ODP Leg 157 drilled into the volcanic apron of Gran Canaria. Low core recovery in depth intervals with high amount of volcanic sediments was compensated by interpretation of continuous downhole measurements. Alongside a reconstruction of the lithology the aim of this investigation is a mass balance of volcanic sediments in the drill holes 953C, 955A und 956B. Three main electrofacies have been distinguished: felsic volcaniclastics, basaltic volcaniclastics, and pelagic sediments. Highly differentiated tephra layers are characterized in the logs by high K and Th concentrations, whereas basaltic volcaniclastics show the highest values for FeO und TiO2. In addition to the logs magnetic susceptibility measurements performed on cores give evidence for enrichment of mafic volcanic clasts in the sediment. High values correlate with the presence of basaltic fragments. Continuous lithologic profiles, so-called electrofacies-logs (EFA-Log), have been established for all three holes. These profiles, and the variation in susceptibility with depth, reflect different depositional condition north and south of Gran Canaria. Thickness estimation of the volcaniclastics calculated from logs and the magnetic susceptibility have been compared to computations done with other methods. Previous investigations include e.g. detailed lithologic core descriptions and high resolution FMS images, estimated results are generally lower than that achieved by the EFA-Log method. This difference is mainly controlled by the core recovery.