The content of Na 2 O increases and that of K 2 O decreases irregularly in the Paleozoic clayey sedimentary rocks of central Bohemia (Czechoslovakia) ranging from Cambrian to Devonian in age. This variation is a function of the presence of porphyritic effusive volcanic rocks. With distance from the volcanic rock masses, a progressive increase of K 2 O and a progressive decrease of Na 2 O are exhibited, both laterally and vertically. In the carbonate sedimentary rocks, the CaO/MgO ratio increases, and the MgO/FeO ratio and absolute FeO amount decrease. These variations are interpreted as reflecting the increasing salinity of the depositional environment and the greater activity of organisms in the carbonate sedimentation.