The generalizations and conclusions presented in the present paper are based on the detailed work of the writer and his assistants on the Maryland Geological Survey, covering the territory from the Susquehanna river to longitude 76 degrees 50 minutes, with the exception of a small area in the immediate vicinity of Baltimore, and on the lines determined by the late Professor Williams, confirmed by a rather detailed reconnaissance by the author, on the territory west of this line to the area of the Frederick and West Washington sheets, where the work of Mr Keith, supplemented by reconnaissance work by the author, is used. The writer has also had access to the field notes, manuscript material, and microscopic sections of Professor Williams, which were prepared by him during his work on the United States Geological Survey. The conclusions reached from the information received from these various sources are, however, the . . .