History of the Study of Mudstones since 1556

In 1556 Agricola published posthumously the first detailed description of a mudstone, but it was not until 1747 that one of the mudstones, a black shale, was first formally defined by William Hooson. A review of the world literature since Hooson's time shows a slow, steady progression in the study of mudstones through the 1800s, followed by an abrupt change to a more rapid pace of advances in the early 1920s. This phase was interrupted by World War II, which caused a sharp drop in activity, but the rate of activity resumed shortly afterwards, until an apparent reversion to slower progress starting in the 1970s. The trends are similar for both conceptual advances and for new techniques, suggesting that the two are closely linked. The break in the 1920s was led by new technologies that expanded observations down to the angstrom level with X-ray diffraction and up to the formation scale with down-hole geophysical logging and seismic reflection. We find it significant that the earliest publications on mudstones come from mining — Agricola and Hooson — and that almost one hundred years later Sorby studied slaty cleavage for the mining industry, followed by Le Chatelier on the heating of clays for ceramics. Thus, practical applications rather than basic research fostered many of the early advances in the study of mudstones. Broadly speaking, the peak in rate of advances in the study of mudstones lagged significantly behind those for igneous and metamorphic rocks. The acceleration of work on crystalline rocks began in the decade of the 1880s, whereas for mudstones, it is in the 1920s, owing, we believe, to the difficulty of working with mudstones in thin section and the difficulty of interpreting their whole-rock chemistry.