is in a large measure true of the Sierra. Nevada plat-
:lorm, which was intensely plicat.ed during the Paleozoic and early meso-
zoic, but which received its present altitude in the late Cenozoic.
Whatever may have been the cause of these great regional uplifts it in
no manner affects the law of isostasy. What the real nature of the up-
lifting force may be is, to my mind, an entire mystery; but I think we
may discern at least one of its attributes, and that is a gradual expansion,
or a diminution of the density, of the subterranean magmas. If the
isostatic force is. operative at all, this expansion is a rigorous conse-
quence; for whenever a rise of the land has taken place one of two things
hats happened; the region affected has either gained an accession of mass
or a mere increase of volume without increase of mass. We l~now of no
cause which could either add to the mass or diminish the density, yet
. . . . . .
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SOME PROBLEMS OF PHYSICAL GEOLOGY 211
one of the two must surely have happened. But the difference of the two
alternatives in respect to' consequences is immense. If the increase of'
volume of an elevated area be due to an accession of matter, the plateau
must be hoisted against its own rigidity and also against the statical
weight of its entire mass lying above the isostat,ic level. But if' the in-
crease of' volume be due to a decrease of density there is no resistance to
be overcome in order to raise the surface. Hence I infer that the cause
which eles-a,tes the land involves an expansion of' the underlying ma~-
mas, and the cause which depresses' it is a shrinkage ol' the magmas. Tl~e
nature of the process is, at present, a complete mystery.
Representative terms from entire chapter:
force acting
