The U.S. Geological Survey recently assessed the potential for natural gas resources in the Middle Devonian Marcellus Shale of the Appalachian Basin Province. The Marcellus Shale was assessed as a continuous gas accumulation using a methodology identical to that used in the assessment of shale and other continuous-type assessment units throughout the United States. This preliminary report provides some additional geologic information used in the Marcellus Shale assessment. The Appalachian Basin Province encompasses rocks of the Paleozoic passive margins, the foreland basins, and fold and thrust belts formed during several episodes in the Paleozoic. The Marcellus Shale is one of many marine shales deposited in the area that is now encompassed by the Appalachian Basin Province.