A summary of the geologic history and structure of the British Isles is needed as a background for the more detailed contributions on numerous aspects of British geology which follow. In keeping with the topic of the symposium, the summary is limited to pre-Permian events, and thus attention is focused on northern and western areas.

The pre-Permian geology of the British Isles is dominated by the arcuate but essentially NE-SW-trending Caledonian mobile belt which involves (mainly) late Precambrian and early Paleozoic rocks in a zone approximately 400 km wide. The mobile belt is bounded on the northwest by a foreland of high-grade metamorphic gneiss of Precambrian age unconformably overlain by a late Precambrian and early Paleozoic sedimentary cover. The gneiss is dated isotopically at 2,600-1,200 m.y. Southeast of the mobile belt is a platform on which a lower Paleozoic shelf facies overlies volcanic, metamorphic, and sedimentary Precambrian rocks that contain intrusives (with minimum ages up to 680 m.y.). Within the Caledonian belt itself, a complex history of deformation and metamorphism is recognizable, beginn ng with late Precambrian events (740 m.y. ago or earlier), extending over perhaps as much as 400 m.y., and merging (in the south) with events in the E-W-trending Hercynian mobile belt in southern England, where orogenic movements culminated during Carboniferous time.

The Caledonian and Hercynian belts can be traced eastward into continental Europe. Mobile belts of comparable age and trend are present in North America and (in the case of the Caledonian) in east Greenland. The earlier history of the northwest Caledonian foreland is analogous to that of part of the Canadian-Greenland and Baltic shields. Rocks of the age of the youngest (Grenville) province of the Canadian shield (1,100-800 m.y.) apparently are absent from the British Isles, but may have been reworked during a later orogeny.