Having first presented his theory in public as an abstract in 1785 to the Royal Society of Edinburgh, Hutton set-out to prove it over the next three years on four epic journeys throughout Scotland with John Clerk of Eldin, who recorded their findings in skilfully executed geological drawings. Hutton was in his late fifties. He had utmost faith in the evidence of rocks and what they tell us “Let us therefore open the book of Nature and read in her records”. He was able to prove his theory through accurate observation of them, and acute interpretation.
The surface of the Earth is composed mainly of rocks in layers. Detailed examination of what composed these rock layers led Hutton to conclude that they were the sediments derived from the weathering and erosion of an earlier surface. As he describes: “From the top of the mountain to the shore of the sea, everything is in a state of change; the rock dissolving, breaking and decomposing for the purpose of becoming soil; the soil travelling along the surface of the earth, in its way to the shore; and the shore wearing and wasting by the agitation of the sea…”
In hardened and compacted rock-layers or strata (the word first coined by Hutton), he recognised particles of sand and grit identical to that which he had observed in soil and also on the sea shore. Likewise larger pieces of gravel and pebble. He realised that the collection of sediment at the bottom of the sea would rest in layers and also there must be an uplifting of these layered compressed sediments to bring them above the surface of the oceans to form dry land; “We are led to conclude that all the strata of the earth have had their origin at the bottom of sea by collection of sand and gravel, of shells, and of earths and clays….consolidating those collections in various degrees and either elevating those consolidated masses or lowering the level of that sea. The spoils of wreck of an older world are everywhere visible in the present. The strata which now compose our continents are all formed out of strata more ancient than themselves”
Hutton recognised more than one cycle of erosion and uplift.
Hutton also needed to explain how rock layers beneath the surface of the ocean became uplifted so that they than became the surface of land. This he did through another leap of his imagination, that the Earth was a heat engine; that its internal heat fuses loose eroded debris into hard rock layers and raises them from the ocean floor to make new land. Hutton sought his evidence for the Earth’s internal heat from rock which had been melted and was once fluid. He found his evidence in igneous intrusions ie solidified melted rock (or magma) which has been forced under pressure from below, into fissures and cracks of other solid sedimentary rock layers, where it cools and solidifies. He gathered many examples of intrusions in his travels to prove his theory eg the Salisbury Crags and the Castle rock – on his doorstep in Edinburgh.
As evidence of the uplifting power of the Earth’s heat engine, Hutton looked again to the nature of sedimentary rock layers and observed that they were often “broken, twisted and counfounded, as might be expected from the operation of subterranean heat and violent expansion”. Had there been no uplift they would have remained as they had been deposited at the bottom of the sea – in undisturbed horizontal layers. He also cited the scale and power of volcanic activity with the widespread occurrence of igneous rocks as supporting evidence.
Having dealt with the issues of sedimentation, consolidation and uplift of rock layers, what of time? As these processes are barely measurable on a human timescale they were certainly impossible to demonstrate. Hutton postulated that they happened very slowly over a long period of time and that the earth was very ancient “with no vestige of a beginning”. He was able intellectually to conceptualise and argue his case because of his theory on time.