The Day the Universe Changed - James Burke [122]
Hutton watched the action of the wind, weather and frost on his own land and concluded that decay attacked landscapes just as it attacked organisms. Land would mature, erode and disappear. Such changes as were sudden could only be the result of underground upheavals. In 1785 Hutton found granite thrusting into younger rocks in Glen Tilt, Scotland, where these dikes cut through schist. The rocks were so irregular in shape that they could only have cooled from a molten state, and the mineral veins in them so ‘foreign’that they must have come from great depth. In 1787 he also found evidence of angular unconformity where strata had been forced upwards by earthquake or volcanic action. If this form of upheaval were continuous, as Hutton thought it was, then the earth was behaving now as it had always behaved.
After studying the effects of weathering and soil erosion, as well as the action of rivers in cutting out valleys, Hutton became convinced that, given enough time, the mechanisms at work in the everyday environment would have been sufficient to produce those phenomena which the diluvialists claimed were caused by catastrophic floods. In defence of the time his weathering processes would take, Hutton indicated the relatively unaltered state of ancient Roman roads. In brief, he claimed that erosion and volcanic activity could account for all possible present states of the landscape.
This view presumed slow and uniform processes at work, and because of this Hutton’s theories became known as ‘uniformitarianism’. ‘A theory,’he said, ‘which is limited to the actual constitution of this Earth, cannot be allowed to proceed one step beyond the present order of things.’
Hutton’s ideas were received unfavourably at first. The French Revolution had aroused conservative reactions in Britain and new scientific ideas were viewed with suspicion. It was not until the early years of the nineteenth century that uniformitarianism began to excite interest. Hutton was attacked by the diluvialists because his theory of river valley erosion did not make sense if the earth were only six thousand years old. It was not until the end of the first quarter of the century that Hutton was to find support, from an amateur geologist later turned politician called George Poulett Scrope, nicknamed ‘Pamphlet Scrope’.
In 1825 Scrope visited central France and conducted an exhaustive survey of the Puy-de-Dôme volcanic formation in the Auvergne, particularly near Puy de Dome itself, west of Clermont-Ferrand, as well as the nearby Limagne valley. Scrope avoided catastrophic explanations where more reasonable ones would do. He concluded that the lava-flows showed evidence of activity over a great period of time, and formulated the theory that the earth had once been extremely hot. This would have been followed by a gradual, though at times violent, cooling down stage.
In 1827 he published his findings in The Geology of Central France. In the same decade the theory of a cooling earth found support from physicists. It had long been known from practical mining experience that lower levels were increasingly hot. Louis Cordier now showed that the geothermal gradient was the same everywhere except near volcanoes. This subterranean heat also dissipated extremely slowly.
Jaujac, in Ardèche, illustrated in Scrope’s book on the volcanoes of central France. A cone and crater can be seen behind the village.
Adolphe Brongniart’s careful drawing of the fronds of a fossil tree-fern from the coal period (left), with a modern tropical tree-fern for comparison.
Jean Fourier’s physics showed that