Sandstone and conglomerate cemented by zeolite crystals in a humid mountainous area in Japan are weathered, and also weakened or strengthened, by chemical interaction with percolating water that was originally in equilibrium with the atmosphere. Near the ground surface, carbonic acid generated by the dissolution of carbon dioxide from the atmosphere and biological activity in the soil dissolves zeolite, and thereby weakens the rocks. At the oxidation front, the base of the oxidized zone, oxygen from the atmosphere reacts with pyrite in the rock to form sulfuric acid and iron oxide and/or hydroxide. The iron oxide and/or hydroxide precipitates on zeolite surfaces, and strengthens the rocks by cementing the grains to each other. Beneath the oxidation front and extending to the dissolution front, sulfuric acid migrates downward, dissolves zeolite, and thus weakens the rocks. This weathering mechanism and the resulting changes in mechanical properties of rocks generally are to be expected in marine sandstone and conglomerate with small amounts of clay in the matrix because the most common cementing mineral is carbonate which is also easily dissolved by acid, and further because pyrite is a common rock-forming mineral in marine sedimentary rocks. The effects of chemical weathering on the mechanical properties of sandstone and conglomerate are different from those of mudstone, because the latter contains much larger amounts of clay minerals. The cementation of constituent grains by iron oxide and/or hydroxide is inhibited by clay minerals, because clay minerals have high specific surface areas, and also because large amounts of iron can be incorporated in the minerals.