Extract

The study of the Lewisian is entering a third phase. The first was the mapping of the complex by the Geological Survey. The second was the dating of the main events which demonstrated the long time span, almost one third of geological time, which separated the early Scourian crustal accretion from the latest Laxfordian igneous activity. This chronology and comparable advances in Canada, Greenland and Scandinavia made it possible for the first time to view the Lewisian not as an isolated fragment of Precambrian, which was the situation when we first took up this problem nearly forty years ago, but as a component in the Precambrian of the northern hemisphere. The third phase, which is now under way, is providing information on the processes that produced these remarkable rocks. We can now begin an attempt to reconstruct the settings in which Scourian and Laxfordian events took place. This is going to be a very difficult task; it involves extracting from the rocks evidence as to the nature of the crust and mantle below the sections of the complex visible today and reconstructing the overlying material, at times 45 kilometres thick, which has now vanished. The key to the Lewisian lies in understanding the conditions under which the accretion of the Scourian occurred between 2700 and 2900 million years ago. For these rocks (the ‘Old Boy’ of the Geological Survey) make up the bulk of the complex, although we now know that they were profoundly modified and received many further additions