One established bit of dogma accepted by both Catholic and Protestant into the 18th century and beyond was the biblical description of origin of the earth and its early history presented in the book of genesis.  Important to this story is the description in the bible of generations from which it was possible to estimate that the earth is about 6000 years old. Today this age remains an established fact for many Protestant sects and is important to both Creationism and its more recent spawn – Intelligent Design.  It is interesting to note that the Catholic Church no longer uses the bible to refute scientifically determined findings like the age of the earth and accepts that the earth is ≈ 4.5 billion years old.  Perhaps the Catholic hierarchy learned from their experience with Galileo.
   
    The scientific test of the Biblical story came as the Natural Philosophers, as the early scientists were called, began to look at the earth and study how it changed.  There are many “heroes” who are part of the story, but I’d like to focus on just one, James Hutton, because he made what I consider to be the most important and most difficult intellectual leap and so made the greatest contribution to our understanding of the earth and its history. (Image 1)
   
    Hutton was a wealthy Scot and so did not have to work.  Instead he took as his vocation a study of the rocks and geology of Scotland, cataloguing the features that he saw, making meticulous notes and drawings and most importantly trying to synthesize his findings, that is make the connections between them and begin to make sense of the many disparate facts that were his observations. One of his discoveries was that although rocks are not all the same but they can be classified as belonging to one of three important groups.  One important distinction among the groups is that two are layered and one is not.  The unlayered rocks are usually composed of crystals arrange in a random pattern (Image 2) but some have very few if any crystals.  These latter ones may appear frothy in places (Image 3), be associated with volcanoes and look very much like the slag that is the once molten rock produce from iron furnace as iron ore is reduced to pig iron. This led Hutton to one of his very important insights or syntheses.  
   
    Hutton applied a principle which he named uniformitarianism to the geological problems he was studying.  It states that the laws of natures (or laws of physics, chemistry, biology and geology) are the same everywhere and throughout time.  In some way we all know this. For example, we are not surprised to see a ball that has been thrown into the air fall back to earth.  And, I imagine if I asked you what would happen to a ball thrown up in the air on Mars, you would say it would fall back to the Martian surface.  But what would happen if you could go back in time 3 million years and throw a ball into the air on that ancient earth?  I’d be very surprised if you answered, “it would keep rising forever”, or “it would float as if it were a cloud.” No you would expect that it would fall to earth because we are all uniformitarianists at heart – it is the very nature of our experience to expect the expected.
   
    Back to Hutton – he looked at the rocks that looked like slag and that were associated with volcanoes and even though he had never seen a volcano erupt, he decided that the rocks – the lava – must have been once molten like the slag from the iron furnace and that the molten rock had flowed out of the volcano just as the slag flowed from the furnace. Because they were similar to the rocks with crystals in that they had no layers, and because some had some crystals randomly distributed through their mass, he also concluded that rocks like granite had once been molten. Finally he found examples of granite that cut across layers of other kinds of rocks as if it had filled a crack while it was still a liquid.  This supported his conclusion but also told him that the granite was younger than rocks it cut. It is just impossible to cut something that is not there, so the layered rock had to be there before its layers could be broken or interrupted by the crack that the granite filled. This was a second BIG IDEA and is so important to geology it is now called the Law of Crosscutting Relationships.  Take a minute to convince yourself it makes sense and to understand that its validity is also dependent on the principle of uniformitarianism.

    Now among the layered rocks were many that look very much like sand or mud or pebbles – that pieces of rock that had somehow accumulated and then been recemented to form new rocks.  Studying this, Hutton realized that you could almost always find pieces of rocks in rivers and streams that transported them toward the ocean where the movement of the water ceased and the particles settled to formed mud or sand (Image  4) or pebble beaches.  If you dug into these beaches you could see that the sand, for example, had been deposited in nearly horizontal layers. It was a much smaller leap then to draw the conclusion that these rocks were comprised of sediments, that is particles of rock that had been transported and then settled to ocean floor to form great flat layers. If you have ever made a sand castle and seen it flattened by the waves, you know that sand in water always tends to flatten and spread.  You can create a temporary sharp edge to the castle or dig a moat, but when the tide comes back, the ocean will flatten all your work.

    Studying the layers of sediment, Hutton drew another important conclusion relating to the relative age of the layers.  Geologists speak now of relative and absolute ages.  Relative means in comparison and relative age is spoken of as older or younger. Absolute age is a number of years.  Your age whether it is 18 or 25 or 55 is an absolute age.  That the fifty-five year old is older than the other two is a relative age. Hutton realized that the layer on top had to be the youngest because it could not have settled on something that was not already there.  So in the photo (Image  5) top. This is the Law of Superposition.

    For this story there is one other aspect of Hutton’s work that is important to the problem of the age of the earth.  He traced the source of the sand, mud and pebbles upstream into the mountains of Scotland.  There he found that their source was often layered rocks made of sand, mud or pebbles that were being broken apart by weather and gravity. Not only that the layers that were so flat and horizontal on the beach were now often tilted and sometimes folded in great bends. He went onto associated tilted and folded layers of sedimentary rock with mountains.

Siccar Point
    One of the outcrops that Hutton studied was a series of layered sedimentary rocks exposed on the North Sea coast near Edinburgh, Scotland called Siccar Points (Image  6).  In the photos you can see the rocks that Hutton saw – two sets of layers, a lower set with the layers nearly vertical and an upper set that is nearly horizontal and mostly eroded or washed away by the waves. Hutton understood what he saw, but I imagine it must have been very difficult to convince himself that his conclusions were correct because his conclusions had revolutionary implications. Let us work through the analysis.

    First we can determine the relative ages of the two sets using either the Law of Superposition or the Law of Crosscutting Relationships.  Both laws require that the rocks on top are younger because they are on top and because the cut across the layers of the lower set. This implies that there was time when the upper set was not there, so we will remove them from the picture.

    This leaves a set of sedimentary layers that are far from horizontal and that end abruptly.  Neither is possible as a result of the formation of the rocks as the sediments were deposited in the ocean and so must reflect things that have happened to them since they formed. Hutton’s experiences led him to interpret the events as a process that had tilted the rocks and lifted them up into a mountain range that had been eroded back down to near sea level. It was this eroded surface that became the base on which the sediments of the upper set were deposited.  So in summary, the lower set had been originally deposited on the sea floor as horizontal layers of sandy sediments.  Subsequent events had taken those sedimentary rocks, tilted and folded them up as part of a mountain range that had then been eroded back to near sea level, and the upper set of sediments had then been deposited on top of the eroded layers.

    While it all seems quite simple and straight forward written out like this, we must remember that for Hutton, his conclusion would have seemed absolutely impossible.  Everyone knew that the world was only 6000 years old, and Hutton knew that what he was describing as the history of the rocks at Siccar Point could not possibly occur in just 6000 years.  This is why I think of him as a hero – he accepted the outcome of his analysis even though it threatened his system of beliefs. He made a great intellectual leap to determine that the earth must much older than was accepted. For the rock formations at Siccar Point to have formed in manner consistent with the principle of uniformitarianism, the earth was millions of years old and possibly hundreds of millions of years old. He abandoned the old idea in favor of a new idea based on scientific observation and analysis. His accepting the conclusion of his analysis and reporting his findings began one of the great scientific revolutions and reshaped our understanding of the earth and its history. Hutton's conclusions are still, as I noted in the beginning, being objected to and fought against by those unwilling to allow their belief systems to be challenged by fact.

    It is important to realize that in some ways the age of the earth is a secondary issue to the creationists who argue against an old earth and evolution.  However, the question of age was from the very first key to Darwin’s intellectual development and his description of evolution through natural selection and has been used over the last century and a half as a test of Darwin’s idea because a young earth would be inconsistent with the origin of species through evolution and natural selection. (Image 7)
 
    As to the relationship between Darwin and Hutton, it is relatively direct. Hutton like all good scientist shared his ideas, first in a series of discussions with friends in Edinburgh and then in a book, the first modern geologic text.  Unfortunately for many scientists communication through writing is not a strength and by all accounts, the Hutton's writing was nearly unreadable. It fell to a companion named Playfair to rewrite his ideas in a readable and understandable fashion.  Playfair’s book begat a third generation text by Lyell which was given to the young, religious Charles Darwin for reading when he set sail as the ship’s naturalist on the Beagle. Darwin was at first skeptical of the conclusions presented by Hutton and Lyell, but by the time he had reached the Andes and found sea shells in the rocks of the highest peaks he recognized the power of their ideas to explain what he was seeing.  Most importantly in accepting their conclusion that the earth must be very old, it became possible for him to imagine a gradual process of evolution producing the huge variety of life he observed during his travels with the Beagle.

    So to me Hutton represents the very best of science -- the willingness to challenge our own "knowledge" and beliefs when confronted with new information that does not fit our view of the world.  Each change in world view, small or large is likely to have repercussions leading to a chain of events unforseen by the women and men who make the discoveries. Hutton could not know that his analysis was laying the groundwork for future discoveries in many fields of science from medicine to astronomy that would affect the lives of all of us.

Reading question:  Explain the importance of Hutton's realization that the earth is millions of years old to Charles Darwin as he developed the Theory of Evolution.

Image sources:
(1) James Hutton -- http://www-history.mcs.st-and.ac.uk/Mathematicians/Hutton_James.html

(2) Granite, Sierra Nevada, CA: J. Alcock, rights reserved.

(3) Basalt, Craters of the Moon, ID: J. Alcock, rights reserved.

(4) Sandstone, Grand Canyon, AZ: J. Alcock, rights reserved.

(5) Mudstsones, Mojave Desert, CA: J. Alcock, rights reserved.

(6) Sandstones, Siccar Point, North Sea Coast of Scotland: J. Alcock, rights reserved.

(7) Charles Darwin -- http://www.nmm.ac.uk