The sandstones of the Charny Formation (Cambrian) near Quebec City contain about 15% detrital feldspar which is practically all albite. Other constituents are quartz grains (50%), fine-grained chloritic matrix (20%) and patches of calcite. Most of the feldspar is untwinned and contains numerous "dust" inclusions. Many grains show a lamellar microstructure of sub-parallel inclusion-rich and -poor lamellae which make an angle of 75 degrees with 001. The only known microstructure in feldspar with this orientation is film perthite. The sandstones contain 1.0 to 3.5% K 2 O and the potash/soda ratio is close to unity; thus, they are somewhat enriched in soda compared with the average sandstone. It seems improbable that sandstones so rich in potash could have been derived from a source in which potash feldspar was almost absent and in which untwinned albite was abundant. It is concluded that most of the untwinned feldspar grains were originally potash feldspar or perthite which has been altered to albite during diagenesis by the action of sodium-rich pore waters (connate sea water).