Archaean Geochemistry 1972 - 1984 The realisation that the continental crust contains well-preserved relics which date as far back as 4/5 of the Earth's age has given a great impetus to the study of early Precambrian terrains. As late as the mid-sixties the Archaean still constituted the 'terra-in cognita' of earth science. High metamorphic grades, poor out crop, and not least a widely assumed obliteration of early crustal records by convective recycling and thermal reworking had com bined to discourage research in this field. Many excellent local studies existed, notably around gold mining centres, but remained unrelated to a broader regional and theoretical understanding. This situation has changed as the consequence of two inter-related factors: (1) advances in isotopic methods and their application to Precambrian rocks, and (2) the recognition that some of the oldest terrains have retained a wealth of primary igneous and sedi mentary textures and even geochemical characteristics.

'What are we going to do with a parcel of old stones?' wrote the director of an African museum a century and a half ago, when one of my ancestors presented him with a splendid collection of fossils of mammal-like reptiles. Old stones, however intriguing, are difficult to interpret, dusty, and do not fit well in the neatly ordered contents of a house of learning. Archaean geology, which is the study of the Earth's history in the period from after 9 the end of planetary accretion (4.5-4.4 x 10 years ago) up to the beginning 9 of the Proterozoic (2.5 x 10 years ago) is much the same - a parcel of old stones seemingly impossible to understand. Yet these stones contain the history of our origins: they can tell us a story that is interesting not just to the geologist (for whom this book is primarily written) but instead addresses the human condition in general.