No important changes in the volume of mudrocks occur across the mudstone-to-slate transition in the Martinsburg Formation at Lehigh Gap, Pennsylvania. Mass-balance calculations based on chemical analyses and specific-gravity measurements of 48 mudstones and 26 graywackes from the Martinsburg Formation across this 130-m strain gradient show constant ratios of Al2O3, TiO2, FeOT, MgO, MnO, Y, V, and Zr. This reflects the relatively low differential mobility of these components during diagenesis and slaty cleavage development, as well as the uniformity of compositions of the mudstones and graywackes at deposition. Using minimum-strain samples and Al2O3 as references, the calculations show no loss in SiO2, but large losses of CaO, Na2O, Ba, and Sr from the outcrop that are directly proportional to strain, and reflect an open system on the scale of kilometers. Analyses of graywacke-mudstone metagraywacke-slate) pairs show that losses in SiO2, Na2O, and volume, and gains in K2O and Ba in the graywacke/metagraywacke are balanced by opposite changes in the adjacent mudstone/slate. This documents a local mobility of SiO2 on the scale of centimeters. The inertness of most major components over the 100-m outcrop (especially SiO2), however, requires (1) that the composition of the fluid passing through the rocks was closely buffered by the quartz-albite-muscovite-chlorite assemblage, (2) that the volume of the fluid was small, and (3) that the P-T gradient down which it traveled was gentle. The changes in chemical composition identified can be explained by the inferred replacement reaction plag + 1M musc + K+ = 2M musc + qz + Ca+2 + Na+ + Sr+2 + Ba+2 The strong correlation of minor-element chemistry and strain indicates that this reaction was strain induced and that changes in bulk chemical composition are defined by the progress of this reaction.