_Introduction
The Intellectual Viewpoint in 1818._
In 1818, the year of the founding of the Journal, the natural sciences
were still in their infancy in Europe. Geology was still subordinate to
mineralogy, was hardly recognized as a distinct science, and consisted
in little more than a description of the character and distribution of
minerals and rocks. America was remote from the Old World centers of
learning. The energy of the young nation was absorbed in its own
expansion, and but a few of those who by aptitude were fitted to
increase scientific knowledge were even conscious of the existence of
such a field of endeavor. Under these circumstances the educative field
open to a journal of science in the United States was an almost virgin
soil. Original contributions could most readily be based upon the
natural history of the New World, and the founder of the Journal showed
insight appreciative of the situation in stating in the “Plan of the
Work” in the introduction to the first volume that “It will be a leading
object to illustrate AMERICAN NATURAL HISTORY, and especially our
MINERALOGY and GEOLOGY.”
At this time educated people were still satisfied that the whole
knowledge of the origin and development of the earth so far as man could
or should know it was embraced in the Book of Genesis. They were
inclined to look with misgiving at attempts to directly interrogate the
earth as to its history. Philosophers such as Descartes and Liebnitz,
the cosmogonists de Maillet and Buffon had been less instrumental in
developing science than in fitting a few facts and many speculations to
their systems of philosophy. By the opening of the nineteenth century,
however, men of learning were coming to appreciate that the way to
advance science was to experiment and observe, to collect facts and
discourage unfounded speculation. Silliman’s insight into the needs of
geologic science is shown in the following quotation (=1=, pp. 6, 7,
1818):
“Our geology, also, presents a most interesting field of inquiry. A
grand outline has recently been drawn by Mr. Maclure, with a masterly
hand, and with a vast extent of personal observation and labour: but
to fill up the detail, both observation and labour still more
extensive are demanded; nor can the object be effected, till more good
geologists are formed, and distributed over our extensive territory.
To account for the formation and changes of our globe, by excursions
of the imagination, often splendid and imposing, but usually
visionary, and almost always baseless, was, till within half a
century, the business of geological speculations; but this research
has now assumed a more sober character; the science of geology has
been reared upon numerous and accurate observations of _facts_; and
standing thus upon the basis of induction, it is entitled to a rank
among those sciences which Lord Bacon’s Philosophy has contributed to
create. Geological researches are now prosecuted by actually exploring
the structure and arrangement of districts, countries, and continents.
The obliquity of the strata of most rocks, causing their edges to
project in many places above the surface; their exposure, in other
instances on the sides or tops of hills and mountains; or, in
consequence of the intersection of their strata, by roads, canals, and
river-courses, or by the wearing of the ocean; or their direct
perforation, by the shafts of mines; all these causes, and others,
afford extensive means of reading the interior structure of the globe.
The outlines of American geology appear to be particularly grand,
simple, and instructive; and a knowledge of the important facts, and
general principles of this science, is of vast practical use, as
regards the interests of agriculture, and the research for useful
minerals. Geological and mineralogical descriptions, and maps of
particular states and districts, are very much needed in the United
States; and to excite a spirit to furnish them will form one leading
object of this Journal.”
_The Prolonged Influence of Outgrown Ideas._
Those interested in any branch of science should, as a matter of
education, read the history of that special subject. A knowledge of the
stages by which the present development has been attained is essential
to give a proper perspective to the literature of each period. Much of
the existing terminology is an inheritance from the first attempts at
nomenclature, or may rest upon theories long discarded. Popular notions
at variance with advanced teaching are often the forgotten inheritance
of a past generation.
Gneiss, trap, and Old Red Sandstone are names which we owe to Werner.
The “Tertiary period” and “drift” are relics of an early terminology.
The geology of tourist circulars still speaks of canyons as made by
“convulsions of nature.” Popular writers still attribute to geologists a
belief in a molten earth covered by a thin crust. Within the present
century the eighteenth century speculations of Werner and his
predecessors, postulating a supposed capacity of water to seep through
the crust into the interior of the earth, resulting in a hypothetical
progressive desiccation of the surface, views long abandoned by most
modern geologists, have been revived by an astronomer into a theory of
“planetology.”
A review of the literature of a century brings to light certain
tendencies in the growth of science. Each decade has witnessed a larger
accumulation of observed facts and a fuller classification of these
fundamental data, but the pendulum of interpretative theory swings away
from the path of progress, now to one side, now to the other, testing
out the proper direction. For decades the understanding of certain
classes of facts may be actually retrogressive. A retrospect shows that
certain minds, keen and unfettered by a prevailing theory, have in some
directions been in advance of their generation. But the judgment of the
times had not sufficient basis in knowledge for the separation and
acceptance of their truer views from the contemporaneous tangle of false
interpretations.
An interesting illustration of these statements regarding the slow
settling of opinion may be cited in regard to the significance of the
dip of the Triassic formations of the eastern United States. The strata
of the Massachusetts-Connecticut basin possess a monoclinal easterly dip
which averages about 20 degrees to the east. Those of the New
Jersey-Pennsylvania-Virginia basin possess a similar dip to the
northwest. Both basins are cut by great faults and the dip is now
accepted by practically all geologists as due to rotation of the crust
blocks away from a geanticlinal axis between the two basins. Edward
Hitchcock, whose work from the first shows an interpretative quality in
advance of his time, states in 1823 (=6=,74) regarding the dip of the
Connecticut valley rocks:
“There is reason to believe that Mount Toby, the strata of which are
almost horizontal, exhibits the original dip of these rocks, and that
those cases in which they are more highly inclined are the result of
some Plutonian convulsion. Such irregularity in the dip of coal fields
is no uncommon occurrence.”
In Hitchcock’s Geology of Massachusetts, published in 1833, ten years
later, geological structure sections of the Connecticut Valley rocks are
given, the facts are discussed in detail and the dip ascribed to the
elevatory forces. He says (l. c., pp. 213, 223):
“If it were possible to doubt that the new red sandstone formation was
deposited from water, the surface of some of the layers of this shale
would settle the question demonstrably. For it exhibits precisely
those gentle undulations, which the loamy bottom of every river with a
moderate current, presents. (No. 198.) But such a surface could never
have been formed while the layers had that high inclination to the
horizon, which many of them now present: so that we have here, also,
decisive evidence that they have been elevated subsequently to their
deposition....
The objection of a writer in the American Journal of Science, that
such a height of waters as would deposit Mount Toby, must have
produced a lake nearly to the upper part of New Hampshire, in the
Connecticut Valley, and thus have caused the same sandstone to be
produced higher up that valley than Northfield, loses its force, when
it is recollected that this formation was deposited before its strata
were elevated. For the elevating force undoubtedly changed the
relative level of different parts of the country. In this case, the
disturbing force must have acted beneath the primary rocks. And
besides, we have good evidence which will be shown by and by, that our
new red sandstone was formed beneath the ocean. We cannot then reason
on this subject from present levels.”
[Illustration: Courtesy of _Popular Science Monthly_.]
In 1840, H. D. Rogers, a geologist who has acquired a more widely known
name than Hitchcock, but who in reality showed an inferior ability in
interpretation, made the following statements in explanation of the
regional monoclinal dip of the New Jersey Triassic rocks averaging 15 to
20 degrees to the northwest:[78]
“Their materials give evidence of having been swept into this estuary,
or great ancient river, from the south and southeast, by a current
producing an almost universal dip of the beds towards the northwest, a
feature clearly not caused by any uplifting agency, but assumed
originally at the time of their deposition, in consequence of the
setting of the current from the opposite or southeastern shore.”
In 1842, at the third annual meeting of the Association of American
Geologists both H. D. and W. B. Rogers argued (=43=, 170, 1842) against
Sir Charles Lyell and E. Hitchcock that the present dip of the Triassic
was the original slope of deposition, stating among other reasons that
the footprints impressed upon the sediments often showed a slipping and
a pushing of the soft clay in the direction of the downhill slope. In
1858 H. D. Rogers still held to the same views of original dip,[79]
notwithstanding that a moderate amount of observation on the mud-cracked
and rain-pitted layers would have supplied the proof that such must have
dried as horizontal surfaces. The idea of inclined deposition is not yet
wholly dead as it has been suggested more than once within the present
generation as a means of escaping from the necessity of accepting the
very great thicknesses of this and similar formations. Thus, as Brögger
has remarked in another connection,—the ghosts of the old time stand
ever ready to reappear.
In the present essay on the rise of structural geology as reflected
through a century of publication in the Journal, attention will be given
especially to two fields, that of structures connected with igneous
rocks and that of structures connected with mountain making, and
emphasis will be placed upon the growth of understanding rather than
upon the accumulating knowledge of details. The growth in both of these
divisions of structural geology is well illustrated in the volumes of
the Journal.
_Structures and Relationships of Igneous Rocks._
_Opposed Interpretations of Plutonists and Neptunists._
During the first quarter of the nineteenth century the geologic
controversy between the Plutonists and Neptunists was at its height; the
Plutonists, following the Scotchman, Hutton, holding to the igneous
origin of basalt and granite, the Neptunists, after their German master,
Werner of Freiberg, maintaining that these rocks had been precipitated
from a primitive universal ocean. The Plutonists, although time has
shown them to have been correct in all essential particulars, were for a
generation submerged under the propaganda carried forward by the
disciples of Werner. The “Illustrations of the Huttonian Theory of the
Earth,” a remarkable classic, worthy of being studied to-day as well as
a century ago, was published in 1802 by John Playfair, professor of
mathematics in the University of Edinburgh and a friend of Hutton, who
had died five years previously. This volume was opposed by Robert
Jameson, professor of natural philosophy in the same university, who had
absorbed the ideas of the German school while at Freiberg and published
in 1808 a volume on the “Elements of Geognosy,” in which the philosophy
of Werner is followed throughout and even obsidian and pumice are argued
to be aqueous precipitates. The authority of the Wernerian autocracy
caused its nomenclature to be adopted in the new world, but strong
evidence against its interpretations was to be found in the actual
structural relations displayed by the igneous rocks.
_Contributions on Volcanic and Intrusive Rocks._
The accumulation and study of facts constituted the best cure for an
erroneous theory. The publications of the Journal contributed toward
this end by articles along several lines. The most original
contributions were those which dealt with the areal and structural
geology of eastern North America, but equally valuable at that time for
the broadening of scientific interest were the studies on the volcanic
activities of the Hawaiian Islands, published through many years.
Perhaps most valuable from the educative standpoint were the extensive
republications in the Journal of the more important European researches,
making them accessible to American readers. In volume =13= (1828), for
example, a digest of Scrope’s work on volcanoes is given, covering forty
pages; and of Daubeny on active and extinct volcanoes, running over
seventy-five pages and extending into vol. =14=. Through these
comprehensive studies the nature of volcanic action became generally
understood during the first half of the nineteenth century and the
original publications in the Journal were valuable in giving a knowledge
of the activities of the Hawaiian volcanoes.
Early in the nineteenth century the whole of America still remained to
be explored by the geologist. The regions adjacent to the centers of
learning were among the first to receive attention and the Triassic
basin of Connecticut and Massachusetts yielded information in regard to
the nature of igneous intrusion. This basin, of unmetamorphic shales and
sandstones, is occupied by the Connecticut River except at its southern
end. The Formation contains within it sills, dikes, and outflows of
basaltic rocks which because of their superior resistance to erosion
constitute prominent hills, in places bounded by cliffs.
Silliman in 1806[80] described East Rock, New Haven, Connecticut, as a
whinstone, trap, or basalt, and accounted for its presence on the
supposition that it had
“actually been melted in the bowels of the earth and ejected among the
superior strata by the force of subterraneous fire, but never erupted
like lava, cooling under the pressure of the superincumbent strata and
therefore compact or nonvesicular, its present form being due to
erosion.”
In these conclusions Silliman was correct. With but a limited amount of
experience he was able to discriminate between the intrusive and
effusive rocks and saw that the prominence of this hill was due to the
erosion of the sediments which once surrounded it.
An extensive paper on the geology of this region was published by Edward
Hitchcock in 1823,[81] then just thirty years of age. This paper shows
the evidence of extensive field observations, and his comments in regard
to the trap and granite are of interest. Hitchcock gives five pages to
the subject of “Greenstone Dykes in Old Red Sandstone” (=6=, 56–60,
1823) and makes the following statements:
“Professor Silliman conducted me to an interesting locality of these
in East-Haven. They occur on the main road from New-Haven to
East-Haven, less than half a mile from Tomlinson’s bridge ... (p. 56).
They are an interesting feature in our geology, and deserve more
attention; and it is peculiarly fortunate that they should be situated
so near a geological school and the first mineral cabinet in our
country ... (p. 58).
Origin of Greenstone.
Does the greenstone of the Connecticut afford evidence in favour of
the Wernerian or of the Huttonian theory of its origin? Averse as I
feel to taking a side in this controversy, I cannot but say, that the
man who maintains, in its length and breadth, the original hypothesis
of Werner in regard to the aqueous deposition of trap, will find it
for his interest, if he wishes to keep clear of doubts, not to follow
the example of D’Aubuisson, by going forth to examine the greenstone
of this region, lest, like that geologist, he should be compelled, not
only to abandon his theory, but to write a book against it. Indeed,
when surveying particular portions of this rock, I have sometimes
thought Bakewell did not much exaggerate when he said in regard to
Werner’s hypothesis, that, ‘it is hardly possible for the human mind
to invent a system more repugnant to existing facts.’
On the other hand, the Huttonian would doubtless have his heart
gladdened, and his faith strengthened by a survey of the greater part
of this rock. As he looked at the dikes of the old red sandstone, he
would almost see the melted rock forcing its way through the fissures;
and when he came to the amygdaloidal, especially to that variety which
resembles lava, he might even be tempted to apply his thermometer to
it, in the suspicion that it was not yet quite cool ... (p. 59).
By treating the subject in this manner I mean no disrespect to any of
the distinguished men who have adopted either side of this question.
To President Cooper especially, who regards the greenstone of the
Connecticut as volcanic, I feel much indebted for the great mass of
facts he has collected on the subject. And were I to adopt any
hypothesis in regard to the origin of our greenstone, it would be one
not much different from his” (p. 60).
By 1833 and more clearly in 1841 Hitchcock had come to recognize the
distinction between intrusive and extrusive basaltic sheets in the
Connecticut valley. Dawson also came to regard the Acadian sheets as
extrusive, and Emerson in 1882 recalled again the evidence for
Massachusetts (=24=, 195, 1882). Davis, however, went a step further and
by applying distinctive criteria not only separated intrusive and
extrusive sheets throughout the whole Triassic area, but by using basalt
flows as stratigraphic horizons unraveled for the first time the system
of faults which cut the Triassic system. His preliminary paper (=24=,
345, 1882) was followed by many others.
From 1880 onward begins the period of precise structural field work. The
older geologists mostly conceived their work after reconnaissance
methods. From 1870 to 1880 a group of younger men entered geology who
paid close attention to the solid geometry and mechanics of earth
structures. In their hands physical and dynamical geology began to
assume the standing of a precise and quantitative science. In the field
of intrusive rocks the opening classic was by Gilbert, who in his volume
on the geology of the Henry Mountains, published in 1880, made
laccoliths known to the world. With the beginning of this new period we
may well leave the subject of intrusive rocks and turn to the progress
of knowledge in regard to those deeper and vaster bodies now known as
batholiths. These, since erosion does not expose their bottoms, Daly
separates from intrusives and classifies as subjacent. The batholiths
consist typically of granite and granodiorite, and introduce us to the
problem of granite.
_Views on the Structural Relations of Granite._
Conscientious field observations were sufficient to establish the true
nature of the intrusive and extrusive rocks. The case was very
different, however, with the nature and relations of the great bodies of
granite, which may be taken in the structural sense as including all the
visibly crystalline acidic and intermediate rocks, known more
specifically as granite, syenite, and diorite.
The large bodies of granite, structurally classified as stocks, or
batholiths, commonly show wedges, tongues, or dike networks cutting into
the surrounding rocks. The relations, however, are not all so simple as
this. Granites may cover vast areas, they are usually the older rocks,
they are generally associated with regional metamorphism of the intruded
formations, which metamorphism is now understood to be due chiefly to
the heat and mineralizers given off from the granite magma, associated
with mashing and shearing of the surrounding rocks. The granite was
often injected in successive stages which alternated with the stages of
regional mashing. A parallel or gneissic structure is thus developed
which is in part due to mashing, in part to igneous injection. Where the
ascent of heat into the cover is excessive, or where blocks are detached
and involved in the magma, the latter may dissolve some of the older
cover rocks, even where these were of sedimentary origin.
Thus between mashing, injection, and assimilation the genetic
relationships of a batholith to its surroundings are in many instances
obscure. Nevertheless, attention to the larger relations shows that the
molten magma originated at great depths in the earth’s crust, far below
the bottoms of geosynclines, and consists of primary igneous material,
not of fused sediments. From those depths it has ascended by various
processes into the outer crust, where it crystallized into granite
masses, to be later exposed by erosion. The amount of material which can
be dissolved and assimilated must be small in comparison with the whole
body of the magma. The original composition of the magma was probably
basic, nearer that of a basalt than that of a granite. Differentiation
of the molten mass is thought to cause the upper and lower parts of the
chamber to become unlike, the lighter and more acidic portion giving
rise to the great bodies of granite. With the exception of certain
border zones the whole, however, is regarded as igneous rock risen from
the depths.
The complex border relations, but more particularly certain academic
hypotheses, led to a period of misunderstanding and retrogression in
regard to the nature of granites. It constitutes an interesting
illustration of the possibility of a wrong theory leading interpretation
astray, chiefly through the magnification of minor into major factors.
This history illustrates the dangers of qualitative science as compared
to quantitative, of a single hypothesis as matched against the method of
multiple working hypothesis. This flux of opinion in regard to the
nature of granites may be traced through the volumes of the Journal.
E. Hitchcock in 1824 (=6=, 12) noted that in places granite appeared
bedded, but in other places existed in veins which cut obliquely across
the strata. Silliman, although careful not to deny the aqueous origin of
some basalts, yet held that the field evidence of New England indicates
for that region the igneous or Huttonian origin of trap and granite
(=7=, 238, 1824).
In 1832 the following article by Hitchcock appeared in the Journal
(=22=, 1, 70):
Report on the Geology of Massachusetts; examined under the direction
of the Government of that State, during the years 1830 and 1831; by
Edward Hitchcock, Prof. of Chemistry and Natural History in Amherst
College.
A footnote adds that this is “published in this Journal by consent of
the Government of Massachusetts, and intended to appear also in a
separate form, and to be distributed among the members of the
Legislature of the same State, about the time of its appearance in
this work. It is, we believe, the first example in this country, of
the geological survey of an entire State.”
This article includes a geological map of the state and covers the
subject of economic geology. The report brought forth the following
remarks from a French reviewer in the _Revue Encyclopédique_, Aug. 1832,
quoted in the Journal (=23=, 389, 1833):
“A single glance at this report, is sufficient to convince any one of
the utility of such a work, to the state which has undertaken it; and
to regret that there is so very small a part of the French territory,
whose geological constitution is as well known to the public, as is
now the state of Massachusetts. France has the greater cause to regret
her being distanced in this race by America, from her having a corps
of mining engineers, who if they had the means, would, in a very short
time furnish a work of the same kind, still more complete, of each of
the departments.”
The complete report published in 1833 is a work of 700 pages. Pages 465
to 517 are devoted to the subject of granite. Numerous detailed sketches
are given showing contact relations. Nine pages are given to theoretical
considerations and many lines of proof are given that granite is an
igneous rock, molten from the internal heat of the earth, and intruded
into the sedimentary strata. His statement is the clearest published in
the world, so far as the writer is aware, up to that date, and marks
Edward Hitchcock as one of the leading geologists of his generation in
Europe as well as America. Unfortunately his views were largely lost to
sight during the following generation.
In 1840 the first American edition of Mantell’s Wonders of Geology gave
currency to the idea that granite is proved to be of all geological ages
up to the Tertiary (=39=, 6, 1840). In 1843 J. D. Dana pointed out
(=45=, 104) that schistosity was no evidence of sedimentary origin. He
regarded most granites as igneous as shown by their structural
relations, but considers that some may have had a sedimentary origin.
_Rise and Decline of the Metamorphic Theory of Granite._
Up to 1860 granite was regarded on the basis of the facts of the field
as essentially an intrusive rock, but gneiss as a metamorphic product
mostly of sedimentary origin. It seemed as though sound methods of
research and interpretation were securely established. Nevertheless, a
new era of speculation and a modified Wernerism arose at that time with
a paper by T. Sterry Hunt, marking a retrogression in the theory of
granite which lasted until his death in 1892.
In November, 1859, Hunt read before the Geological Society of London a
paper on “Some Points in Chemical Geology” in which he announced that
igneous rocks are in all cases simply fused and displaced sediments, the
fusion taking place by the rise of the earth’s internal heat into deeply
buried and water-soaked masses of sediments (see =30=, 133, 1860). The
germ of this idea of aqueo-igneous fusion was far older, due to Babbage
and John Herschel, neither of them geologists, but such sweeping
extensions of it had never before been published. Hunt had the advantage
of a wide acquaintanceship with geological literature and chemistry. He
wrote plausibly on chemical and theoretical geology, but his views were
not controlled by careful field observations. In fact he wrote
confidently on regions which apparently he had never seen and where a
limited amount of field work would have shown him to have been
fundamentally in error. A man of egotistical temperament, he sought to
establish priority for himself in many subjects and in order to cover
the field made many poorly founded assertions. Building on to another
Wernerian idea, he held that many metamorphic minerals had a chronologic
value comparable to fossils—staurolite for example indicating a
pre-Silurian age—and on this basis divided the crystalline rocks into
five series. Although there is much of value buried in Hunt’s work it is
difficult to disentangle it, with the result that his writings were a
disservice to the science of geology. Although carrying much weight in
his lifetime, they have passed with his death nearly into oblivion.
Marcou, with a limited knowledge of American geology, and but little
respect for the opinions of others, had published a geologic map of the
United States containing gross errors. In support of his views he read
in November, 1861, a paper on the Taconic and Lower Silurian Rocks of
Vermont and Canada. In the following year he was severely reviewed by
“T,” who states positively in controverting Marcou (=33=, 282, 283,
1862) that “the granites (of the Green Mountains) are evidently strata
altered in place.”
“Mr. Marcou should further be informed that the granites of the Alpine
summits, instead of being, as was once supposed, eruptive rocks, are
now known to be altered strata of newer Secondary and Tertiary age. A
simple structure holds good in the British Islands, where as Sir
Roderick Murchison has shown in his recent Geological map of Scotland,
Ben Nevis and Ben Lawers are found to be composed of higher strata,
lying in synclinals. This great law of mountain structure would alone
lead us to suppose that the gneiss of the Green mountains, instead of
being at the base, is really at the summit of the series....
We cannot here stop to discuss Mr. Marcou’s remark about ‘the
unstratified and oldest crystalline rocks of the White mountains’
which he places beneath the lower Taconic series. Mr. Lesley has shown
that these granites are stratified, and with Mr. Hunt, regards them as
of Devonian Age. (This Journal, vol. =31=, p. 403.) Mr. Marcou has
come among us with notions of mountains upheaved by intrusive
granites, and similar antiquated traditions, now, happily for science,
well nigh forgotten.”
It is seen that Marcou, notwithstanding the general character of his
work, happened to be nearer right in some matters than were his critics,
and that “T” had adopted to the limit the views of Hunt.
The recovery of geology from this period of confusion was partly owing
to the slow accumulation of opposed facts; especially to a recognition
of the fact that the overplaced relation of the granite gneisses in
western Scotland was due to great overthrusts; also to the evidence of
the clearly intrusive nature of many of the Cordilleran granites. The
recovery of a sounder theory was hastened, however, by the application
of criticisms by J. D. Dana in the Journal. In 1866 (=42=, 252) Dana
pointed out that sedimentary rocks in Pennsylvania, in Nova Scotia, and
other regions which had been buried to a depth of at least 16,000 feet
are not metamorphic. Mere depth of burial of sediments was not
sufficient therefore to produce metamorphism and aqueo-igneous fusion.
The baseless and speculative character of the use of minerals as an
index of age and of Hunt’s interpretation of New England geology in
general was shown by Dana in 1872 (=3=, 91). The following year Dana
pointed out clearly that igneous eruptions in general have been derived
from a deep-seated source and did not come from the aqueo-igneous fusion
of sediments. As to gradations between true igneous rocks and fused and
displaced sediments he makes the following statements (=6=, 114, 1873):
“Again, the plastic rock-material that may be derived from the fusion
or semifusion of the supercrust, (that is, of rocks originally of
sedimentary origin,) gives rise to “igneous” rocks often not
distinguishable from other igneous rocks, when it is ejected through
fissures far from its place of origin; while crystalline rocks are
simply _metamorphic_ if they remain in their original relations to the
associated rocks, or nearly so.
Between these latter igneous rocks and the metamorphic there may be
indefinite gradations, as claimed by Hunt. But if our reasonings are
right, the great part of igneous rocks can be proved to have had no
such supercrust origin. The argument from the presence of moisture or
of hydrous minerals in such rocks in favor of their origin from the
fusion of sediments has been shown to be invalid.”
The injected marginal rocks and the post-intrusive metamorphism of most
of the New England granites has, however, obscured more or less their
real igneous nature so that the gradation from metamorphic sediments
through igneous gneisses to granites could be read in either direction.
These features misled Dana who accepted the prevailing idea of the
general metamorphic origin of granite. Dana makes the following
statement (=6=, 164, 1873):
“But Hunt is right in holding that in general granite and syenite (the
quartz-bearing syenite) are undoubtedly metamorphic rocks where not
vein-formations, as I know from the study of many examples of them in
New England; and the veins are results of infiltration through heated
moisture from the rocks adjoining some part of the opened fissures
they fill.”
Granite, although regarded at this time as the extreme of the
metamorphic series and originating from sediments, was looked upon as
typically Archean in age, though in some cases younger. Such a doctrine
permitted such extreme misinterpretations as that of Clarence King and
S. F. Emmons on the nature of the intrusive granite of the Little
Cottonwood canyon in the Wahsatch Range. This body cuts across 30,000
feet of Paleozoic rocks and to the careful observer, as later admitted
by Emmons, shows clear evidence of its transgressive nature. But at that
time it was generally considered that granite mountains were capable of
resisting the erosion of all geological time. Consequently it did not
seem incredible to King and his associates that here a great granite
range of Archean origin had stood up through Paleozoic time until
gradual subsidence had permitted it to be buried beneath 30,000 feet of
sediments.[82]
It may seem to the present day reader that such a misinterpretation,
doing violence to fundamental geologic knowledge as now recognized, was
inexcusable; but in the light of the history of geology as here detailed
it is seen to have been the interpretation natural to that time. It is
true that a careful examination of the facts of that very field would
have proved the post-Paleozoic and intrusive nature of that great
granite body now known as the Little Cottonwood batholith, but Emmons
has explained the rapid and partial nature of the observations which
they were compelled to make in order to keep up to their schedule of
progress (=16=, 139, 1903).
Whitney had found some years earlier that the granites of the Sierra
Nevada were igneous rocks intrusive into the Triassic and Jurassic
strata. The Lake Superior geologists began to show in the eighties that
granite was there an intrusive igneous rock. R. D. Irving and Wadsworth
noted these relations. Lawson in 1887 pointed out emphatically (=33=,
473) that the granites of the Rainy Lake region, although basal, were
younger than the schists which lay above them. The granite gneisses he
held were of clearly the same igneous origin as the granites and neither
gave any field evidence of being fused and displaced sediments. From
this time forward the truly igneous nature of granite became
increasingly accepted until now the notion of its being made of
sedimentary rocks softened and recrystallized by the rise of the
isogeotherms through deep burial is as obsolete as the still older
doctrine of the Neptunists that granite was laid down as a crystalline
precipitate on the floor of the primitive ocean.
The recognition of the truly igneous nature of granites has been
followed in the present generation by a series of studies on their
structural relations and mode of genesis. A number of important initial
articles on various aspects of structure and contact relations have
appeared in the Journal, but this sketch of the history of the subject
may well stop with the introduction to this modern period.
_Orogenic Structures._
_Views of Plutonists and Neptunists._
Orogenic structures are, as the name implies, those connected with the
birth of mountains. Nearly synonymous terms are deformative or secondary
structures. On a small scale this division embraces the phenomena
exposed in the rock ledge or quarry face, or in the dips and
dislocations varying from one exposure to another. These structures
include faults, folds, and foliation. On a larger scale are included the
relations of the different ranges of a mountain system to each other,
relations to previous geologic history, relations to the earth as a
whole, and to the forces which have generated the structures.
In order to see the stage of development of this subject in 1818 and its
progress as reflected through the publications of a century, more
particularly in the Journal, it is desirable to turn again to those two
treatises emanating from Edinburgh at the beginning of the nineteenth
century and representing two opposite schools of thought, the Plutonists
and Neptunists.
Playfair, in 1802, devotes nineteen pages to the subject of the
inflection and elevation of strata.[83] He places emphasis on the
characteristic parallelism of the strike of the folds throughout a
region, as shown through the intersection of the folds by a horizontal
plane of erosion. He contrasts this with the arches shown in a
transverse section and enlarges on our ability to study the deeply
buried strata through the denudation of the folded structure. He argues
from these relations that the structures can not be explained by the
vague appeal of the Neptunists to forces of crystallization, to slopes
of original deposition, or to sinking in of the roofs of caverns. The
causes he argues were heat combined with pressure. As to the directions
in which the pressure acted he is not altogether clear, but apparently
regards the pressure as acting in upward thrusts against the sedimentary
planes, the latter yielding as warped surfaces. His method of
presentation is that of inductive reasoning from facts, but he stopped
short of the conception of horizontal compression through terrestrial
contraction.
Jameson, professor of natural history in the same university, in 1808
contemptuously ignores the work of Hutton and Playfair in what he calls
the “_monstrosities_ known under the name of Theories of the Earth.” In
a couple of pages he confuses and dismisses the whole subject of
deformation. He states:[84]
“It is therefore a fact, that all inclined strata, with a very few
exceptions, have been formed so originally, and do not owe their
inclination to a subsequent change.
When we examine the structure of a mountain, we must be careful that
our observations be not too micrological, otherwise we shall
undoubtedly fail in acquiring a distinct conception of it. This will
appear evident when we reflect that the geognostic features of Nature
are almost all on the great scale. In no case is this rule to be more
strictly followed than in the examination of the stratified structure.
By not attending to this mode of examination, geognosts have fallen
into numberless errors, and have frequently given to extensive tracts
of country a most irregular and confused structure. Speculators
building on these errors have represented the whole crust of the globe
as an irregular and unseemly mass. It is indeed surprising, that men
possessed of any knowledge of the beautiful harmony that prevails in
the structure of organic beings could for a moment believe it
possible, that the great fabric of the globe itself,—that magnificent
display of Omnipotence,—should be destitute of all regularity in its
structure, and be nothing more than a heap of ruins.”
This was the attitude of a leader of British opinion toward the subject
of deformational geology from which the infant science had to recover
before progress could be made. The early maps were essentially
mineralogical and lithological. The order of superposition and the
consequent sequence of age was regarded as settled by Werner in Germany
and not requiring investigation in America. The early examples of
structure were sections drawn with exaggerated vertical scales and those
of Maclure do not show detail.
_Recognition of Appalachian Structures._
Following the founding of the Journal in 1818 there is observable a
growth in the quality and detail of geological mapping. Dr. Aiken,
professor of natural philosophy and chemistry in Mt. St. Mary’s College,
published in the Journal in 1834 (=26=, 219) a vertical section
extending between Baltimore and Wheeling, a distance of nearly 250
miles, on a scale of about 7 miles per inch. The succession of rocks is
carefully shown and the direction of dip, but no attempt is made to show
the underground relations, the stratigraphic sequence, and the folded
structures which are so clear in that Appalachian section. The text also
shows that the author had not recognized the folded structure.
Furthermore, where the folds cease at the Alleghany mountain front, the
flat strata are shown as resting unconformably on the folded rocks to
the east.
R. C. Taylor, geologist, civil and mining engineer, was from 1830 to
1835 the leading student of Pennsylvanian geology as shown by the
publication in 1835 of four papers aggregating over 80 pages in the
Transactions of the Geological Society of Pennsylvania. His work is
noticeable for accuracy in detail and no doubt was influential in
setting a high standard for the state geological survey which
immediately followed.
H. D. and W. B. Rogers have been given credit in this country, and in
Europe also, as being the leading expounders of Appalachian structure.
Merrill speaks of H. D. Rogers as unquestionably the leading structural
geologist of his time.[85] To the writer, this attributed position
appears to be due to his opportunities rather than to scientific acumen.
The magnificent but readily decipherable folded structure of
Pennsylvania, the relationships of coal and iron to this structure, the
considerable sums of money appropriated, and the work of a corps of able
assistants were factors which made it comparatively easy to reach
important results. In ability to weigh facts and interpret them Edward
Hitchcock showed much more insight than H. D. Rogers, while in the
philosophic and comprehensive aspects of the subject J. D. Dana far
outranks him.
H. D. Rogers in his first report on the geological survey of New Jersey,
1836, recognizes that the Cambro-Silurian limestones (lower Secondary
limestones) were deposited as nearly horizontal beds and the ridges of
pre-Cambrian gneiss (Primary) had been pushed up as anticlinal axes (p.
128). He also clearly recognized the distinction between slaty cleavage
and true dip as shown in the Ordovician slates (p. 97). Between 1836 and
1840 he had learned a great deal on the nature of folds as is shown in
his Pennsylvania report for 1839 and the structure sections in his New
Jersey report for 1840.
R. C. Taylor, who had now become president of the board of directors of
the Dauphin and Susquehanna Coal Company, published in the Journal in
1841 (=41=, 80) an important paper entitled “Notice of a Model of the
Western portion of the Schuylkill or Southern Coal Field of
Pennsylvania, in illustration of an Address to the Association of
American Geologists, on the most appropriate modes for representing
Geological Phenomena.” In this paper he calls attention to the value of
modeling as a means of showing true relations in three dimensions. He
condemns the custom prevalent among geologists of showing structure
sections with an exaggerated vertical scale with its resultant
topographic and structural distortions. Taylor was widely acquainted
with the structure of Pennsylvania, Maryland, and Virginia.
_Nature of Forces Producing Folding._
In 1825 Dr. J. H. Steele sent to Professor Silliman two detailed
drawings and description of an overturned fold at Saratoga Lake, New
York. As to the significance of this feature Steele makes the following
statement (=9=, 3, 1825):
“It is impossible to examine this locality without being strongly
impressed with the belief that the position which the strata here
assume could not have been effected in any other way than by a power
operating from beneath upwards and at the same time possessing a
progressive force; something analogous to what takes place in the
breaking up of the ice of large rivers. The continued swelling of the
stream first overcomes the resistance of its frozen surface and having
elevated it to a certain extent, it is forced into a vertical
position, or thrown over upon the unbroken stratum behind, by the
progressive power of the current.”
So far as the present writer is aware this is the first recognition in
geological literature of the evidence of a horizontally compressive and
overturning force as a cause of folding.
To E. Hitchcock belongs the credit of being the first to describe
overturning and inversion of strata on a large scale, but without
clearly recognizing it as such. In western Massachusetts metamorphism is
extreme in the lower Paleozoic rocks in the vicinity of the overthrust
mass of Archean granite-gneiss which constitutes the Hoosic range. The
Paleozoic rocks of the valley to the west are overturned and appear to
dip beneath the older rocks. Farther west the metamorphism fades out and
the series assumes a normal position. Such an inverted relation, up to
that time unknown, is described in 1833 as follows by Hitchcock in his
Geology of Massachusetts (pp. 297, 298):
“But a singular anomaly in the superposition of the series of rocks
above described, presents a great difficulty in this case. The strata
of these rocks almost uniformly dip to the east: that is, the newer
rocks seem to crop out beneath the older ones; so that the saccharine
limestone, associated with gneiss in the eastern part of the range,
seems to occupy the uppermost place in the series. Now as
superposition is of more value in determining the relative ages of
rocks than their mineral characters, must we not conclude that the
rocks, as we go westerly from Hoosac mountain, do in fact belong to
older groups? The petrifactions which some of them contain, and their
decidedly fragmentary character, will not allow such a supposition to
be indulged for a moment. It is impossible for a geologist to mistake
the evidence, which he sees at almost every step, that he is passing
from older to newer formations, just as soon as he begins to cross the
valley of Berkshire towards the west. We are driven then to the
alternative of supposing, either that there must be a deception in the
apparent outcrop of the newer rocks from beneath the older, or that
the whole series of strata has been actually thrown over, so as to
bring the newest rocks at the bottom. The latter supposition is so
improbable that I cannot at present admit it.”
Hitchcock tried to reconcile the evidence by a series of unconformities
and inclined deposition, but finds the solution unsatisfactory.
In this same year, 1833, Elie de Beaumont, a distinguished French
geologist, published his theory of the origin of mountains. He advanced
the idea that since the globe was cooling it was condensing, and the
crust, already cool, must suffer compression in adjusting itself to the
shrinking molten interior. He concluded from the evidence shown in
Europe that the collapse of the crust occurred violently and rapidly at
widely spaced intervals of time. This hypothesis introduced the idea of
mountain folding by horizontal compressive forces. The theoretical paper
of de Beaumont, together with further observations by Hitchcock and
others, led the latter in 1841 to a final belief in the inversion of
strata on a large scale by horizontal compression. His conclusions are
expressed in an important paper published in the Journal (=41=, 268,
1841) and given on April 8, 1841, as the First Anniversary Presidential
Address before the Association of American Geologists. This
comprehensive summary of American geology occupies 43 pages. Three pages
are given to the inverted structure of the Appalachians from which the
following paragraphs may be quoted:
“We have all read of the enormous dislocations and inversions of the
strata of the Alps; and similar phenomena are said to exist in the
Andes. Will it be believed, that we have an example in the United
States on a still more magnificent scale than any yet described?...
Let us suppose the strata between Hudson and Connecticut rivers, while
yet in the plastic state, (and the supposition may be extended to any
other section across this belt of country from Canada to Alabama,) and
while only slightly elevated, were acted upon by a force at the two
rivers, exerted in opposite directions. If powerful enough, it might
cause them to fold up into several ridges; and if more powerful along
the western than the eastern side, they might fall over so as to take
an inverted dip, without producing any remarkable dislocations, while
subsequent denudation would give to the surface its present
outline....
Fourthly, we should readily admit that such a plication and inversion
of the strata might take place on a small scale. If for instance, we
were to press against the extremities of a series of plastic layers
two feet long, they could easily be made to assume the position into
which the rocks under consideration are thrown. Why then should we not
be equally ready to admit that this might as easily be done, over a
breadth of fifty miles, and a length of twelve hundred, provided we
can find in nature, forces sufficiently powerful? Finally, such forces
do exist in nature, and have often been in operation.”
The advanced nature of these conceptions may be appreciated by
contrasting them with those put forth by H. D. and W. B. Rogers on April
29, 1842, before the third annual meeting of the same body (=43=, 177,
1842) and repeated by them before the British Association at Manchester
two months later. In their own words, the Rogers brothers from their
studies on the folds shown in Pennsylvania and Virginia, conceived
mountain folds in general to be produced by much elastic vapor escaping
through many parallel fissures formed in succession, producing violent
propulsive wave oscillations on the surface of the fluid earth beneath a
thin crust. Thus actual billows are assumed to have rolled along through
the crust. They did not think tangential pressure alone could produce
folds. Such pressures were regarded as secondary, produced by the
propagation of the waves and the only expression of tangential forces
which they admitted was to fix the folds and hold them in position after
the violent oscillation had subsided (=44=, 360, 1843). The leading
British geologists De la Beche and Sedgwick criticized adversely this
remarkable theory, stating that they could see no such analogy in
mountain folds to violent earthquake waves and that in their opinion the
slow application of tangential force was sufficient to account for the
phenomena (=44=, 362–365, 1843).
H. D. Rogers in the prosecution of the geological survey of Pennsylvania
displayed notable organizing ability and persistence in accomplishment,
even to advancing personally considerable sums of money, trusting to the
state legislature to later reimburse him. Finally, after many delays by
the state, the publication was placed directly in his charge and he
produced in 1858 a magnificent quarto work of over 1,600 pages,
handsomely illustrated, and accompanied by an atlas. It is excellent
from the descriptive standpoint, standing in the first class. Measured
as a contribution to the theory of dynamical geology, the explanatory
portions were, however, thirty years behind the times. The same
hypotheses are put forth in 1858 as in 1842. There is no acceptance of
the views of Lyell concerning the uniformitarian principles expounded by
this British leader in 1830, or of the nature of orogenic forces as
published by Elie de Beaumont in 1833. Rogers rejects the view that
cleavage is due to compression and suggests “that both cleavage and
foliation are due to the parallel transmission of planes or waves of
heat, awakening the molecular forces, and determining their
direction.”[86] Thus a mere maze of words takes the place of inductive
demonstrations already published.
In following the play of these opposing currents of geologic thought we
reach now the point where a period of brilliant progress in the
knowledge of mountains and of continental structures begins in the work
of J. D. Dana. In 1842 Dana returned from the Wilkes Exploring
Expedition and the following year began the publication of the series of
papers which for the next half century marked him as the leader in
geologic theory in America. His work is of course to be judged against
the background of his times. His papers mark distinct advances in many
lines and are characterized throughout by breadth of conception and
especially by clear and logical thinking. His work was published very
largely in the Journal, of which after a few years he became chief
editor. His first contribution on the subject of mountain structures,
entitled “Geological results of the earth’s contraction in consequence
of cooling,” was published in 1847 (=3=, 176). The evidence of
horizontal pressure was first perceived in France as shown by the
features of the Alps. Elie de Beaumont connected it, by means of the
theory of a cooling and contracting globe, with the other large fact of
the increase of temperature with descent in the crust. Dana credits the
Rogers brothers with first making known the folded structures of the
Appalachians, but objects to their interpretation of origin. He showed
by means of diagrams that the folds are to be explained by lateral
pressure, the direction of overturning indicating the direction from
which the driving force proceeded.
The Rogers brothers and especially James Hall, in working out the
Appalachian stratigraphy, had noted that the formations, although
accumulating to a maximum thickness of between 30,000 and 40,000 feet,
showed evidences that the successive formations were deposited in
shallow water. It suggested to them that the weight of the accumulating
sediments was the cause of subsidence, each foot of sediment causing a
foot of down sinking. This idea has continued to run through various
text books in geology for half a century, yet Dana early saw the fallacy
and in 1863 in the first edition of his Manual of Geology (p. 717)
states “whether this is an actual cause or not in geological dynamics is
questionable.” In 1866 in an important article on “Observations on the
origins of some of the earth’s features,” Dana deals more fully and
finally with this subject (=42=, 205, 252, 1866). He shows that such an
effect of accumulating sediment postulates a delicate balance, a very
thin crust and no resistance below. If such a weakness were granted it
would be impossible for the earth to hold up mountains. Furthermore such
subsidence was not regular during its progress and finally in the long
course of geologic time gave place to a reverse movement of elevation.
Hall had pointed out the fact that the sediments were thickest on the
east in the region of mountain folding and thinned out to a fraction of
this thickness in the broad Mississippi basin. Hall argued that the mere
subsidence of the trough would produce the observed folding and that the
folding was unrelated to mountain making or crustal shortening. In
supposed proof he cited the fact that the Catskills consist of unfolded
rock, are higher than the folded region to the south, and nearly as high
as the highest metamorphic mountains to the east.[87] Hall and all his
contemporaries were handicapped in their geological theories by a
complete inappreciation of the importance of subaërial denudation. For
subscribing to these errors of their time even the ablest men should not
be held responsible. Hall was the most forcible personality in geology
in his generation. His contributions to paleontology were superb. His
perception of the relation existing between troughs of thick sediments
and folded structures was a contribution of the first importance; yet in
the structural field his argument as to the production of the
Appalachian folds by mere subsidence during deposition indicates a
remarkable inability to apply the logical consequences of his hypothesis
to the nature of the folds as already made known by the Rogers. Dana
pointed out in reply to Hall that the folding did not correspond to the
requirements of Hall’s hypothesis, especially as the folding took place
not during, but after the close of the vast Paleozoic deposition. Dana
states in conclusion on Hall’s hypothesis (=42=, 209, 1866) that “It is
a theory of the origin of mountains with the origin of mountains left
out.”
_The Theory of Geosynclines and Geanticlines._
The fact that systems of folded strata lie along axes of especially
thick sediments and that this implied subsidence during deposition was
Hall’s contribution to geologic theory, but curiously enough he failed,
as shown, to connect it with the subsequent nature of mountain folding.
He did not see why such troughs should be weak to resist horizontal
compression. The clear recognition of this relationship was the
contribution of Le Conte, who in a paper on “A theory of the formation
of the great features of the earth’s surface” (=4=, 345, 460, 1872),
reached the conclusion that “mountain chains are formed by the mashing
together and the up-swelling of sea bottoms where immense thicknesses of
sediment have accumulated.”
As to the cause why mashing should take place along troughs of thick
sediments Le Conte adopts the hypothesis of aqueo-igneous fusion
proposed independently long before by Babbage and Herschel and
elaborated into a theory of igneous rocks by Hunt. Under this view, as
the older sediments became deeply buried, the heat of the earth’s
interior ascended into them, and since they included the water of
sedimentation a softening and metamorphism resulted. Dana had shown,
however, six years previously (=42=, 252, 1866), as the following
quotation will indicate, that metamorphism of sediments required more
than deep burial and that no such weakening as was postulated by
Herschel had occurred:
“The correctness of Herschel’s principle cannot be doubted. But the
question of its actual agency in ordinary metamorphism must be decided
by an appeal to facts; and on this point I would here present a few
facts for consideration.
The numbers and boldness of the flexures in the rocks of most
metamorphic regions have always seemed to me to bear against the view
that the heat causing the change had ascended by the very quiet method
recognized in this theory....
But there are other facts indicating a limited sufficiency to this
means of metamorphism. These are afforded by the great faults and
sections of strata open to examination. In the Appalachian region,
both of Virginia and Pennsylvania, faults occur, as described by the
Professors Rogers, and by Mr. J. P. Lesley, which afford us important
data for conclusions. Mr. Lesley, an excellent geologist and
geological observer, who has explored personally the regions referred
to, states that at the great fault of Juniata and Blair Cos.,
Pennsylvania, the rocks of the Trenton period are brought up to a
level with those of the Chemung, making a dislocation of at least
16,000, and probably of 20,000, feet. And yet the Trenton limestone
and Hudson River shales are not metamorphic. Some local cases of
alteration occur there, including patches of roofing slate; but the
greater part of the shales are no harder than the ordinary shales of
the Pennsylvania Coal formation.
At a depth of 16,000 feet the temperature of the earth’s crust,
allowing an increase of 1° F. for 60 feet of descent, would be about
330° F.; or with 1° F. for 50 feet, about 380° F.—either of which
temperatures is far above the boiling point of water; and with the
thinner crust of Paleozoic time the temperature at this depth should
have been still higher. But, notwithstanding this heat, and also the
compression from so great an overlying mass, the limestones and shales
are not crystalline. The change of parts of the shale to roofing slate
is no evidence in favor of the efficiency of the alleged cause; for
such a cause should act uniformly over great areas.”
The next contribution to the theory of orogeny was a series of papers
published in 1873 by Dana, entitled “On some results of the earth’s
contraction from cooling, including a discussion on the origin of
mountains and the nature of the earth’s interior.”[88] This
contribution, viewed as a whole, ranks among the first half dozen papers
on the science of mountains. The following quoted paragraphs give a view
of the scope of this article:
“_Kinds and Structure of Mountains._”
“While mountains and mountain chains all over the world, and low
lands, also, have undergone uplifts, in the course of their long
history, that are not explained on the idea that all mountain
elevating is simply what may come from plication or crushing, the
_component parts_ of mountain chains, or those simple mountains or
mountain ranges that are the product of one _process of making_—may
have received, _at the time of their original making_, no elevation
beyond that resulting from plication.
This leads us to a grand distinction in orography, hitherto neglected,
which is fundamental and of the highest interest in dynamical geology;
a distinction between—
1. A simple or _individual_ mountain mass or range, which is the
result of _one process of making_, like an individual in any process
of evolution, and which may be distinguished as a _monogenetic_ range,
being _one in genesis_; and
2. A composite or _polygenetic_ range or chain, made up of two or more
monogenetic ranges combined.
The Appalachian chain—the mountain region along the Atlantic border of
North America—is a _polygenetic_ chain; it consists, like the Rocky
and other mountain chains, of several _monogenetic_ ranges, the more
important of which are: 1. The Highland range (including the Blue
Ridge or parts of it, and the Adirondacks also, if these belong to the
same process of making) pre-Silurian in formation; 2. The Green
Mountain range, in western New England and eastern New York, completed
essentially after the Lower Silurian era or during its closing period;
3. The Alleghany range, extending from southern New York southwestward
to Alabama, and completed immediately after the Carboniferous age.
The making of the Alleghany range was carried forward at first through
a long-continued subsidence—a _geosynclinal_ (not a _true_ synclinal,
since the rocks of the bending crust may have had in them many true or
simple synclinals as well as anticlinals), and a consequent
accumulation of sediments, which occupied the whole of Paleozoic time;
and it was completed, finally, in great breakings, faultings and
foldings or plications of the strata, along with other results of
disturbance.
These examples exhibit the characteristics of a large class of
mountain masses or ranges. A geosynclinal accompanied by sedimentary
depositions, and ending in a catastrophe of plications and
solidification, are the essential steps, while metamorphism and
igneous ejections are incidental results. The process is one that
produces final stability in the mass and its annexation generally to
the more stable part of the continent, though not stable against
future oscillations of level of _wider range_, nor against denudation.
It is apparent that in such a process of formation elevation by direct
uplift of the underlying crust has no necessary place. The attending
plications may make elevations on a vast scale and so also may the
shoves upward along the lines of fracture, and crushing may sometimes
add to the effect; but elevation from an upward movement of the
downward bent crust is only an incidental concomitant, if it occur at
all.
We perceive thus where the truth lies in Professor Le Conte’s
important principle. It should have in view alone _monogenetic_
mountains and these only _at the time of their making_. It will then
read, plication and shovings along fractures being made more prominent
than crushing:
Plication, shoving along fractures and crushing are the true sources
of the elevation that takes place _during the making_ of geosynclinal
monogenetic mountains.
And the statement of Professor Hall may be made right if we recognize
the same distinction, and, also, reverse the order and causal relation
of the two events, accumulation and subsidence; and so make it read:
Regions of monogenetic mountains were, previous, and preparatory, to
the making of the mountains, areas each of a slowly progressing
geosynclinal, and, _consequently_, of thick accumulations of
sediments.
The prominence and importance in orography of the mountain
individualities described above as originating through a geosynclinal
make it desirable that they should have a distinctive name; and I
therefore propose to call a mountain range of this kind a
_synclinorium_, from _synclinal_ and the Greek ὄρος, mountain.
This brings us to another important distinction in orographic
geology—that of a second kind of monogenetic mountain. The
_synclinoria_ were _made through a progressing geosynclinal_. Those of
the second kind, here referred to, were _produced by a progressing
geanticlinal_. They are simply the upward bendings in the oscillations
of the earth’s crust—the geanticlinal waves, and hardly require a
special name. Yet, if one is desired, the term _anticlinorium_, the
correlate of _synclinorium_, would be appropriate. Many of them have
disappeared in the course of the oscillations; and yet, some may have
been for a time—perhaps millions of years—respectable mountains.
The geosynclinal ranges or synclinoria have experienced in almost all
cases, since their completion, true elevation through great
geanticlinal movements, but movements that embraced a wider range of
crust than that concerned in the preceding geosynclinal movements,
indeed a range of crust that comes strictly under the designation of a
polygenetic mass.”
“_The Condition of the Earth’s Interior._”
“The condition of the earth’s interior is not among the geological
results of contraction from cooling. But these results offer an
argument of great weight respecting the earth’s interior condition,
and make it desirable that the subject should be discussed in this
connection. Moreover, the facts throw additional light on the
preceding topic—the origin of mountains.
It seems now to be demonstrated by astronomical and physical
arguments—arguments that are independent, it should be noted, of
direct geological observation—that the interior of our globe is
essentially solid. But the great oscillations of the earth’s surface,
which have seemed to demand for explanation a liquid interior, still
remain facts, and present apparently a greater difficulty than ever to
the geologist. Professor Le Conte’s views, in volume iv, were offered
by him as a method of meeting this difficulty; yet, as he admits in
his concluding remarks, the oscillations over the interior of a
continent, and the fact of the greater movements on the borders of the
larger ocean, were left by him unexplained. Yet these oscillations are
not more real than the changes of level or greater oscillations which
occurred along the sea border, where mountains were the final result;
and this being a demonstrated truth, no less than the general solidity
of the earth’s interior, the question comes up, how are the two truths
compatible?
The geological argument on the subject (the only one within our
present purpose) has often been presented. But it derives new force
and gives clearer revelations when the facts are viewed in the light
of the principles that have been explained in the preceding part of
this memoir.
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:
C. E. Dutton, Critical observations on theories of the earth’s
physical evolution, The Penn Monthly, May and June, 1876.
Footnote 92:
B. Willis, Research in China, vol. 2, 1907.
Footnote 93:
Joseph Barrell, Science, 39, 259, 260, 1909; Jour. Geol., 22, 672–683,
1914.
Footnote 94:
T. C. Chamberlin, Geology, vol. 1, pp. 541, 542, 1904.
Footnote 95:
W. M. Davis, The geological dates of origin of certain topographic
forms on the Atlantic slope of the United States, Geol. Soc. Am.
Bull., 2, 541–542, 545–586, 1891.
V
A CENTURY OF GOVERNMENT GEOLOGICAL SURVEYS