18th century in Berwickshire, showing the gently tilting red sandstone layers above vertically tilted greywacke rocks
Abraham Gottlob Werner (1749–1817) proposed
Neptunism, where
strata represented deposits from shrinking seas
precipitated onto primordial rocks such as
granite. In 1785
James Hutton proposed an opposing, self-maintaining infinite cycle based on natural history and not on the
Biblical account. Hutton then sought evidence to support his idea that there must have been repeated cycles, each involving
deposition on the
seabed, uplift with tilting and
erosion, and then moving undersea again for further layers to be deposited. At
Glen Tilt in the
Cairngorm mountains he found granite penetrating
metamorphic schists, in a way which indicated to him that the presumed primordial rock had been
molten after the strata had formed. He had read about
angular unconformities as interpreted by Neptunists, and found an
unconformity at
Jedburgh where layers of
greywacke in the lower layers of the cliff face have been tilted almost vertically before being eroded to form a level plane, under horizontal layers of
Old Red Sandstone. In the spring of 1788 he took a boat trip along the
Berwickshire coast with
John Playfair and the geologist
Sir James Hall, and found a dramatic unconformity showing the same sequence at
Siccar Point. Playfair later recalled that "the mind seemed to grow giddy by looking so far into the abyss of time", and Hutton concluded a 1788 paper he presented at the
Royal Society of Edinburgh, later rewritten as a book, with the phrase "we find no vestige of a beginning, no prospect of an end". Both Playfair and Hall wrote their own books on the theory, and for decades robust debate continued between Hutton's supporters and the Neptunists.
Georges Cuvier's
paleontological work in the 1790s, which established the reality of
extinction, explained this by local catastrophes, after which other fixed species repopulated the affected areas. In Britain, geologists adapted this idea into "
diluvial theory" which proposed repeated worldwide annihilation and creation of new fixed species adapted to a changed environment, initially identifying the most recent catastrophe as the
biblical flood.
19th century meeting in Glasgow 1840 From 1830 to 1833
Charles Lyell's multi-volume
Principles of Geology was published. The work's subtitle was "An attempt to explain the former changes of the Earth's surface by reference to causes now in operation". He drew his explanations from field studies conducted directly before he went to work on the founding geology text, and developed Hutton's idea that the earth was shaped entirely by slow-moving forces still in operation today, acting over a very long period of time. The terms
uniformitarianism for this idea, and
catastrophism for the opposing viewpoint, was coined by
William Whewell in a review of Lyell's book.
Principles of Geology was the most influential geological work in the middle of the 19th century.
Systems of inorganic earth history Geoscientists support diverse systems of Earth history, the nature of which rests on a certain mixture of views about the process, control, rate, and state which are preferred. Because geologists and
geomorphologists tend to adopt opposite views over process, rate, and state in the inorganic world, there are eight different systems of beliefs in the development of the terrestrial sphere. All geoscientists stand by the principle of uniformity of law. Most, but not all, are directed by the principle of simplicity. All make definite assertions about the quality of rate and state in the inorganic realm.
Lyell Lyell's uniformitarianism is a family of four related propositions, not a single idea: • Uniformity of law – the laws of nature are constant across time and space. • Uniformity of methodology – the appropriate hypotheses for explaining the geological past are those with analogy today. • Uniformity of kind – past and present causes are all of the same kind, have the same energy, and produce the same effects. • Uniformity of degree – geological circumstances have remained the same over time. None of these connotations requires another, and they are not all equally inferred by uniformitarians.
Gould explained Lyell's propositions in ''
Time's Arrow, Time's Cycle (1987), stating that Lyell conflated two different types of propositions: a pair of methodological assumptions with a pair of substantive hypotheses''. The four together make up Lyell's uniformitarianism.
Methodological assumptions The two methodological assumptions below are accepted to be true by the majority of scientists and geologists. Gould claims that these philosophical propositions must be assumed before you can proceed as a scientist doing science. "You cannot go to a rocky outcrop and observe either the constancy of nature's laws or the working of unknown processes. It works the other way around." You first assume these propositions and "then you go to the outcrop." :*
Uniformity of law across time and space: Natural laws are constant across space and time. is necessary in order for scientists to extrapolate (by inductive inference) into the unobservable past. :*
Uniformity of process across time and space: Natural processes are constant across time and space. ::Though similar to uniformity of law, this second
a priori assumption, shared by the vast majority of scientists, deals with geological causes, not physicochemical laws. The past is to be explained by processes acting currently in time and space rather than inventing extra esoteric or unknown processes
without good reason, otherwise known as parsimony or
Occam's razor.
Substantive hypotheses The substantive hypotheses were controversial and, in some cases, accepted by few. ::Gould explained Hutton's view of uniformity of rate; mountain ranges or grand canyons are built by the accumulation of nearly insensible changes added up through vast time. Some major events such as floods, earthquakes, and eruptions, do occur. But these catastrophes are strictly local. They neither occurred in the past nor shall happen in the future, at any greater frequency or extent than they display at present. In particular, the whole earth is never convulsed at once. :*
Uniformity of state across time and space: Change is evenly distributed throughout space and time. ::The uniformity of state hypothesis implies that throughout the history of our earth there is no progress in any inexorable direction. The planet has almost always looked and behaved as it does now. Change is continuous but leads nowhere. The earth is in balance: a dynamic
steady state. He dismissed the first principle, which asserted spatial and temporal invariance of natural laws, as no longer an issue of debate. He rejected the third (uniformity of rate) as an unjustified limitation on scientific inquiry, as it constrains past geologic rates and conditions to those of the present. So, Lyell's uniformitarianism was deemed unnecessary. Uniformitarianism was proposed in contrast to
catastrophism, which states that the distant past "consisted of epochs of paroxysmal and catastrophic action interposed between periods of comparative tranquility" Especially in the late 19th and early 20th centuries, most geologists took this interpretation to mean that catastrophic events are not important in geologic time; one example of this is the debate of the formation of the
Channeled Scablands due to the catastrophic
Missoula glacial outburst floods. An important result of this debate and others was the re-clarification that, while the same principles operate in geologic time, catastrophic events that are infrequent on human time-scales can have important consequences in geologic history.
Derek Ager has noted that "geologists do not deny uniformitarianism in its true sense, that is to say, of interpreting the past by means of the processes that are seen going on at the present day, so long as we remember that the periodic catastrophe is one of those processes. Those periodic catastrophes make more showing in the stratigraphical record than we have hitherto assumed." Modern geologists do not apply uniformitarianism in the same way as Lyell. They question if rates of processes were uniform through time and only those values measured during the
history of geology are to be accepted. The present may not be a long enough key to penetrating the deep lock of the past. Geologic processes may have been active at different rates in the past that humans have not observed. "By force of popularity, uniformity of rate has persisted to our present day. For more than a century, Lyell's rhetoric conflating axiom with hypotheses has descended in unmodified form. Many geologists have been stifled by the belief that proper methodology includes an a priori commitment to gradual change, and by a preference for explaining large-scale phenomena as the concatenation of innumerable tiny changes." The current consensus is that
Earth's history is a slow, gradual process punctuated by occasional natural catastrophic events that have affected Earth and its inhabitants. In practice it is reduced from Lyell's conflation, or blending, to simply the two philosophical assumptions. This is also known as the principle of geological actualism, which states that all past geological action was like all present geological action. The principle of
actualism is the cornerstone of
paleoecology. == Social sciences ==