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Pre-scientific geology: 15th - 18th century

From the days of ancient Greece men wonder how rocks were formed, and how fossils came to be embedded in them - even, in places, high among mountains. Theories abound, but there is no way of checking them except by using common sense to weed out absurdities.

As ever, Leonardo da Vinci is a pioneer in the application of some measure of scientific rigour. He is able to refute the popular theory that Noah's flood lifted sea creatures up the hillsides; he points out that fossils are embedded right through rocks and not only on what might have been the surface during a temporary flood. Leonardo concludes that they must have been deposited on some ancient sea bed. How they moved up from there remains as yet a mystery.

During the 17th century links begin to be made between sedimentary deposits in the oceans and the strata observable in rock formations. But any concept as to how they have been transformed into rocks and mountains is much handicapped by the prevailing view as to the age of the world.

This derives from a chronology published by James Ussher, archbishop of Armagh, in 1650. Ussher calculates, by assigning exact periods to the succession of events in the Old Testament, that God began his hectic week of creation on the morning of Sunday, Octber 23rd, 4004 BC.

With less than 6000 years available for the formative events of geology, the concept develops that all rocks must derive from sedimentary deposits - and that they have been flung up into the form of mountains in a few moments of catastrophic upheaval.

This view is expounded persuasively by Abraham Gottlob Werner, a distinguished professor at the school of mining in Freiberg. But from 1785 an alternative explanation is put forward by James Hutton in Edinburgh. The rival camps become known as the neptunists (believing with Werner that all rocks were formed in the sea) and the plutonists (following Hutton's theory that some derive from erupting lava, Pluto being god of the underworld).

James Hutton and Strata Smith: 1768-1815

James Hutton is a doctor and farmer who retires in 1768 and then devotes himself to his hobby of studying the rocks around Edinburgh. His observations lead him to disagree with the orthodoxy of the time, the theory propounded by Werner and the 'neptunists' that all rocks have been formed in the sea.

Hutton sees evidence of rocks being formed by volcanic eruption and by erosion as well as by sedimentation. And he is convinced that these processes, far from being the result of isolated catastrophes, are still continuing. In a much quoted phrase, he sees 'no vestige of a beginning, no prospect of an end'.

The paper which Hutton reads to the Royal Society of Edinburgh in 1785 is entitled Theory of the Earth, or an Investigation of the Laws Observable in the Composition, Dissolution and Restoration of Land Upon the Globe. In this concept of a continuing process, in which the long slow story of the earth's development can be read, Hutton lays the basis for scientific geology.

The next important observation results from the practical work of canal building in England. William Smith is an engineer employed in carrying out surveys for canals. He becomes interested in the rock strata revealed in the excavations and begins to chart them, listing the fossils which they contain (and acquiring the nickname Strata Smith).

He observes that the same rock strata can be identified in different regions, that they always lie in the same sequence and that each kind of rock contains its own range of fossils. His findings are first assembled in a document of 1799 entitled Order of the Strata, and Their Imbedded Organic Remains, in the Neighbourhood of Bath. Smith later compiles a complete geological map of England, Wales and part of Scotland which he publishes in 1815.

Related work is carried out at much the same time in northern France by the great naturalist Georges Cuvier. With the realization that the rocks and their fossils give a coherent and reliable account of the earth's past, geology becomes the scientific craze of the early 19th century.

While amateurs are out with their hammers, knocking at the rock face in any area rich in fossils, the professionals continue to chart, date and name the identifiable strata and their periods. Several of the names are based on British places (Cambrian, Devonian) or Celtic tribes (Ordovician, Silurian) because much of the work is done in Britain by Adam Sedgwick and Roderick Murchison.

The completed chart of Geological periods, international in its application, opens the door to palaeontology and palaeobotany (the scientific study of animals and plants through their fossil remains). And discoveries in those sciences provide the evidence leading to the theory of evolution.

This History is as yet incomplete.