The mineralogical composition of marly rocks from deep marine Lower Cretaceous clastic formations of the Gerecse Mts., Hungary, was determined by X-ray diffraction. The most abundant clay mineral is a corrensite-like regular inter-stratification of chlorite, vermiculite and smectite layers, d(001) of untreated material is about 29 Å. This phase and hematite are interpreted as alteration products of basic volcanic rocks. The majority of the relatively abundant plagioclases can also be volcanogenic. Illite, illite/smectite, kaolinite and probably a part of chlorite can be of terrigeneous clastic origin. Layer pairs of macroscopically different appearance and calcite contents do not differ in their non-carbonatic mineralogical composition. There are slight variations in the type of the 29 Å regularly interstratified clay minerals. Mineralogical variations of the intercalated sandstone layers are not reflected in the composition of the fine-grained material but the overall strong volcanogenic character of the pelitic sediments is similar to the coarse-grained ones. Regularly mixed-layer corrensite-like phases indicate medium strong diagenetic transformation of basic igneous source material corresponding to a burial temperature of slightly above 100 °C. Analogous formations mainly from the Alpine-Carpathian-Dinaric area are discussed.