The Day the Universe Changed - James Burke [124]
Scrope’s descriptions of central France intrigued Lyell. The Auvergne was a volcanic area, formed of basalt-capped hills, old craters and deep river valleys. The sedimentary strata were freshwater, sometimes covering, sometimes covered by volcanic deposits, often lying at heights which varied as much as 1500 feet. It looked as if there had originally been early valleys filled by lava flow, after which rivers had carved new valleys out of the lava.
In 1828 Lyell arrived in the Auvergne together with a fellow-enthusiast, Roderick Murchison. Near Aurillac they found a range of low hills formed of layers of marl, sometimes as thin as one-thirtieth of an inch. In each layer were the flattened stems of chara algae, freshwater shells and tiny marsh animals. Each layer was formed by a year’s deposit. The depth of the marl gave evidence of steady unchanging processes at work over thousands of years. The same was true of deep fissures cut into lava by the rivers.
After they had left the Auvergne for Nice, Murchison became ill. When he had written up his Auvergne notes Lyell left Murchison and headed south, where he sought for evidence of the passage of time in the volcanic areas round Vesuvius and Etna. On the island of Ischia in the Bay of Naples he saw high on the side of the central peak a stratum of clay with thirty species of marine shells, all of which were identical to modern Mediterranean types. These were young fossils elevated hundreds of feet by recent volcanic activity. Outside Syracuse harbour, in Sicily, Lyell found more of the same. In a marl outcrop halfway up a cliff, he again found corals and shell fossils of the modern type. Here, however, the marl was deposited beneath very old limestone. Finally, at Enna, in the centre of Sicily, he found an immense escarpment made up of all the strata he had already seen and filled with fossils of the modern variety. The strata stood no less than 3000 feet above sea-level.
Lyell’s choice of the mollusc as a ‘clock’is explained by these shells. Above, a fossil from the earliest times. Below, its virtually unchanged descendant.
Lyell’s suspicion that geological processes involved vast stretches of time was confirmed on the plain of Catania. There, he found limestone strata with marine fossils similar to present-day organisms. These layers of rock passed under Etna. Lyell had already seen the dozens of secondary cones around the side of the main volcano and on the basis of historical evidence had concluded that they had taken at least 12,000 years to form. The Bove Valley, cutting deep into the mountain flank, revealed more buried cones. There must be thousands more, hidden by lava from the main volcano. Lyell recognised that all the cones and the central peak had been gradually built up by single laval flows, and that the entire mass, now 10,000 feet high and ninety miles wide, must have taken millions of years to form. The fact that the limestone layer which passed under Etna contained fossil organisms virtually identical with their modern descendants convinced Lyell that the earth was immeasurably old.
In February 1829 Lyell was back in London, where he immediately began writing. In June the following year the first volume of his three-volume work, Principles of Geology, appeared. In this first book Lyell included a history of geology and a description of the inorganic physical processes at work in the modern world. In the second volume he dealt with processes such as the type of climatic change which might cause species to appear and disappear. In the final volume he put forward a theory which was to shatter the biblical complacency of the Victorian intellectual world.
His aim was to reconstruct the history of the earth, based on processes that were still continuing and on an ‘adequate’time-scale. For Lyell, uniform