The Theory of Earth

An endlessly varied, picturesque and rigorous walk around the colourful north coast of Arran ended with a bit of geekery that, of course, I found irresistible 

At the end of the 18th century it was generally accepted by the church, the geological community and society in general that all the rocks in earth were precipitated at the same time out of the waters of the biblical great flood as they evaporated, and that this happened roughly 6000 years ago

A child of the Scottish Enlightenment called James Hutton did not belive this and had his own ideas. He began searching Scotland for evidence to support his case, and this is the first example he found. The rocks on the top and right of the picture exhibit layers that fall away gently to the right. The ones at the bottom left have layers that fall steeply to the left. The two orientations of rock abut directly against one another whilst having layers more or less at right angles to one another and neither one is level

Hutton argued that this, along with other evidence, showed that these rocks formed at different times, separated by an immeasurably long period. More generally, he created a convincing case that the rocks we observe now were formed long ago by processes that we can also observe now (from sediments in the oceans and volcanic activity), occurring at the same rate, and that this process has been continuous since the distant past and will continue. 

This is the fundamental principle on which all modern geology is built. He expressed this with typical Scottish eloquence and economy with words: "The result, therefore, of our present inquiry is that we find no vestige of a beginning and no prospect of an end". How terrifying for the 18th century worldview

We now know that the rocks on the bottom left are 540 million year old schist and those on the right are 350 million year old sandstone. He was right. 'The Theory of Earth' was the title of the paper he produced that outlined his ideas. What a title to be able to use!