Proceedings of the Geologists' Association 1999/4 [antikvár]

Measurement, portrayal and analysis of orientation data and the origins of early modern structural geology (1670-1967) Richard J. Howarth HOWARTH, R. J. 1999. Measurement, portrayal and analysis of orientation data and the origins of early modem structural geology (1670-1967). Proceedings of the Geologists' Association, 110, 273-309. Dip and strike measurements were made by miners as early as 1505 and by the 1770s took place in a 'geognostical' context. Early portrayal of dip and strike in maps and the emergence of cross-sections and block-diagrams is discussed. In the 1820s, recordings of cleavage and fohation, lineation, joints, faults and shckensides began. By the 1840s interest in Élie de Beaumont's Systemes de Montagnes encouraged graphical summaries of two-dimensional orientation data (e.g. the strike of bedding, mineral veins or joints). Towards the close of the nineteenth century, primary effort was devoted to regional mapping, resulting in improved understanding of the nature of structures, particularly 'Alpine' thrust tectonics. The emergence of Hans Cloos' school of 'granite tectonics' in Germany in the 1920s, and its subsequent uptake in the United States, with the emigration of Robert Balk and Ernst Cloos in 1923 and 1930, is noted. Meanwhile, in Austria, Walter Schmidt and Bruno Sander were developing 'structural petrology,' dominated by the microscopic analysis of grain-orientation. Its enthusiastic take-up in Europe, Scandinavia and the USA popularized statistical analysis of three-dimensional orientation data. As scepticism grew regarding kinematic interpretations made by structural petrologists, interest in smictoral analysis based on field mapping returned in the 1940s. The subsequent emergence of early 'modem' structiu-al geology in Britain, with its emphasis on statistical analysis of small-scale structures, is discussed. The turning point is placed at about 1952. Department of Geological Sciences, University College London, Gower Street, London WCIE 6BT, (email: [email protected]). 1. INTRODUCTION 'It is with extreme distrust that the geologist regards all attempts to apply the exact methods of mathematics to the subject of his studies' (Suess, 1883-1909: transl. IV, p. 601) 'Field work is physically and mentally healthier than statistical laboratory analysis, but both are essential' (E. Cloos, 1946, p. 32). This paper reviews the development of methods for the display and analysis of orientation data from the earliest days up to the late 1960s, set against a background of growth in understanding of the meaning of structures, and something of the personalities involved. The geological map per se, is excluded here, since its development has been described elsewhere (e.g. North, 1934; Ireland, 1943; Boud, 1975; Butcher, 1983; Herries Davies, 1983; Oldroyd, 1990, 1996). The measurement of dip and strike and its portrayal in early cross-sections, block diagrams and maps is first considered. The growth of ideas concerning the nature of cleavage and foliation; lineation; joints, veins and faults; and thrust tectonics is next discussed, since this provided the motivation for collection and analysis of quantitative data. Early methods for the display of two-dimensional orientation data are next reviewed, followed by the work of Hans Cloos' school of 'granite tectonics' and the development of structural petrology, which were both Proceedings of Ihe Geologists' Association, 110, 273-309. major influences on the change to a three-dimensional approach. Finally, the transition from micro- to macro-scale studies is discussed, with emphasis on mapping and statistical analysis of small-scale structures, which has underpinned the 'modem' approach to structural geology. 2. OBSERVATION OF DIP AND STRIKE Terminology The importance of obtaining knowledge regarding vein orientation and inclination was emphasized in De Re Metallica, the first comprehensive treatise on metal mining and metallurgy, by the Saxon physician Georg Bauer (1494-1555), best known by his Latinized name, Agricola (1556, pp. 44-74). According to Hoover & Hoover (1912, pp. 610-11), the terms strike (.stricken), dip (fallen) and outcrop (.ausgehen) first occur in Eyn Nützlich Bergbüchlin von allen Metal (A useful booklet on the mining of all metals), published anonymously, possibly in Ausberg, Germany, in 1505 (a later edition was printed in Erfurt in 1527). The author was probably the Saxon physician and mining engineer Ulrich Rühlein von Kalbe (d. 1523). Over a century later, George Sinclair (or Sinclar, fl. 1630-96), Professor of philosopy at the University of Glasgow was 'ejected in 1662, for declining to comply with the episcopalian form of church government' (Chambers, 1837: IV, p. 238). In order to make a living, he moved to 0016-7878/99 $15 00 © 1999 Geologists' Association