FAMOUS GEOLOGISTS/Hutton 201 But it was obvious that soil was constantly being washed into the sea, and, since it was essential for human well-being, it had somehow to be replenished As a deist, rather than a biblical literalist, Hutton could take a grand view of time The Earth could be millions of years old, but in that case the land would eventually be eroded to a plain and the good soil would end up as sediments in the seas So Hutton asked himself how high ground could be regenerated to provide a source of new soil Hutton’s Theory of Cyclic Earth Processes In 1764, Hutton made a journey into the Highlands and began to collect geological information and specimens in a systematic manner His farms were by then profitable and the sal ammoniac business was prospering So he began to think of returning to Edinburgh, now as a gentleman–farmer Probably in 1767, he rented out his farms and returned to Edinburgh (his old scandal had by then been forgotten or forgiven) to enjoy the pleasures of intellectual life in one of the great cities of the eighteenth-century Enlightenment Among Hutton’s new friends were the economist Adam Smith, the chemist Joseph Black, who conducted experiments on heat, and the engineer and steam-engine inventor James Watt It is likely that Watt’s engines encouraged Hutton to think of heat as an agent of geological change Perhaps the Earth had a central source of heat that might somehow drive the cycle essential for a theory of the Earth that provided for a renewal of soil? The Earth’s internal heat could be analogous to the fire of Watt’s engine, which drove the complicated mechanism of the engine and the machinery of a factory But Hutton did not imagine that the Earth’s internal heat was due to combustion Hutton’s theory of the Earth was, then, developed as follows The Earth, he thought, had a central reservoir of heat, the source or means of maintenance of which was unspecified Rocks on the surface were broken down by weathering and erosion to form soils Sediment was deposited in the seas by rivers, which also carved valleys Sediments accumulated in layers on the ocean floors, and the lower layers were compressed and consolidated by the sediments deposited on top of them, assisted by the Earth’s internal heat The rock-salt deposits of Cheshire seemed to Hutton to have been melted at some time Likewise, the grains of sand in quartzites seemed to show evidence of fusion at their edges in the process of consolidation by heat In time, the consolidated materials, under pressure, might become so hot that they would melt Veins of crystalline rock, dykes or sills, could be emplaced Moreover, Hutton supposed, great masses of molten material (which we would call magma) could be intruded into the Earth’s crust, heaving it up On cooling, this magma might crystallize to form subterranean masses of granite, which might subsequently be exposed by weathering and erosion Thus the land would be renewed and Hutton’s Earth, ‘designed in wisdom’, would continue indefinitely as a place suited to human habitation The upheaval of strata was confirmed by the presence of marine fossils in strata well above sea-level However, at the time of the first public presentation of his theory, Hutton appeared to have personal knowledge only of mineral or metallic ore veins, not granitic veins, and he did not then describe any personal examinations of large granitic bodies He went looking for these systematically only after the preliminary presentation of his ideas Be that as it may, the whole process envisaged by Hutton was cyclic, for the upheaved strata would be eroded to form a new surface, on which other sediments might subsequently be deposited So one might hope to find places where the lower layers were inclined to the horizontal and the overlying ones lie over them horizontally Such a structure came to be known as an unconformity, and the subsequent discovery of unconformities was considered a triumph for Hutton’s theory, as he apparently had the idea of such structures before he actually saw them An unconformity could be taken to mark the end of one cycle and the commencement of the next Hutton’s cyclic Earth processes were continuous and open ended He did not say that the Earth was infinitely old, but as he put it in a famous sentence: ‘we find no vestige of a beginning—no prospect of an end’ Hutton (1788 p 304) His cycle has been called the ‘geostrophic cycle’ (see Figure 1, which explicates Hutton’s notion of unconformity) Hutton’s theory was formally read before the Royal Society of Edinburgh in 1785 and published in 1788 It appeared in expanded form in two volumes in his Theory of the Earth in 1795 Two further incomplete volumes remained unpublished in his lifetime, but the manuscripts were found in the nineteenth century and published in 1899 as Volume This book described Hutton’s fieldwork after the presentation of his 1785 paper Hutton’s 1785/1788 paper did not explain the Earth’s internal heat, but he tried to use field specimens to support its existence Many of the materials that bind sediments together, such as calcareous spar, silica, etc., are not themselves water soluble