. . . And Still We Evolve
A Handbook on the
History of Modern Science
[This handbook, which has been prepared by Ian
Johnston of Malaspina University-College, Nanaimo, BC, for Liberal Studies
students, is in the public domain and may be used without charge and without
permission, released May 2000]
Section
Two: The Early Development of Modern Geology
Geology
and Religion: The Problem of Historical Science
The
major 18th century challenge to orthodox religion and traditional order came
not from astronomy or physics, which, as we have seen, generally supported
natural theology (except in their extreme materialistic version), but from the
growing interest in geology, the study of the earth (1).
And the enquiries undertaken by the geologists, which proved so essential to
19th century biology, not least of all to Darwin, were to have
revolutionary consequences for religion and society, in many respects
consequences far more fundamental than those brought about by the
Copernican-Newtonian revolution in physics.
One
of the major reasons why geology and biology proved so devastating had to do
with the way in which they introduced a new element into the human
understanding of the world, the study of phenomena historically, that is, as
the unique results of a process of change. As Stephen Jay Gould has remarked,
once the astronomers set the stage, the geologists and biologists then set about
attending to the plot.
For
the most part, constructing a narrative of the earth's origins had not been an
issue raised by the great astronomers. They had been primarily concerned with
establishing the eternal design of the cosmos and the permanent mathematical
laws of motion. How the universe developed from a more primitive state to its
present order they were content to ascribe to myth or to religion.
One
instructive exception to the above remarks is Descartes. In his theory of
vortices he had offered a hypothesis to explain the development of the solar
system in naturalistic terms. However, Newton demonstrated that Descartes's
idea violated Kepler's laws, and in so doing Newton stressed that the only
satisfactory account for the present structure of our solar system was that it
had been created by God in its present form:
. . . it is unphilosophical to seek for
any Origin of the World [except divine creation], or to pretend that it might
arise out of a Chaos by the mere Laws of nature; though being once form'd, it
may continue by those Laws for many Ages.
Thus,
Newton's physics did not, in itself, promote a historical approach to
scientific problems; indeed, it was hostile to such a hypothesis.
It
is easy to see why the revolution in our conceptions of space which the
astronomers effected should have had less challenging impact than the
revolution in our conceptions of time which the geologists and biologists
introduced. Placing the sun rather than the earth in the centre of our solar
system and insisting on the infinite distances of space were certainly
potentially disturbing ("The eternal silences of the infinite frighten
me," wrote Pascal in Fragment 206 of the Pensées). However, they
did not directly challenge revealed religion or established order in so
dramatic and fundamental a way as did a theory which insisted that Biblical
accounts of the creation of the world and of human beings were historically
impossible.
For
the same reasons, one can see why people were less disturbed by the unwelcome
realization that they were inhabitants of a minuscule planet somewhere in space
than they were by the announcement that human beings had existed for only a
minuscule portion of the history the earth (2).
Historical
Critiques and Progress Through Science
Historical
arguments in the 17th century were commonly closely linked to proposals for
social reform. Hobbes, for example, appealed to an original state of nature
(before the development of society) in order to advance his idea of a social
contract. Following Hobbes's lead a century later, Rousseau anchored his
political arguments on the same historical thinking. While neither Hobbes nor
Rousseau made strong claims about the historical existence of such a state of
nature, the tactics had obvious implications for those interested in developing
a critical understanding of society and nature in terms of process, something
powerfully encouraged by the discovery of so many "primitive"
societies in the New World of the Americas.
Throughout
the 18th century, appealing to historical developments from the Classical World
to the Middle Ages, invoking, that is, the notion of a unique historical
process, became an increasingly popular and persuasive method to analyze social
institutions, particularly with a view to undermining present arbitrary
authority by showing its historically based character (as opposed to all
official notions about the eternal rightness of the present power
arrangements). The most famous example in England was Edward Gibbon's Decline
and Fall of the Roman Empire (1776), a devastating historical critique of
early Christianity (3).
Linked
to the rationality of the new science, such historical arguments often offered
a reassuring sense of human progress, the modern idea that the growth of
scientific knowledge led to social and political reform and was, in fact,
society's best hope for an improved and improving future. For the most
part, such optimism in the 18th century was cautious, but it fostered a belief
in science as the enterprise most likely to lead to appropriate solutions to
social and personal ills. This Enlightenment idea, which gained
increasing support during the 19th century in all aspects of human life, was to
have enormous implication for Western thought (only in recent years have we
begun to entertain widespread doubts about this reassuring scenario).
In
the spirit of this hope, during the French Revolution the Marquis de Condorcet,
in a book with the revealing title Sketch for a Historical Picture of the
Progress of the Human Mind (1795), could look back proudly on the
achievement of his century:
[The progress of the sciences has]
destroyed prejudices and . . . redirected the human intelligence, which had
been obliged to follow the false directions imposed on it by the absurd beliefs
that were implanted in each generation in infancy with the terrors of
superstition and the fear of tyranny. All errors in politics and morals are
based on philosophical errors and these in turn are connected with scientific
errors. There is not a religious system nor a supernatural extravagance that is
not founded on ignorance of the laws of nature. The inventors, the defenders of
these absurdities could not foresee the successive perfection of the human
mind.
Ironically,
as Condorcet was writing these words, one of France's greatest scientists,
Antoine Laurent Lavoisier (1743-1794) was led to the guillotine, after being
informed that the Republic "had no need of scientists" (4).
The
History of the Earth
For
all of Newton's emphasis that, seen in its entirety, the universe was static,
in his era a completely different view started to emerge, one that stressed the
dynamic, developing, changing nature of both the heavens and the earth. In
other words, the rising popularity of historical accounts of society extended
itself to natural philosophy (science).
As
we have seen, Rene Descartes (1596-1650) proposed as part of his
mechanical philosophy the idea that there had been a history of the earth's
formation. The Creation did not fix the present state of the earth, a condition
which was the result of natural laws governing the development of the original
matter over a long time span.
Descartes
was very careful to make this suggestions with extreme tact (for he was on
risky ground), arguing that it was only a hypothetical suggestion, a
demonstration of how God, had He so wished, might have produced the
earth and its life very slowly by natural secondary causes, rather than an
account of how, Descartes assured his readers, God really did create the world
(in six days) (5).
One
of the first attempts to narrate a story about the earth's surface was
undertaken by Nicolaus Steno (1648-1686), a Danish
natural philosopher and later a bishop. In Prodromus (1699) Steno sought
to reconstruct the history of the earth's surface on the basis of present
empirical evidence. The task was to find out "in what way the present
condition of any thing discloses the past condition of that same thing."
The basic assumption here, and a fundamental difference between the historical
approach and Newton's method, was that originally the earth had had a very
different surface. In Steno's theory, this surface had originally been a smooth
pile of sediments. Somehow, over time, disruption had taken place. The major
scientific task was to provide a coherent narrative account of what had
happened and why.
Steno
relied heavily on scripture, but he had also undertaken field work in Italy and
recognized the marine origin of fossils. His theory introduced into science a
narrative of mechanically caused events (crustal collapse) to account for
present phenomena. Thus, Steno's work is traditionally considered to mark the
beginning of modern geology (there are, however, other claimants).
In
the same period, Thomas Burnet, an Anglican clergyman and private
chaplain to William III, produced in four volumes his Sacred Theory of the
Earth (1680-1690). Burnet attempted, like Steno, to provide a natural
explanation for the historical narrative given about the Creation in the Bible.
He assumed the literal truth of the scriptural account and sought to explain
the physics of those events. Burnet was confident that reason and scripture
could be reconciled, because apparent contradictions, which were not compatible
with the nature of God, would be resolved by future discoveries. Thus, he urged
readers to be careful about too readily using scripture to deny scientific
findings:
'Tis a dangerous thing to engage the
authority of scripture in disputes about the natural world, in opposition to
reason; lest time, which brings all things to light, should discover that to be
evidently false which he had made scripture to assert. . . . We are not to
suppose that any truth concerning the natural world can be an enemy to religion;
for truth cannot be an enemy to truth, God is not divided against himself.
One
of Burnet's central claims was that the present uneven features of the earth's
surface were created at the time of Noah's Flood, which took place when the
earth cracked open and waters rose from the abyss and subsided leaving the
original perfection of the earth's surface forever altered.
Traditionally
Burnet's book has been treated (especially by historians of geology) as an
example of pseudo-scientific mumbo-jumbo and not considered significant.
However, Newton (who corresponded with Burnet on the topic) praised the book,
and a recent re-evaluation has called attention to the importance of Burnet's
methodology, which refused to rest a scientific argument on miraculous interventions
but which stressed the operation of physical laws in the service of a narrative
explanation (6).
Such
facts seemed more and more to suggest that the earth had gone through different
stages of development. So, for example, Benoît de Maillet (1659-1738) in
Telliamed, or Conversations between and Indian Philosopher and a French
Missionary on the Diminution of the Sea, the Formation of the Earth, the Origin
of Man, etc. (1748) proposed that the world had undergone a series of
epochs, a history incompatible with the literal scriptural accounts. According
to de Maillet, the earth had been formed by natural causes and species of life
had evolved over time, with land animals arising originally from creeping
marine life.
Similar
views were advanced by Georges Leclerc, Comte de Buffon (1707-1788),
director of the royal botanical collection in Paris and one of the outstanding
scientists of the century. His Histoire Naturelle, published from 1749
onward, used evidence of horizontal strata and marine fossils to infer that the
earth had been formed by the slow action of natural causes. Buffon also
proposed a theory for the historical origin of the planets as a result of a
near collision of the sun and a comet (later called the dynamic encounter
hypothesis).
While
not totally denying scripture, Buffon nevertheless seriously offended the
religious establishment, and in 1753, when the fourth volume of his work
appeared, he disassociated himself from his own views, affirming (in an echo of
Galileo) that he held to the literal Mosaic account and would give up his
theory of the formation of the planets, which had been only "a purely philosophical
supposition" (i.e., a thought experiment). These statements were probably
ironic, since Buffon's later efforts continued his former work.
Buffon's
emphasis on the essential similarity between the work of the historian and that
of the natural scientist illustrates just how firmly entrenched the narrative
approach to science had become within a century of the first triumph of
Newton's ahistorical method:
Just as in Civil History, one consults
titles, searches for medals, and deciphers ancient inscriptions in order to
determine the epochs of human revolutions and to establish the dates of human
or civil events, so in the same way in Natural History, it is necessary to dig
into the archives of the world, drawing ancient monuments from the entrails of
the earth, collecting their debris, and gathering together in one body of
proofs all the clues of physical changes which can enable us to regain the
different ages of Nature. (1779)
In
Buffon's narrative view of nature, constant changes were an inherent part:
It is not possible to doubt . . . that
there have occurred an infinity of revolutions, of upheavals, of particularly
changes, and of alterations on the surface of the earth, as much by the natural
movement of the waters of the sea as by the action of rains, of frosts, of
running waters, of winds, of subterranean fires, of earthquakes, of floods,
etc. (1749)
To
support this notion, Buffon proposed that the earth was 75,000 years old (and
perhaps much older, even millions of years) and that this history was
characterized by distinct epochs (he described seven). In each of these epochs
the earth had had a significantly different form. In tracing the narrative of
these epochal changes, Buffon placed great stress on the importance of fossils
(7).
One
can get a sense of the impact of this new idea by considering the different
meanings of the world revolution. In Newtonian astronomy revolution meant an
eternally recurring regular motion (like a planetary orbit), an unvaryingly
repetitive pattern set at the creation and continuing indefinitely. In natural
history, by contrast, revolution was increasingly being used to refer to a
series of unique, irreversible, catastrophic, and still operating changes in a
constant process of transformation. Thus, in the new scientific accounts there
was less and less firm a basis for maintaining a faith in the permanent
rightness in any present state of things (scientific or otherwise). The
political implications of this new sense of time were not insignificant (8).
By
the end of the 18th century and early in the 19th, a detailed historical
approach was taken to Newton's astronomy itself. The nebular hypothesis,
derived from the independent work of the Marquis de Laplace (1749-1827),
the French mathematician, who was, as we have mentioned in Section One, very
enthusiastic about denying any place for God in scientific explanations, of Immanuel
Kant (1724-1804), and of William Herschel (1738-1822), the
discoverer of the planet Uranus (in 1781) (9).
The
development of this theory, which completed the mechanistic view of the cosmos
by extending it back to primordial time, provided evolutionary biologists in
the early 19th century with support from the most prestigious discipline of all
(10).
Historical Science, Politics, and Scripture
The
problems initiated by the historical approach arose, in large part, because
orthodox Judaeo-Christian religion is a historically based faith which rests
upon a belief in God's actions in history, so that the Biblical accounts of the
creation of the world and of the life in it and of the relationship between the
Creator and his creations are absolutely central to what it means to be a
devout Christian. In addition, Christian religion was anchored upon a single
codified text (a historical narrative) which had the status of the revealed
word of God. While symbolic or allegorical interpretations of the Bible had
long been an important part of Christian theology, the literal truth of the
many parts of the story, especially of Genesis, was for many crucial to the
faith.
Challenges
to the literal authority of the scriptures were not new to the 18th century. A
particularly strong attack on conventional interpretation of scripture had been
launched by Hobbes (1651), and another followed soon after on the Continent in Tractatus
Theologico-Politicus (1670) by Spinoza. The way in which these two writers
were widely despised and vilified as atheists in the 18th century is a clear
indication of the general hostility to any radical tampering with the narrative
established by the revealed word in the Bible.
One
does not have to probe very deeply to appreciate the political implications of
any serious challenge to traditional interpretations of scripture. Sorting out
the appropriate relationship between historical investigations of the earth's development
and orthodox interpretation of scripture was obviously much more than simply a
doctrinal dispute. It involved, and was clearly seen to involve, significant
social and political implications for both conservatives and those demanding
reform.
The
first immediate challenge posed by the development of a historical interest in
the development of the earth was twofold: (a) Did geological investigation
confirm or contradict Genesis (especially the time frame of creation and the
flood)? (b) Did the events in Genesis occur as a result of natural laws and, if
not, what evidence was there for a divine purpose?
In
response to these questions a number of different theoretical positions were
adopted by 18th century geologists. At one extreme, advocates of naturalism or
materialism held that eternal natural laws rule and have always ruled all
nature, without divine design or final causes, and with only a material
causation. Not surprisingly, such a position tended to be linked with radical
and revolutionary politics.
At
the other (non-scientific) extreme, the supernaturalists, especially those
concerned about unwelcome social consequences latent in natural history,
abolished all faith in rational and material connections between events, since
the only adequate cause of all events in the world was the work of God, who, in
His freedom, might produce order or depart from it, as His Divine Will chose.
Reason must therefore adapt itself to revelation, not the other way around.
According to some who held this view, the fossil record was unreliable since it
was the work of Satan and intended to deceive human beings about revealed
truth.
Intermediate
between these two extremes were the deists, who held, as we have seen in
Section One, that God created matter and laws according to a plan fixed at the
time of creation and that the design of nature was the only sure evidence human
beings had of God's activity (since there were no more divine interventions).
This position corresponded with Charles Darwin's original stance in his work.
The semi-deists were closer to religious orthodoxy. They permitted God
occasional "unnatural" interventions, usually catastrophic.
In
England, with some exceptions, the main arguments did not generally involve
scriptural literalists or even very conservative Anglicans (who, for all the
influence they exerted over scientific patronage, were not strongly represented
in scientific circles, except in the universities) (11).
The main disputes took place between moderate Anglicans and
deists. The former maintained that the evidence of geology could be compatible
with an interpretation of the Bible, and some devout Christian geologists
maintained that geology, like Newton's astronomy, provided evidence for
Christian belief. In the words of Richard Kirwan, president of the Royal
Academy of Dublin, in 1799:
Recent experience has shown that the
obscurity in which the philosophical knowledge of this [original] state has
hitherto been involved, has proved too favorable to the structure of various
systems of atheism or infidelity, as these have been in their turn to
turbulence and immorality, not to endeavor to dispel it by all the lights which
modern geological researches have struck out. Thus it will be found that
geology naturally ripens . . . into religion, as this does into morality.
The
main challenge to this position came from deists like James Hutton
(1726-1797) and Charles Lyell (1797-1875), who believed in a deity but
not in divine intervention or in the revelations of scripture.
We
know less about the relationship between geology and religion on the Continent.
It appears that outside Britain, geologists did not concern themselves so much
about religion, perhaps because the advanced state of historical biblical
scholarship in Germany had already convinced many that there was no great need
to take the bible literally (in 1835 D. F. Strauss's Life of Jesus
Critically Examined proposed that the Gospels were second-century
mythological writings; the book was translated into English by George Eliot in
1846).
Moreover,
in Germany there was a growing tendency by the end of the 18th century to base
Christian belief less on concepts of God or on evidence of God's handiwork in
the natural world than on the personal experience of faith. As S. T. Coleridge,
one of the first importers of German thought into England, later put it,
Evidences of Christianity! I am weary of
the word. Make a man feel the want of it, rouse him, if you can, to the
self-knowledge of his need of it; and you may safely trust it to its own
Evidence. (12).
Fossils
Fossils
are the evidence in rocks of earlier animal and plant life. They may consist of
actual remains (bones, teeth) or be casts or impressions of the hard parts. At
times the bone or shell can be replaced by something harder, like silica. Other
fossils remains can include tracks or molds of tracks and trails.
For
a long time the origin of fossils was a very disputed matter. Some people
thought they grew from seed (sperm) deposited on rock. Others saw in fossils
evidence of Noah's flood. On the other hand, Leonardo da Vinci concluded that
fossils now on high inland mountains could not have reached their present location
during the Deluge (Noah's Flood).
Steno's
work with fossils encouraged further studies of this key subject. Martin
Lister (1638-1712) wrote the first book devoted to fossils based on their
organic nature and proposed a geologic map. Buffon relied extensively upon
fossil evidence. This growing interest in fossils was prompted by, among other
things, an accelerating accumulation of fossil evidence throughout the 18th
century, the result of field work, explorations, and the rapidly increasing
rate of construction projects, particularly rock quarries (for building
materials) and canals.
William
Smith (1769-1839), a civil
engineer who worked on the construction of canals in England, kept an extensive
record of the fossils he observed in his tireless inquiries. Smith produced the
first coloured geological map (1815) and in Stratigraphic System of
Organized Fossils (1817) proposed the sedimentary rock units could be
classified according to the fossil remains they contained and that, with the
aid of the fossil record, geologists could assign relative ages to rocks. The
identification and classification of fossils at that time provided the only
sure means of comparing the ages of different rocks (other than the Law of
Superposition, mentioned below).
For
some scriptural geologists, fossils were convincing evidence of the truth of
the Biblical account of the flood. One such inquirer was delighted to find
fossils with Hebrew characters on them. Only when he discovered one containing
his own name did he realize that his students had been playing a practical joke
on him. However, the growing evidence for the different ages of sedimentary
rocks (rocks formed in water by the deposit of small particles) and therefore
of the fossils they contained gradually convinced most geologists that one
single universal deluge was not a satisfactory explanation for the empirical
evidence.
The
accumulating facts brought out, too, certain difficulties in the narrative
sequence of fossils. A particularly vexing problem for geologists and biologists
was the apparently erratic nature of the fossil record. This appeared to
contain confusing evidence about the varieties and frequency of fossils in
successive layers of rock. A good deal of this evidence seemed to point to a
discontinuous and progressive history of the earth and the life in it, with
more frequent and more complex organisms in the younger strata (i.e., the
strata closest to the surface of the earth).
Darwin
himself relied upon the fossil record to back up his notion of descent with
variation. However, since he was well aware that the inconsistent facts in the
fossil record were a weak point in his theory of a constant rate of change, he
appealed to the future: continued research would yield further evidence for his
ideas by filling in the gaps in the fossil record (13).
James Hutton and Uniformitarianism
James
Hutton (1726-1797), a Scotsman
who is generally acknowledged (at least in British science) as the founder of
the modern study of geology, in his work adopted a deistic position and a firm
faith in the argument from design. He sought to find evidence of a wise and
benevolent deity in the order of nature by research into the earth's crust,
"to acknowledge an order, not unworthy of Divine wisdom, in a subject
which, in another view, has appeared as the work of chance, of an absolute
disorder and confusion" (1788).
Thus,
Hutton, like Boyle before him, linked his work, which, unlike many of the
theories of his contemporaries on the Continent, he conducted with a strong
empirical emphasis (continuing the tradition recommended by Bacon), to an
investigation which he hoped would reveal final causes. In discovering the laws
of the earth's crust, he would be demonstrating "the purpose of a habitable
world . . . a world peculiarly adapted to the purposes of man."
Hutton's
fundamental assumption, which has remained the most important working principle
of historical geology and biology every since, is the principle of
uniformitarianism (often referred to, by a direct translation of the French
term for the idea, as the actualistic principle). This notion was not
original to Hutton. Leonardo da Vinci had anticipated it three hundred years
earlier, and it had been explicitly formulated by French physician Georges
Fushsel (1722-1773), as follows: "the manner in which nature at the
present time is still acting and producing things must be assumed as the rule
in our explanation."
In
other words, the uniformitarian principle committed the scientist to explaining
the past events of the earth's history in terms of processes still at work
(without miraculously powerful events unknown today). This assumption is, in
effect, making the same initial claim about time that Newton made about space.
Just as the physical laws of motion are the same everywhere in the universe, so
they have been the same throughout the history of the universe.
Uniformitarianism,
as it later developed, involved at least three important assumptions about the
continuity of physical processes of the present and the past: (a) that they
were the same in kind and proceeded in the same way, (b) that they were the
same in intensity, and (c) that they occurred at the same rate (14).
On
the basis of this principle and his observations, Hutton developed a picture of
the earth as undergoing an endless mechanical steady-state cycle: oceans became
land, land sunk down to lie under the ocean. The system was always undergoing
change, but the change was cyclical and repetitive. There was a creative and
omnipotent God, who had set this process in motion in accordance with natural
laws and who did not intervene.
If we believe that there is almighty
power and supreme wisdom employed for sustaining that beautiful system of
plants and animals . . . we must certainly conclude that the earth, on which
this system of living things depends, has been constructed on principles
adequate to the end proposed, and procure it a perfection which it is our
business to explore. Therefore, a proper [theory] should lead us to see that
wise construction by which this earth is made to answer the purpose of its
intention, and to preserve itself from every accident by which the design of
this living world might be frustrated. For, as this world is a material
machine, we must see how this machine is so contrived as either to have parts
to move without wearing or decay, or to have those parts which are wasting and
decaying again repaired.
Hutton's
most lasting and best known symbol of this eternal process is the rock cycle:
rocks are weathered and broken down to form sediments which are then buried in
water. As they get buried (by more material being deposited on top), they are
subjected to pressure and heat, so that the rocks melt or undergo
transformations of some kind. Then by uplift they become mountains, subject to
erosion and a new start to the endless cycle.
This
same process, Hutton wrote, "is also manifest in the animal and vegetable
kingdoms," and, in his most disputatious (although frequently
misunderstood) claim, he concluded
If the succession of worlds is
established in the system of nature, it is in vain to look for anything higher
in the origin of the earth. The result, therefore, of our present enquiry is,
that we find no vestige of beginning,--no prospect of an end.
Hutton's
notion of an endlessly repetitive cycle, in contrast to the ideas of Buffon,
bore an obvious similarity to the regularity of Newton's cosmology. In a sense,
Hutton's theory drew much of the revolutionary threat out of historical
geology, at least to the extent of denying its uniquely irreversible character.
To Buffon's idea of constant change, Hutton added the notion of constant repair
and renewal, so that the narrative of the earth's crust was more reassuringly
cyclical and permanent.
It
is not, therefore, perhaps surprising that his theory (which appeared just
before the outbreak of the French Revolution) should have been given such an
honoured place in British science in the early 19th century, at a time when the
establishment was trying with all the means at its disposal to counter
Continental notions of historical process and change as progressive, the
favoured ideology of radicals in Paris, Edinburgh, and London (15).
The Neptunist-Plutonist/Vulcanist Controversy
At
the end of the 18th century the dominant school of geology was at Freiburg (in
present day Germany) where Abraham Werner (1750-1817) was the inspector
of mines and professor of mining and mineralogy at the Mining Academy. Many of
Werner's students came from Great Britain and were early members of the
Geological Society of London (founded in 1807 as a society committed to no
particular theory but rather to cooperative fact gathering on the Baconian
model) (16).
In his Short Classification and Description of Rocks (1787) and in his immensely popular lectures,
Werner insisted that geologists should classify rocks according to the age of
their formation (i.e., arrange them in chronological order) rather than
according to mineral types (the traditional method). In other words, Werner
placed at the centre of geological inquiry a reconstruction of historical
processes. This is an important example of how an apparently minor change in a
system of classification can have significant effects on how people understand
the processes which produced the material being classified.
The
major problem facing any historical reconstruction of the processes involved in
the earth's surface were obvious enough to anyone who had undertaken geological
observations. Some of the major questions were the following: (a) How is it
that the greater part of the land is made up of material formed in the sea? (b)
How did the uniform strata come to be broken and intersected by fissures and
intrusions of various kinds? (c) How did rocks formed under the sea come to
make up some of the highest portions of the earth's crust? (d) Where did rock
come from?
In
his response to these and other basic questions about the history of the earth,
Werner championed what has come to be called the Neptunist theory of the
earth. This concept holds, in brief, that the earth was originally a great body
of water containing suspended matter. The earth's core was formed by
sedimentation and was exposed when the waters retreated to form continents.
Noah's flood overwhelmed the continents and added new rock layers. Subsequent
volcanic activity disrupted the continents and added more sediment. Later
floods added more layers. Thus, all rock formations, according the Neptunist
theory, were deposited by water (with a minor exception, volcanic rocks).
Werner
based his theory on very wide observations which led him to the generalization
that there was a basic similarity in all formations as one moved from the
mountains to the lowlands, with clearly demarcated transitions from granite
(the oldest and highest rocks), to loose or cemented aggregates, marked by an
increasing frequency of fossils and a decrease in metals. On the basis of this
theory of rock formation, Werner constructed a classification system.
In
contrast to this Neptunist hypothesis was the idea that rocks were formed in
fire, a theory which came to be called the Plutonist (or Vulcanist) theory
of the earth. Originally proposed by Abbé Anton Moro (1687-1750) on the
basis of a study of volcanic islands, this alternative to Werner's ideas was
taken up by James Hutton. In response to the Neptunist theory of the
Wernerians, Hutton advocated a view that the surface of the earth was subject
to two basic processes: rocks were worn away by weathering and erosion, and
then they were re-formed and uplifted by heat and pressure.
A
key question in the dispute between the Neptunists and the Plutonists was the
origin of basalt, a common dark fine-grained rock. Werner denied any volcanic
origin for basalt, and Neptunists claimed to have observed fossils in basalt, a
sure sign of a rock's origin as a sedimentary deposit. Hutton, together with
his close associate, friend, and popularizer, John Playfair, asserted that
basalt never contained fossils, that it was always insoluble, hard, and
crystalline, and that masses of basalt often cut across other rocks, all of
which pointed to its origin from magma, or molten rock under the earth's crust
(magma which reaches the surface is called lava). If the disputants had had
access to areas of active volcanic activity, the argument would have been
resolved quickly, since basaltic lava is easily observed in such places (e.g.,
Iceland).
The
most significant difference between the disputants here was not their evidence
but their initial assumptions. Werner and his followers were committed to a
historical understanding of the earth and to the notion of gradual processes of
transformation and to a qualified uniformitarianism. They were not, however,
prepared to accept the full principle of uniformitarianism, and their theory
depended upon certain causes (for the sudden movement of the oceans) which no
longer operated. For Neptunists, the present might be the key to the past, but
that did not mean for them, as it did for Hutton, that present causes must
account for all past processes.
For
some of the Plutonists, on the other hand, Werner and his followers were simply
trying too hard to accommodate historical geology to the writings of Moses and
were, by compromising uniformitarian principles, merely speculating about a
model which did not work in practice and abandoning the idea of natural laws governing
all physical processes.
The
controversy came to an end gradually in the 19th century, and the verdict of
history has come down on the side of Hutton and the Plutonists (at least
insofar as the origin of rocks is concerned) (17).
Catastrophism and Uniformitarianism
Early
in the development of modern geology it became generally agreed that stratified
rocks (i.e., rocks made up of particles in horizontal layers), because they
frequently contained remains of aquatic life, had been formed as deposits in
water. This theory gave rise to the corollary (proposed by Steno in the 17th
century) that each stratum of such rocks was younger than the ones beneath it
(later this notion came to be called the Law of Superposition).
Initially,
the presence of such stratified rocks (called sedimentary rocks) prompted
certain scriptural geologists to claim that all stratified rock had been
created at the time of the flood described by Moses in Genesis 7. This idea
received a death blow from the work of Georges Cuvier (1769-1832), the
most important and influential figure in French science in the early 19th
century. His study of the different layers of the Paris Basin led him to
propose a series of sudden inundations and retreats of the sea.
Cuvier's
study of the composition and fossil record of the different strata convinced
him that
the breaking to pieces and overturning
of the strata [show] plainly enough that [earlier revolutions] were sudden and
violent like the last; and the heaps of debris and rounded pebbles which are
found in various places among the solid strata demonstrate the vast force of
the motions excited in the mass of waters by these overturnings.
For
Cuvier the only satisfactory explanation for such evidence was a series of huge
catastrophes (hence his theories came to be called catastrophism).
Present processes produced effects too small and slow, and volcanic action,
although violent, was too local. The evidence required the action of some
extremely unusual and overwhelming force: "The thread of operations is
here broken; the march of Nature is changed; and none of the agents which she
now employs would have been sufficient for the production her ancient
works." Cuvier could suggest no physical cause for such repeated catastrophes,
but his work effectively demonstrated that the geological record could not be
explained by reference to simply one major flood.
Cuvier's
work was translated and published in England in 1813, where the catastrophic
theory became very popular, not least of all because part of his theory gave
strong support to the notion that the last such catastrophic flood could be
dated at roughly the same time as the flood described in Genesis. To make this
point more persuasively, Cuvier had included in this theory a survey of
mythological accounts of a great flood from many cultures and had traced them
back to an approximate common date.
The
scriptural geologist William Buckland (1784-1856) drew on Cuvier's
theory, which he supported with the discovery in 1821 of the Kirkdale Caves in
Yorkshire, which contained the bones of many extinct animals which had
apparently died in a sudden flood, to confirm the truth of the Biblical
account: "had we never heard of such an event [Noah's Flood] from
scripture . . . Geology of itself must have called in the assistance of some
such catastrophe to explain the phenomenon" (18).
Catastrophism,
of course, violated the principle of strict uniformitarianism, since it
appealed to occasional unknown catastrophic processes. This meant that the
history of the earth was, in part, a record of unique events rather than
constant evidence for the operation of the same physical processes in a dynamic
but steady-state system. Hence catastrophism aroused the opposition of the
followers of Hutton. Prominent among them was John Fleming (1785-1857),
who insisted that Buckland's evidence indicated only localized flooding and not
a universal deluge. Moreover, he argued, catastrophism contradicted rather than
confirmed Genesis.
Charles
Lyell
The
heated controversy between these two historical explanations acted as a great
spur to geological field work, an activity which culminated early in the 19th
century in the work of Charles Lyell, whose Principles of Geology first
appeared in 1830. Lyell brought the book out in separate volumes and in many
different editions, eleven during his own life time (the last one appeared in
1872). Lyell's book reasserted very strongly the principle of strict
uniformitarianism, for Lyell insisted that, given sufficient time, present-day
forces operating at present rates could have produced all observable geological
phenomena.
Lyell
further emphasized in his famous definition of geology its essentially
historical character, announcing at once a significant difference between his
work and Hutton's:
Geology is the science which
investigates the successive changes that have taken place in the organic and
inorganic kingdoms of nature; it enquires into the causes of these changes, and
the influence which they have exerted in modifying the surface and external
structure of the planet.
Lyell
was an important, complex, controversial, and influential scientist (and
lawyer) writing about crucial theories at a time when the politics of science
was in England marked by particularly fierce disputes. As one of the best known
and most articulate spokesmen for the conservative establishment, he devoted
his energies to fighting Continental theories of progression, especially the
materialistic evolutionary science of Lamarck, "the Orang Outang"
concept, which he, along with many of the contemporaries, detested (see Section
Three below). His geology, therefore, one writer has recently observed,
"was a singular reaction to the threat of bestialization" (19).
To
resolve the apparent contradiction presented between this theory and the fossil
record (which seemed to indicate an uneven and progressive development, with
more frequent and more complex organisms in younger strata), Lyell proposed a
complex statistical distribution of fossils which, in his view, enabled him to
argue for a generally steady-state view of living populations, in spite of the
apparent evidence to the contrary.
This increase in existing species, and
gradual disappearance of the extinct, as we trace the series of formations from
the older to the newer, is strictly analogous . . . to the fluctuation of a
population such as might be recorded at successive periods, from the time when
the oldest of the individuals now living was born to the present moment. (20)
This effort to prevent the progressive reformers
from using fossil history for their politically unacceptable materialistic
theories of change may have satisfied Lyell, but his denial of progressive
developments in the history of the earth and life in it bewildered many of his
conservative colleagues, simply because for them the evidence was so
overwhelmingly clear. They appreciated the anti-reform implications but were
unwilling to outlaw the progression so obviously indicated by the fossil
record, especially with recourse to a theory which, many felt, was not Christian
(21).
Nevertheless,
by mid-century, on the eve of the publication of The Origin of Species
(1859) the modified catastrophist view generally was the more favoured of the
two theories, not least of all because catastrophism had adapted itself to
theories of progression over geological time. True to the original spirit of
Hutton, Lyell and the strict uniformitarians held to the notion of an
unchanging, repetitive steady-state process: "we can see neither the
beginning nor the end of that vast series of phenomena which it is our business
as geologists to investigate" (22). Not only was this out of touch with the progressive spirit of
the time, but, more importantly, and quite apart from the problem of fossils,
the non-progressive view of the earth had difficulty answering the growing body
of evidence about the earth's heat loss, which suggested that the earth's
temperature must, over geological time, be changing and that, therefore, the
earth could not remain in the same state indefinitely, certainly not long
enough to fit the time frame demanded by strict uniformitarianism.
The
conflict between the uniformitarians and the catastrophists had an important
religious dimension. Strict uniformitarianism, as practiced by Lyell and
Darwin, emphasized that of paramount concern were the permanent laws of nature
always at work. God never intervened with an inexplicable event in the process.
The Catastrophists' insistence that the history of the earth could only be
understood by such interventions meant that the catastrophists were able to use
their science to support a faith in a living God, an immanent Presence, for
sudden, otherwise inexplicable events in the history of the earth provided
evidence for the continuing action of a God not bound by natural laws.
Darwinian
evolution, from one perspective, turned out to be a workable compromise between
catstrophism and uniformitarianism. It proposed essentially a historical
development (e.g., the world was not undergoing a constantly repeated, stable
process, and thus each stage of the story was unique, as the fossil record
suggested), and yet the forces at work in the process were, as Darwin insisted,
uniformitarian.
What
remained constant, in this view, was not the state of the world or of the
species in it, but the processes by which irreversible changes took place.
Whether or not such changes represented a progress (i.e., the movement towards
higher or better life forms) was a matter of dispute. Although Darwin was, on
this point, ambiguous, his theory was widely interpreted as a
"progressive" view: evolution led to higher life forms (23).
Seen in this light, we can interpret Darwinian
evolution as, for Lyell, in part, what Gould calls a "fallback"
position. It enabled him to continue to maintain uniformitarianism in a
modified form (uniformity of rate) and yet to take account of the growing
evidence for changes in the history of the earth. Lyell himself admitted that,
on the basis of the facts, there was little to determine a clear choice between
uniformitarian evolution and catastrophism, but the different implications for
his work in geology were clear enough (24).
To say this is not to claim that uniformitarians
won the day and drove catastrophism from the field. The issue is still alive in
the continuing debates about the rates of change and time spans in the history
of the development of species (25).
Postscript: Continental Drift and Plate Tectonics
One
of the most dramatic theories of modern geology, the idea that the earth's
continents were once joined and have gradually drifted apart, first made its
appearance in 1858, in a paper published by an American scriptural geologist, Antonio
Snider-Pelligrini. His major evidence was the remarkable fit between the
coastlines on either side of the Atlantic. In Snider's hypothesis (which was
not a particularly scientific argument), America had once been the island of
Atlantis, broken off from the single mass of dry land by volcanic action and by
Noah's Flood. The shift had occurred in a single violent movement.
The
modern arguments about continental drift began later, with papers published by F.
B. Taylor in 1910 and by Alfred Wegener in 1915. These versions of
the theory arose, in part, as a way of explaining the odd distribution of
ancient climates (e.g., fossils of warm-climate plants in coal deposits near
the poles) (26).
In
the 20th century, the theory of continental drift has been incorporated into a
more comprehensive theory called plate tectonics. This posits that on
the surface of the earth there are number of large, thin, relatively brittle
plates, which move, under pressure from the molten rock of the earth's
interior, over the upper mantle. These plates interact with each other to
produce a number of major geological features (mountains, marine trenches,
inland volcanoes, earthquakes, and so on). The continents ride on these plates
and thus move with them (27).
(1) The term geology appears to have been first introduced in 1779 by
H. B. de Saussure (1740-1799) of Geneva, the founder of modern mountaineering.
[Back to text]
(2) One way to appreciate dramatic impact of a historical
understanding is to consider the following analogy about the history of the
earth. According to our present understanding, if we were to compress the
history of the earth into a single year, then single-celled organisms would
appear early in April, the first vertebrates towards the end of May, and the
first land plants about the middle of July. Early in August the seas would be
filled with fish, and in early September the first insects would turn up.
October would mark the flourishing of the dinosaurs and the emergence of the
first animals with the ability to fly and of simple mammals. Sometime around noon
on December 31 would appear a stooped creature of human-like proportions. At
about 11 p.m., Neanderthal humans would emerge, at 11:30 p.m. Neolithic human
beings. Between approximately 11:56 p.m. and 11:59 p.m. the story of the
Egyptians, Babylonians, Greeks, and Romans would take place. Columbus would
discover America at approximately 11:59:40; the American War of Independence
would occur at 11:59:53 p.m. [Back to text]
(3) Gibbon's book was for many decades a source of
considerable chagrin to the Anglican establishment, for this extremely
scholarly, authoritative, and popular history of the early Church was the work
of an anti-Christian. It was embarrassing to have the most reliable history of
the early church so hostile to Christian doctrine. [Back to text]
(4) This famous blow against French science does not accurately
reflect the developments promoted by the French Revolution and Napoleon. Early
in the 19th century, the French government reformed entirely the structure of
the scientific establishment, including education, appointments, and
promotions. The changes really stressed merit and made French science early in
the 19th century the leader in Europe and very much the envy of many
reform-minded English researchers and teachers, several of whom went to France
for their education. For more details on this issue see Guerlac, Chapter 31,
"Some Aspects of Science During the French Revolution." [Back to text]
(5) There is no reason to suppose that Descartes was being
deliberately ironical on this important point, and the hypothetical nature of
his historical speculations created certain problems for his readers. The key
passage is worth quoting in full: "I do not doubt that the world was
created in the beginning with all the perfection which it now possesses: so
that the Sun, the Earth, the Moon, and the Stars existed in it, and so that the
Earth did not only contain the seeds of plants but was covered by actual
plants: and that Adam and Eve were not born as children but created as adults.
The Christian faith teaches us this, and natural reason convinces us that this
is true; because, taking into account the omnipotence of God, we must believe
that everything He created was perfect in every way. But, nevertheless, just as
for an understanding of the nature of plants or men it is better by far to
consider how they can gradually grow from seeds than how they were created by
God in the very beginning of the world; so, if we can devise some principles
which are very simple and easy to know and by which we can demonstrate that the
stars and the Earth, and indeed everything which we perceive in this visible
world, could have sprung forth as if from certain seeds (even though we know
that things did not happen that way); we shall in that way explain their nature
much better than if we were merely to describe them as they are now. . .
." (Principles of
Philosophy, 1644). The historical
scientific proposal here, even if hypothetical, was influential. See Sloan 298
ff. [Back to text]
(6) Stephen Jay Gould's Time's
Arrow Time's Cycle offers an
interesting and sympathetic reassessment of Burnet's and Steno's work, with a
helpful comparison between the two early theories. Gould provides not only an
analysis of Burnet's arguments but also a selection of comments from later
historians of geology ridiculing Burnet's book. Thus, Gould's analysis is a
useful reminder of the dangers of some snap judgments in historical science. [Back to text]
(7) For a useful and brief discussion of Buffon's
ideas in the context of his religious beliefs, see Scientific Progress and Religious Dissent 55-60. [Back to text]
(8) For a further analysis of this point, together
with a discussion of the influence of the political revolutions in America and
France, see Cohen, Section IV (p. 195 ff), "Changing Concepts of
Revolution in the Eighteenth Century." [Back to text]
(9) Kant is, of course, most justly famous as the philosopher who
delivered science from the skepticism of Hume, but it is worth mentioning that
Kant was also a very competent physicist who "anticipated Laplace in formulating
a nebular hypothesis to explain the origin of the solar system . . . , was the
first to point out that tidal friction must have a slow retarding effect on the
Earth's rotation, and that by its reaction it has forced the Moon to present
always the same face to the Earth . . . [and who] showed that the different
linear velocities of successive zones on the Earth as it rotates explain the
'trade' winds, and other similar steady currents of air. He wrote also on the
Causes of Earthquakes, on the Different Races of Men, on Volcanoes in the Moon,
and on Physical Geography" (Dampier 195). For a very thorough and
generally hostile study of Kant's contributions to astronomy, see Stanley L.
Jacki's introduction to his translation of Kant's Universal History of Nature and Theory of Heaven. [Back to text]
(10) For a detailed account of this story see Simon Schaffer,
"The Nebular Hypothesis and the Science of Progress," in Moore. For an introduction to Laplace's scientific work
see Robert Fox, "Laplacian Physics," in Olby and others, 278-294. [Back to text]
(11) Those who hold unswervingly to the literal
truth of the Biblical narrative are nowadays often called fundamentalists. This
is hardly the right term to apply to the 18th or 19th century.
"[Fundamentalism] seems to have been first used in 1920 to describe
certain American Christians opposed to the removal of supernatural elements of
Christianity as advocated by certain 'progressive' thinkers at the time. . . .
Since that day, however, the word Fundamentalist has acquired a pejorative
meaning, relating to the position of extreme ultra-conservatism that had been
discredited in the nineteenth century if not before." (Crisis of Evolution 125). [Back to text]
(12) The important shaping influence in this
transformation of Christian faith from arguments about evidence for natural
order and design (as described in Genesis) to questions of inner conviction and
feeling was primarily the work of Friedrich Schleiermacher (1768-1834), whose
major work on theology was published in 1821. Schleiermacher's influence
throughout the 19th century was very significant. [Back to text]
(13) The problem, however, has not gone away.
Inconsistencies in the fossil record are still a problem in evolutionary
biology. For the record of the first 3000 million years, nothing more
complicated than bacteria and slime appears. Then suddenly there seems to have
been an "explosion" of life. Moreover, since that time, according to
David Raupp, curator of the Field Museum in Chicago, "instead of finding
the gradual unfolding of life, what geologists of Darwin's time and geologists
of the present day actually find is a highly uneven or jerky record; that is,
species appear in the sequence very suddenly, show little or no change during
their existence in the record, then abruptly go out of the record." The
arguments about what this erratic record means, especially in relation to
Darwin's theory, is the source of one of the liveliest, best known, and most
interesting contemporary arguments in biology, especially in the exchanges
between Stephen Jay Gould and Richard Dawkins. [Back to text]
(14) Uniformitarianism became in the work of
Charles Lyell a frequently ambiguous term; Lyell used the multiple meanings to
good rhetorical effect to bolster his arguments for a non-progressive geology.
On this point, see Gould, Chapter Four. [Back to text]
(15) For a critical reassessment of Hutton which
stresses that his importance has been overestimated see Rachel Laudan,
"The History of Geology, 1780-1840" in Olby and others, pages
314-325. Sorting out Hutton's contribution to geology creates difficulties for
a number of reasons: (a) Hutton's style is very prolix and difficult (many
people heard of his work through the interpretative summary produced by his
friend John Playfair), (b) the British tradition may have seriously overvalued
and misinterpreted his work for nationalistic-political reasons, and (c) there
is evidence to suggest that Hutton was not as empirical as the tradition
maintains. Stephen Jay Gould's recent analysis of Hutton's work maintains that
Hutton, far from conducting himself in an appropriately Baconian way
"presents his theory as the a
priori solution to a problem in
final causation, not as an induction from field evidence" and with a
disregard for the historical basis of geological inquiry (Gould 76). In Gould's
view Hutton's work was mythologized by Charles Lyell, "one of the most
flagrant mischaracterizations ever perpetuated by the heroic tradition in the
history of science." [Back to text]
(16) This society was one of the more conservative
of the learned societies at the time. Its rules tacitly prohibited divisive
arguments over cosmological theories and discussions of origins. Charles Darwin
found the atmosphere here far more congenial than at the stormier meetings of the
Zoological Society. Darwin had been a member of the Wernerian Society while a
young student at Edinburgh. [Back to text]
(17) For a more detailed study of this
controversy, with illustrations to clarify each theory, see Binn, Chapter 2. [Back to text]
(18) Cuvier's work was attractive to those who
wished to stem the radical implications of much of the new historical science.
For Cuvier, as Cohen points out (p. 276), emphasized that the sort of
revolutions which Buffon had talked about as causing changes to the earth's
crust were, in fact, violent catastrophic events which not only altered the
topography but which exterminated whole species. There was nothing particularly
orderly or progressive (or welcome) about such events. Cuvier's work in
biological taxonomy also was a major inspiration to English conservatives
working in biology (see Section Three below). Buckland's orthodoxy
("Geology is the efficient auxiliary and handmaid of religion") was
an important influence in getting the suspect science of geology into Oxford's
classic-dominated clerical curriculum. [Back to text]
(19) Desmond 328. Desmond, for whom social and
political context is all important in understanding the history of science,
stresses the points that Lyell's book was written at the height of the
agitation over reform in England (1831-2) and that Lyell had visited France and
had witnessed the fighting in Paris during the July Revolution in 1830.
Moreover, his family on his father's Scottish estate had been harassed by
roving gangs of reformers (see pages 328-9). [Back to text]
(20) For a useful analysis of Lyell's attempts to
deal with the difficult problem of the fossil record in his uniformitarian
theory see Gould, p. 150 ff. [Back to text]
(21) Lyell himself, with a good deal of
reluctance, finally gave up this part of his argument. The tenth edition of his
most famous book (published in 1866, that is, seven years after the publication
of The Origin of Species) and his On the Geological Evidences of the
Antiquity of Man (1862) declared this change of mind: progress in life's
history is "an indispensable hypothesis . . . [which] will never be
overthrown." Gould has an insightful analysis of this change of mind
(which has been, in Gould's persuasive view, too easily ascribed to the
influence of Darwin, Lyell's close friend. See Gould 168 ff. [Back to text]
(22) Identifying both Hutton and Lyell as
uniformitarians should not obscure some important differences between their two
theories. One important point is that in Hutton's theory the different stages
of the cycle operated sequentially and globally; whereas, in Lyell's view the
processes were constant and local. Thus, for Hutton the earth as a whole went
through distinctively different stages. For Lyell any particular locality might
go through such sequences, but the earth as a totality remained basically the
same. See Gould 150 ff. [Back to text]
(23) For a more detailed account of the
Catastrophism-Uniformitarianism debate see Chapter 3 of Binn. [Back to text]
(24) Lyell, in fact, not only finally accepted
evolution; he also reconciled himself to its progressive character, something
not strictly necessary. Gould (p. 170) makes an important point in this matter:
"No logical necessity can extract an implication of progress from the fact
of evolution. Darwin himself had maintained a very ambiguous attitude toward
the idea of progress, accepting it provisionally as a feature of parts of the
fossil record, but denying that the theory of natural selection . . . required
organic advance. Nonetheless, many evolutionists have always viewed the
concepts of progress and transmutation as necessarily connected, and Lyell, for
whatever reason, certainly adopted this view. Therefore, for him, a decision to
embrace evolution also entailed progress as a fact." Thomas Huxley warned
Darwin that linking his theory to uniformitarian principles would create
problem: "You have loaded yourself with an unnecessary difficulty,"
Huxley wrote on the day The
Origin of Species was published.
And, in fact, the problems created by the fact that Darwin's uniformitarianism
demanded huge expanses of time for natural selection to work raised major
objections to his theory from physicists, who claimed that the earth's
temperature could not have sustained life for such a long period. [Back to text]
(25) In various modified ways, catastrophism has
made something of a comeback in recent years (especially in the theory that the
extinction of the dinosaurs was brought about by a single violent catastrophe,
a collision between the earth and a huge comet). For a brief discussion of some
of the recent theories about mechanisms of evolutionary change (hopeful
monsters, embryonic change, chromosomal jumps, punctuated equilibria, and so on),
see Hitching, Chapter Six. And for a very spirited and thorough defense of
Darwinism against such challenges see Dawkins. [Back to text]
(26) The credit for first suggesting that the
continents have moved has prompted a long argument. Francis Bacon pointed out
the interesting conformity to the shapes of Africa and South America, as did
Alexander von Humboldt almost two hundred years later. But neither proposed
that the two continents had once been together. For a discussion of Wegener's
thesis in context, see Cohen, Chapter 29. [Back to text]
(27) This very incomplete account is intended to
serve as only the briefest mention of a complex subject. For a fuller
description of the principles of plate tectonics see Spencer or any relatively
recent textbook introduction to Physical Geology. [Back to text]
[Back
to johnstonia Home Page]