S capolite is a common rock-forming mineral in parts of the Belt Series of Precambrian age in southern Shoshone County and adjoining parts of Clearwater County, Idaho. It is most abundant in moderately metamorphosed calcareous shaly layers of the Wallace Formation but occurs also in their highly metamorphosed equivalents and in the lowest part of the Prichard Formation. The mode of occurrence varies with distance from the Idaho batholith, with bulk composition, and with grade of metamorphism. In the northern part of the area where rocks were metamorphosed to the epidote-amphibolite facies, the highest concentration of scapolite is in layers rich in calcite, biotite, or hornblende and diopside. In biotite-rich layers interbedded with quartzite, scapolite is in round holoblasts; in carbonate granofels, crystals are euhedral to subhedral; and in hornblende- and diopside-bearing layers, small anhedral grains are common. In rocks metamorphosed to the amphibolite facies, such as diopside gneiss and calcium-magnesium-aluminum silicate rocks (here named “camalsite”), scapolite occurs in small anhedral grains. In diopside gneiss, scapolite is in thin layers that extend long distances parallel to the bedding. In camalsite, small masses exceptionally rich in scapolite are common. The mode of occurrence and the distribution parallel to the bedding suggest that scapolite crystallized from a sedimentary rock that contained saline minerals. The scapolite contains much Cl but only a little SO 3 , suggesting that halite was the chief source mineral. The local high concentrations of scapolite occur in a zone where elements have been redistributed, either because of metamorphism of dispersed or layered saline minerals with accompanying migration of chlorine or because of metamorphism of primary local masses of such minerals.

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 . . .