Chapter XII: A Century of Zoology in America. Wesley R. Coe 391 (8)

The Appalachian subsidence in the Alleghany region of 35,000 to 40,000
feet, going on through all the Paleozoic era, was due, as has been
shown, to an actual sinking of the earth’s crust through lateral
pressure, and not to local contraction in the strata themselves or the
terranes underneath. But such a subsidence is not possible, unless
seven miles—that is, seven miles in maximum depth and over a hundred
in total breadth—unless seven miles of _something_ were removed, in
its progress, from the region beneath.

If the matter beneath was not aërial, then liquid or viscous rock was
pushed aside. This being a fact, it would follow that there existed,
underneath a crust of unascertained thickness, a sea or lake of mobile
(viscous or plastic) rock, as large as the sinking region; and also
that this great viscous sea continued in existence through the whole
period of subsidence, or, in the case of the Alleghany region, through
all Paleozoic time—an era estimated on a previous page to cover at
least thirty-five millions of years, if time since the Silurian age
began embraced fifty millions of years.

The facts thus sustain the statement that lateral pressure produced
not only the subsidence of the Appalachian region through the
Paleozoic, but also, cotemporaneously, and as its essential
prerequisite, the rising of a sea-border elevation, or geanticlinal,
parallel with it; and that both movements demanded the existence
beneath of a great sea of mobile rock.”

The recognition of regional _warping_ as a major factor in the larger structure of mountain systems, and the expression of that factor in the terms geosyncline and geanticline forms a notable advance in geologic thought. Subsequent folding on a regional scale results in the development of synclinoria and anticlinoria. Van Hise has given these latter terms wide currency, but apparently inadvertently has used synclinorium in a different sense than that in which Dana defined it. Dana gave the word to a mountain range made by the mashing and uplift of a geosyncline, Van Hise defines it as a downfold of a large order of magnitude, embracing anticlines and synclines within it; anticlinorium he uses for a corresponding up fold.[89] Rice has called attention to this change of definition,[90] but Van Hise’s usage is likely to prevail, since they are needed terms for the larger mountain structure and do not require a determination of the previous limits of upwarp and downwarp,—of original denudation and deposition. Furthermore, a geosyncline in mountain folding may have one side uplifted, the other side depressed and there are reasons for regarding the folds of Pennsylvania, Dana’s type synclinorium, as representing but the western and downfolded side of the Paleozoic geosyncline. Under that view the folded Appalachians of Pennsylvania constitute a synclinorium in both the sense of Dana and Van Hise.

_The Ultimate Cause of Crustal Compression._

The next important advance in the theory of mountains was made by C. E. Dutton who in 1874 published in the Journal (=8=, 113–123) an article entitled “A criticism upon the contractional hypothesis.” Dutton gives reasons for holding that the amount of folding and shortening exhibited in mountain ranges, especially those of Tertiary date, is very much greater in magnitude and is different in nature and distribution from that which would be given by the surficial cooling of the globe. The following quotations cover the principal points in the argument:

“The argument for the contractional hypothesis presupposes that the
earth-mass may be considered as consisting of two portions, a cooled
exterior of undetermined (though probably comparatively small) depth,
inclosing a hot nucleus.... The secular loss of heat, it is assumed,
would be greater from the hot nucleus than from the exterior, and the
greater consequent contraction of the nucleus would therefore
gradually withdraw the support of the exterior, which would collapse.
The resulting strains upon the exterior would be mainly tangential.
Owing to considerable inequalities in the ability of different
portions to resist the strains thus developed, the yielding would take
place at the lines, or regions of least resistance, and the effects of
the yielding would be manifested chiefly, or wholly, at those places,
in the form of mountain chains, or belts of table lands, and in the
disturbances of stratification. The primary division of the surface
into areas of land and water are attributed to the assumed smaller
conductivity of materials underlying the land, which have been left
behind in the general convergence of the surface toward the center.
Regarding these as the main and underlying premises of the
contractional argument, it is considered unnecessary to state the
various subsidiary propositions which have been advanced to explain
the determination of this action to particular phenomena, since the
main proposition upon which they are based is considered untenable.

There can be no reasonable doubt that the earth-mass consists of a
cooled exterior inclosing a hot nucleus, and a necessary corollary to
this must be secular cooling, probably accompanied by contraction of
the cooling portions. But when we apply the known laws of thermal
physics to ascertain the rate of this cooling, and its distribution
through the mass, the objectionable character of the contractional
hypothesis becomes obvious.

That Fourier’s theorem, under the general conditions given, expresses
the normal law of cooling, is admitted by all mathematicians who have
examined it. The only ground of controversy must be upon the values to
be assigned to the constants. But there seem to be no values
consistent with probability which can be of help to the contractional
hypothesis. The application of the theorem shows that below 200 or 300
miles the cooling has, up to the present time, been extremely
little.... At present, however, the unavoidable deduction from this
theorem is that the greatest possible contraction due to secular
cooling is insufficient in amount to account for the phenomena
attributed to it by the contractional hypothesis.

The determination of plications to particular localities presents
difficulties in the way of the contractional hypothesis which have
been underrated. It has been assumed that if a contraction of the
interior were to occur, the yielding of the outer crust would take
place at localities of least resistance. But this could be true only
on the assumption that the crust could have a horizontal movement in
which the nucleus does not necessarily share. A vertical section
through the Appalachian region and westward to the 100th meridian
shows a surface highly disturbed for about two hundred and fifty
miles, and comparatively undisturbed for more than a thousand. No one
would seriously argue that the contraction of the nucleus had been
confined to portions underlying the disturbed regions: yet if the
contraction was general, there must have been a large amount of slip
of some portion of the undisturbed segment over the nucleus. Such a
proposition would be very difficult to defend, even if the premises
were granted. It seems as if the friction and adhesion of the crust
upon the nucleus had been overlooked. Nor could this be small, even
though the crust rested upon liquid lava. The attempts which some
eminent geologists have recently made to explain surface corrugation
by this method clearly show a neglect on their part to analyze
carefully the system of forces which a contraction of the nucleus
would generate in the crust. Their discussions have been argumentative
and not analytical. The latter method of examination would have shown
them certain difficulties irreconcilable with their knowledge of
facts. Adopting the argumentative mode, and in conformity with their
view regarding the exterior as a shell of insufficient coherence to
sustain itself when its support is sensibly diminished, the tendency
of corrugation to occur mainly along certain belts, with series of
parallel folds, is not explained by assuming that these localities are
regions of weakness. For a shrinkage of the nucleus would throw each
elementary portion of the crust into a state of strain by the action
of forces in all directions within its own tangent plane. A relief by
a horizontal yielding in one direction would by no means be a general
relief.”

Dutton’s criticisms robbed the current hypothesis of mountain-making of its conventional basis without providing a new foundation. It was a quarter of a century in advance of its time, has been seldom cited, and seems to have had but little direct influence in shaping subsequent thought. It, however, gave direction to Dutton’s views, and his later papers were far-reaching in their influence.

If contraction from external cooling is not the cause of the compressive forces it is necessary to seek another cause. Two years later, in 1876, Dutton attempted to provide an answer to this open question.[91] A review of this paper, evidently by J. D. Dana, is given in the Journal. The following explanations of Dutton’s theory and of Dana’s comments upon it are contained in a few paragraphs from this review (=12=, 142, 1876).

“Captain Dutton presents in this paper the views brought out in his
article in volume viii of this Journal, with fuller illustrations, and
adds explanations of his theory of the origin of mountains. The
discussion should be read by all desiring to reach right conclusions,
it presenting many arguments from physical considerations against the
contraction-theory, or that of the uplifting and folding of strata
through lateral pressure. There is much to be learned before any
theory of mountain-making shall have a sufficient foundation in
observed facts to demand full confidence, and Captain Dutton merits
the thanks of geologists for the aid he has given them toward reaching
right conclusions. His discussions are not free from misunderstandings
of geological facts, and if they fail to be finally received it will
be for this reason.

We here give in a brief form, and nearly in his own words, the
principal points in his theory of mountain-making as explained in the
later part of his memoir.

Accepting the proposition that there is a plastic condition of rock
beneath the earth’s crust and that metamorphism is a ‘hydrothermal
process,’ and believing that ‘the penetration of water to profound
depths [in the earth’s crust] is a well sustained theory,’ he says
that great pressure and a temperature approaching redness are
essential conditions of metamorphism.... ‘The heaviest portion would
sink into the lighter colloid mass underneath, protruding it laterally
beneath the lighter portions where, by its lighter density, it tends
to accumulate.’ He adds: ‘The resulting movements would be determined,
first, by the amount of difference in the densities of the upper and
lower masses, and, second, by inequalities in the thickness of the
strata: the forces now become adequate to the building of mountains
and the plication of strata, and their modes of operation agree with
the classes of facts already set forth as the concomitants of those
features.’

The views are next applied to a system of plications. ‘It has been
indicated that plications occur where strata have rapidly accumulated
in great volume and in elongated narrow belts; that the axes of
plications are parallel to the axes of maximum deposit; and that the
movements immediately followed the deposition’—the case of the
Appalachians being an example in which the accumulations averaged
40,000 feet. He observes: ‘Wherever the load of sediments becomes
heaviest, there they sink deepest, protruding the colloid magma
beneath them to the adjoining areas, which are less heavily weighted,
forming at once both synclinals and anticlinals.’

With regard to this new theory, we might reasonably question the
existence of the colloid magma—a condition fundamental to the
theory—and his evidence that water penetrates to profound depths in
the earth’s crust sufficient to make hydrous rocks. We might ask for
evidence that the rocks beneath the Cretaceous and Tertiary, and other
underlying strata of the Uintahs, were in such a colloid state, and
this so near the surface, that the ‘beds subsided by their gross
weight as rapidly as they grew.’

Again, he says that the movements of mountain-making ‘immediately
followed the deposition.’ ‘Immediately’ sounds quick to one who
appreciates the slowness of geological changes. The Carboniferous age
was very long; and somewhere in that part of geological time, either
before the age had fully ended, or some time after its close, the
epoch of catastrophe began.”

We see foreshadowed in this paper the theory of isostasy, or condition of vertical equilibrium in the crust which Dutton published in 1889. This theory has borne remarkable fruit, but Dutton attempted to link to it the horizontally compressive forces which have produced folding and overthrusting. Willis in 1907[92] and Hayford in 1911, overlooking Dana’s objections, have attempted to make a lateral isostatic undertow the cause of all horizontal movements in the crust, adopting the mechanism of Dutton. The present writer, although accepting the principle of isostasy as an explanation of broad vertical movements, has published papers which go to show the inadequacy of this hypothesis of lateral pressure; inadequate in time relation, in amount, and in expression.[93]

In 1903 it was determined by several physicists that the materials of the earth’s crust were radioactive and must generate throughout geologic time a quantity of heat which perhaps equalled that lost by radiation into space. By 1907 this had become demonstrated. The remarkable conclusion had been reached that the earth, although losing heat, is not a cooling globe. Dutton’s contentions against mountain growth through external cooling and contraction were thus unexpectedly, through a wholly new branch of knowledge, demonstrated to be true.

Nevertheless, all students of orogeny are agreed that profound compressive forces have been the chief agents in developing mountain structures. Chamberlin was the first to arrive at the idea that the shrinkage may originate in the deeper portions of the earth under the urgency of the enormous pressures, apparently by giving rise to slow recombinations of matter into denser forms.[94]

_The New Era in the Interpretation of Mountain Structures._

In the meantime, between 1874 and 1904, another advance in the knowledge of mountain structures was taking place in Europe. Suess studied the distribution of mountain arcs over the earth and dwelt upon the prevalence of overthrust structures; the backland being thrust toward and over the foreland, the rise of the mountain arc or geanticline depressing the foredeep or geosyncline. Bertrand and Lugeon from 1884 to 1900 were reinterpreting the Alpine structures on this basis. They showed that the whole mountain system had been overturned and overthrust from the south to an almost incredible degree. Enormous denudation had later dissevered the northern outlying portions and given rise to “mountains without roots,”—isolated outliers, consisting of overturned masses of strata which had accumulated as sediments far to the southward in another portion of the ancient geosyncline.

On a smaller scale similar phenomena are exhibited in the Appalachians. Willis showed that the deep subsidence of the center of the geosyncline gave an initial dip which determined the position of yielding under compression. Laboratory experiments brought out the weakness of the stratigraphic structure to resist horizontal compression. The nature of the stratigraphic series was shown to determine whether the yielding would be by mashing, competent folding, or breakage and overthrust. The problem of mountain structures was thus brought into the realm of mechanics. These results were published in three sources in 1893,—the Transactions of the American Institute of Mining Engineers, the thirteenth annual report of the United States Geological Survey, and the Journal (=46=, 257, 1893).

Finally should be noted the contributions of the Lake Superior school of geology, in which the work of Van Hise stands preeminent. Under the economic stimulus given by the discovery and development of enormously rich bodies of iron ore, hidden under Pleistocene drift and involved in the complex structures of vanished mountain systems of ancient date, structural geology and metamorphism have become exact sciences to be drawn upon in the search for mineral wealth and yielding also rich returns in a fuller knowledge of early periods of earth history.

_Crust Movements as Revealed by Physiography._

During the last quarter of the nineteenth century another division of geology, dominantly American, was taking form and growth,—the science of land forms,—physiography. The history of that development is treated by Gregory in the preceding chapter but some of its bearings upon theory, in so far as they affect the subject of mountain origin, are necessarily given here.

Powell, Dutton, and Gilbert in their explorations of the West saw the stupendous work of denudation which had been carried to completion again and again during the progress of geologic time. The mountain relief consequently may be much younger than the folding of the rocks, and may be largely or even wholly due to recurrent plateau movement, a doctrine to which Dana had previously arrived. But the introduction of the idea of the peneplain opened up a new field for exploration in the nature and date of crust movements. Davis by this means began to study the later chapters of Appalachian history, the most important early paper being published in 1891.[95] Since then Davis, Willis, and many others have found that, girdling the world, a large part of the mountainous relief is due to vertical elevatory forces acting over regions of previous folding and overthrust. In addition, great plateau areas of unfolded rocks have been bodily lifted one to two miles, or more, above their earlier levels. They may be broad geanticlinal arches or bounded by the walls of profound fractures.

The linear mountain systems made from deep troughs of sediments have come then to be recognized as but one of several classes of mountains. This class, from its clear development in the Appalachians, and the fact that many of the laws of mountain structure pertaining to it were first worked out there, has been called by Powell the Appalachian type (=12=, 414, 1876). A classification of mountain systems was proposed by him in which mountains are classified into two major divisions, those composed of sedimentary strata altered or unaltered, and those composed in whole or in part of extravasated material. The first class he subdivides into six sub-classes of which the folded Appalachians illustrate one. It appears to the writer that Powell’s classification gives disproportionate importance to certain types which he described; but nevertheless, the fact that such a classification was made, indicates the growth of a more comprehensive knowledge of mountains,—their origin, structure, and history.

_Relations of Crust Movements to Density and Equilibrium._

A recent important development in the fields of geophysics and major crust movements consists in the incorporation into geology of the doctrine of isostasy. The evidence was developed in the middle of the nineteenth century by the geodetic survey of India which indicated that the Himalayas did not exert the gravitative influence that their volume called for. It was clear that the crust beneath that mountain system was less dense than beneath the plains of India and still less dense than the crust beneath the Indian Ocean. This relation between density and elevation indicated some approach to flotational equilibrium in the crust, comparable in its nature though not in delicacy of adjustment to the elevation of the surface of an iceberg above the ocean level owing to its depth and its density, less than that of the surrounding medium. This important geological conception was kept within the confines of astronomy and geodesy, however, until Dutton in 1876, but especially in 1889, brought it into the geologic field. A test of isostasy was made for the United States by Putnam and Gilbert in 1895 and much more elaborate investigations have since been made by Hayford and Bowie. These investigations demonstrate the importance and reality of broad warping forces acting vertically and related to the regional variations of density in the crust.

There are consequently two major and unrelated classes of forces involved in the making of mountain structures,—the irresistible horizontal compressive forces, arising apparently from condensation deep within the earth, and vertical forces originating in the outer envelopes and tending toward a hydrostatic equilibrium. In this latter field of investigation, America, since the initial paper by Dutton, has taken the lead.

_Conclusion on Contributions of America to Theories of Orogeny._

The sciences arose in Europe, but those which treated of the earth were still in their infancy when transplanted to America. The first comprehensive ideas on the nature of mountain structures arose in Great Britain and France. These ideas served as a guide and stimulus to observation in the recognition of deformations in the strata of the Appalachian system. Since 1840, however, America has ceased to be a pupil in this field of research but has joined as an equal with the two older countries. New ideas have been contributed, new and striking illustrations cited, first by the scientists of one nation, next by those of another. The composite mass of knowledge has grown as a common possession. Nevertheless, a review of the progress since 1840 as measured by the contribution of new ideas shows on the whole America at least equal to its intellectual rivals, and at certain times actually the leader. This is true of the science of geology as a whole and also of the subdivision of orogeny.

Thus far no mention has been made of German geologists, with the exception of Suess, an Austrian. German geology is voluminous and the names of many well-known geologists could be cited. But this article has sought to trace the origin and growth of fundamental ideas. The Germans have been assiduous observers of detail; preeminent as systematizers and classifiers, seldom originators. Even petrology, which might be regarded as their especial field, was transplanted from Great Britain. In the science of mountains they have followed in their fundamental ideas especially the French.

Turning to the mediums of publication through which this progress of knowledge in earth structures has been recorded, the American Journal of Science stands foremost as the only continuous record for the whole century in American literature, fulfilling for this country what the Quarterly Journal of the Geological Society has done for Great Britain since 1845, and the Bulletin de la Société Géologique for France since 1830.

_Notes._

Footnote 78:

H. D. Rogers, Geology of New Jersey, Final Report, p. 115, 1840.

Footnote 79:

H. D. Rogers, Geology of Pennsylvania, vol. 2, pt. II, pp. 761, 762,
1858.

Footnote 80:

Connecticut Academy of Arts and Sciences, 1810; quoted by G. P.
Merrill in Contributions to the History of North American geology,
Ann. Rpt. Smithsonian Institution for 1904, p. 216.

Footnote 81:

A Sketch of the geology, mineralogy, and scenery of the regions
contiguous to the river Connecticut; with a geological map and
drawings of organic remains; and occasional botanical notices, the
Journal, 6, 1–86, 201–236, 1823; 7, 1–30, 1824.

Footnote 82:

Clarence King, U. S. Geol. Exploration of the Fortieth Parallel, vol.
1, pp. 16, 44–48, 1878.

Footnote 83:

Illustrations of the Huttonian Theory of the Earth, pp. 219–238, 1802.

Footnote 84:

Robert Jameson, Elements of Geognosy, pp. 55–57, 1808.

Footnote 85:

G. P. Merrill, Contributions to the History of American Geology.
Report of the U. S. National Museum for 1904, p. 328.

Footnote 86:

H. D. Rogers, Geology of Pennsylvania, vol. 2, p. 916, 1858.

Footnote 87:

James Hall, Natural History of New York, Paleontology, vol. 3, pp.
51–73, 1859.

Footnote 88:

The Journal, 5, 423–443, 474, 475; 6, 6–14, 104–115, 161–172, 304,
381, 382, 1873.

Footnote 89:

C. R. Van Hise, Principles of North American Pre-Cambrian Geology, U.
S. Geol. Surv., 16th Ann. Report, pt. I, pp. 607–612, 1896.

Footnote 90:

W. N. Rice, On the use of the words synclinorium and anticlinorium,
Science, 23, 286, 287, 1906.

Footnote 91: