Hutton noticed that rocks consisted of strata, parallel orderly layers of consolidated sediment. The layers were composed of different materials that must have been derived from rocks even older than themselves. He thought this was similar to what was currently happening on the ocean floor, where a new layer of sediment was forming. This new layer of sediment contained bits and pieces of material that had been worn away from the land of some preexisting continent and carried out to sea by the natural flowing of waters. Hutton believed the subterranean heat emanating from the interior of the Earth transformed these layers into solid structures. He was aware of the roles that pressure also played, the compaction from upper layers, and the prevention of volatile substances from escaping. Thus the layers of sediment became consolidated after being compacted and cooked for long periods of time.
How could this mechanism account for the formation of mountains? If strata were formed under the sea, what about landmasses that existed thousands of feet above sea level? Hutton again believed that some force underneath the surface of the Earth was responsible. He witnessed powerful volcanic eruptions that he thought were the result of great expansion of the burning igneous matter in the interior of the Earth. He proposed that these great expansions also occurred in the geological past. They caused convulsions that ripped up through the ground and forced rock and crust upward, causing bending and folding, forming mountains and hills. Magma that had not penetrated the Earth's surface during volcanic activity cooled and solidified, forming granite or other crystalline rocks. This in itself was a novel proposition, since at the time the existence of igneous rocks as a type of rock completely separate from sedimentary rock was not recognized. If this were all true, then he predicted that arrangements should exist in the strata such that some upturned strata would be vertical or tilted relative to undisturbed layers. One would expect the slanted strata to have been eroded, and then eventually be overlaid by a new layer of horizontal sedimentary rock. Such structures, called unconformities, are quite prevalent. One famous locality, called Hutton's Unconformity, was located near his home in Berwickshire, along the west coast at Siccar Point.
Erosion played a key role in Hutton's theory of how the Earth was sculpted. Dry land decayed unremittingly. Flowing water and pounding waves ate away at rock beds. Wind and weathering acted on exposed surfaces of mountains, producing new soils. Glaciers broke loose and transported chunks of rocky matter with them. Loose soil containing mineral components and organic matter was washed away by rain, and silt was carried by rivers. Chemical reactions in water caused particulate matter to precipitate out of solution. Eventually all of the loose particulate matter made its way to the oceans, where the sediment settled and was compacted to form new layers of strata, completing the geological cycle.
Hutton was the first to recognize that many igneous rocks were younger than the rocks in which they were found. Veins of unstratified rock-filling fractures were described from many locations. Hutton had predicted these would occur if the granite in these veins was once melted, and that during the forceful convulsions it was pushed up and outward into cracks that resulted from the violent tremors. The problem with this idea was that no one understood the origin or composition of granite.
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