INTRODUCTION The age of the Glenarm series in general and the Wissahickon schist in particular has been under discussion for over 30 years, and in spite of much work done in the areas underlain by this series its age is still left in doubt. Some workers assign it to the pre-Cambrian because of lack of evidence to the contrary, and others hold it to be the equivalent of a metamorphic facies of the Ordovician Martinsburg shale. As an interpretation, the “Martic overthrust” has added a new element to the discussion. Obviously a good store of facts is needed on which all workers can agree and which should help all concerned. The problem involved is fundamental for the interpretation of width and depth of the Appalachian geosyncline, its deformation, the participation of the basement, and the role played by the intrusions. The area reaches roughly from New Jersey to Alabama and accompanies the Appalachians along their southeast side as a belt of “pre-Cambrian” crystalline rocks. This would leave the Appalachians without definite basement and also almost without accompanying intrusions found in other orogenic units like the Caledonians, Variscians, or the Alps. The authors are not particularly interested as to whether the Wissahickon schist is pre-Cambrian or Paleozoic or whether the Martic overthrust does or does not exist. They have tried an application of all modern structural and petrologic methods at their command in a controversial area in which detailed information is lacking, hoping that this may contribute to . . .