The belt of ancient crystalline rocks bordering along the east side of the Appalachian system south of the glacial limit affords abundant opportunity for the study of rock disintegration and decay as manifested through the somewhat complex processes commonly grouped under the term “ weathering.” The small area comprised within the northwestern portion of the District of Columbia is particularly favorable to the observation and study of the chemical and physical processes involved. This is due, first, to the fact that in numerous instances one is enabled to study all phases of the transition from sound, fresh rock to arable soil in a single outcrop, where all danger of admixture of foreign material is reduced to a minimum, and, second, to the equally interesting if not important fact that the time-limit of such disintegration can be drawn with a considerable degree of accuracy. The investigations here detailed were undertaken . . .