later events overprinted pre-existing structures veiling the earlier history and the nature of protoliths. Although the Alpine overprint does not facilitate the recognition of older struc­ tures, pre-Mesozoic basement units were recognized during the first beginnings of geological observations in the Alps, about 200 years ago. Fifty percent of the Alpine domain is underlain by basement units that have been unconformably covered since Permian and Mesozoic times. This basement appears today in a complex pattern among the Alpine structures. The history of their discovery and explanation, parallel with a growing sophistication of research methods, are the subject of the introductory chapter of Part II.