The Potsdam sandstone of Late Cambrian age crops out along the N., E., and SE. borders of the Adirondack Mountains in northeastern New York state. It has generally been described as an orthoquartzite. In the Mooers area, however, the Potsdam is mainly a subarkose or arkose. Detailed analysis of 10 thin sections of the Potsdam sandstone from the Mooers Quadrangle provide data for grain-count composition, median grain size, cumulative size curves for individual samples, mean grain size, coefficient of sorting, roundness, and sphericity. The examination of 25 additional thin sections provided data for a lithofacies map. In the Mooers area, the Potsdam can be distinguished from the sandstone in the overlying Theresa dolomite, which, unlike the Potsdam, contains no zircon, very little feldspar, and very well rounded quartz grains. Mean grain roundness in the Potsdam ranges from 0.4 to 0.5; sphericity ranges from 0.73 to 0.79. The relation of roundness to sphericity is similar to that reported in other published studies. The median size decreases and sphericity increases to the N., away from outcropping Precambrian rocks. Sorting is best and median grain size smallest where the beds dip least, suggesting that the dip is probably primary. Sedimentary analysis by thin-section techniques gives abundant, fairly reliable quantitative data for sedimentary studies.