Granitic gneiss with swirling leucosomes and irregular biotite-rich restite layers is the dominant lithology and constitutes approximately 75 to 80 percent of the exposed rocks. The remaining 20 to 25 percent comprises hornblende-biotite gneiss, amphibolite with or without pyroxene, and pegmatite. Granitic gneiss is composed of quartz, plagioclase, biotite, and microcline. Minor and accessory minerals are garnet, muscovite, magnetite, ilmenite, sphene, apatite, and zircon. The hornblende gneiss contains plagioclase, quartz, hornblende, and biotite with/without orthopyroxene. Accessory minerals are garnet, muscovite, clinozoisite, perthitic orthoclase, iron-titanium oxides, sphene, and apatite. Amphibolites are composed of subequal amounts of hornblende and plagioclase with minor quartz, biotite, clinopyroxene, and orthopyroxene.
In Delaware, predominantly an impure quartzite and garnet-sillimanite-biotite-microcline schist. Major minerals include microcline, quartz, and biotite with minor plagioclase, and garnet. Muscovite and sillimanite vary with metamorphic grade. Accessory minerals are iron-titanium oxides, zircon, sphene, and apatite. Microcline is an essential constituent of the quartzites and schists and serves to distinguish the Setters rocks from the plagioclase-rich schists and gneisses of the Wissahickon Formation.
The Delaware Academy of Science has been instrumental in informing Delaware citizens about science and utilization of local resources. Since 1970 the annual meeting of the Delaware Academy of Science has been used as a time for presentation of ongoing research in various areas of science in the Delaware region. The proceedings of these meetings have resulted in publication of transactions of the Delaware Academy of Science. The 1976 annual meeting focused on aspects of the geology of Delaware. Members of the Delaware Geological Survey and the Geology Department at the University of Delaware contributed papers in their specific disciplines. This volume presents an overview of studies of geological features and processes of evolution of the geology of Delaware. Although this collection of papers does not represent an all-inclusive study of the subject, the selections included in this volume highlight past, present, and future trends in the study of Delaware's geology. It is hoped that the combined bibliographies of all the papers will provide a comprehensive view of the literature for further investigation into the geology of Delaware.