188 HISTORY OF GEOLOGY FROM 1900 TO 1962 a few chemists and geologists, including, at the turn of the century, Frank Clarke In the 1920s, attention turned to attempting to understand the distributions and abundances of the elements in the Earth’s crust, led by Goldschmidt in Germany and Vladimir Vernadsky in Russia Goldschmidt’s Geochemistry (published in 1957) brought together the essentials of the subject, although it had already moved on with the studies of isotopes begun at the University of Chicago, immediately after the Second World War, by Harold Urey and his co-workers An international Geochemical Society was formed in 1955, and important seminars were held in 1957 and 1958 at the Carnegie Institution The study of organic geochemistry also developed, with practical work related to deleterious substances in coal, mine dusts, and petroleum, and their dispersal into the atmosphere Stratigraphy and Sedimentology Stratigraphy and the attempt to write the history of the Earth’s crust had been the essence of geology since the late eighteenth century But the rock layers and their enclosed fossils called for further study During the first part of the twentieth century, the field mapping of the late nineteenth century was extended to many parts of the Earth that had been previously untouched A major improvement in the maps was the attention to structural and sedimentological detail Around 1920, it was shown that features such as cross-bedding and graded bedding could indicate the order of superposition, particularly in vertical and highly folded non-fossiliferous strata The difference between cleavage and bedding in metamorphic rocks also had to be recorded Thus previous interpretations of important regions, such as the Scottish Highlands, had to be re-examined The structural geologist Marland Billings wrote ‘‘one clear contact or key bed is more valuable than a hundred petrofabric diagrams’’ To make sense of the observable history worldwide, international agreement was needed about naming certain features and presenting them on maps and crosssections Stratigraphical commissions were established to set down rules for such things as identifying ‘type localities’ The study of facies – the total nature of a volume of strata (rock composition, fossil content, type of bedding, sedimentary structures, etc., usually reflecting the conditions of origin) – was developed by Johannes Walther and others Walther recognized that facies relations were dynamic He stated that only rock types that can be deposited side-by-side can overlie each other directly in a vertical sequence This led to the study of time-stratigraphical units, with the recognition that rock boundaries did not necessarily correspond with time In this work, and in sedimentation studies in general, Amadeus Grabau was an acknowledged leader Statistical analysis, long neglected by geologists and first used in petrology by Paul Niggli in 1924, became widely applied in stratigraphy, with information about rock-layer thickness (isopach) and lithofacies variation (e.g sand–shale ratios) being presented in graphical form Closely related to stratigraphical studies was the examination of the materials of sedimentary rocks Pioneers were Johann Udden and Chester Wentworth, who, in about 1920, studied size distributions and the shapes of grains, devising a quantification chart that was useful in the interpretation of sediment histories Ralph Bagnold’s study of the physics of blown sand in 1941 brought together considerable earlier research Between 1900 and the 1950s, the heavy accessory detrital minerals of sedimentary rocks were widely used for stratigraphical correlation by William Rubey, Percy Boswell, and others Laboratory work began to supplement field studies From the 1930s flumes were widely used by Henry Milner, Paul Krynine, and others to examine the behaviour of sedimentary materials under different conditions One problem, not always adequately addressed but worked on by Francis Shepard, one of the first US marine geologists, and M King Hubbert, was the scaling from the actual geological dimensions to the laboratory dimensions In the early 1940s, Krynine attempted to set up a sediment research laboratory in the USA, bringing together the skills of oil companies, academics, and government geologists This did not eventuate, and separate laboratories continued In 1950 experimental flume work by Phillip Kuenen and Carlo Migliorini showed how ‘turbidity currents’ occurred, carrying materials rapidly downslope from a shallow source This led to a ‘turbidite revolution’ in interpreting many types of stratigraphical occurrences, particularly greywackes, graded bedding, and other sedimentary features, and to an understanding of how some submarine cables might have been cut on continental shelves following earthquakes Palaeontology The study of invertebrate fossils continued apace during the first half of the twentieth century Initially palaeontologists were concerned mostly with taxonomy and classification, but additional fields opened up with the study of the evolution of particular groups The Jurassic ammonites provided an example of a rapidly evolving fauna, which helped to pin down time zones, while the Ordovician and Silurian graptolites facilitated correlation between sedimentary rocks from these periods in widely separated parts of the