The Mälar Sandstone in the Granholmen-—Vargholmen area is an arkose with 20–25 % feldspars and minor admixtures of sericite matrix, rock fragments, and accessory minerals. The petrology suggests that Jotnian igneous activity was an agent in sandstone cementation. Solutions emanating from dolerites supplied mainly iron, calcium, and silica, resulting in the formation of chlorite, calcite, epidote, laumontite, prehnite, and cryptocrystalline quartz cements in the sandstone dikes in dolerite and in the sandstones vicinal to basic igneous bodies. The relations between dolerite and sandstone on Granholmen and Vargholmen islands show that sandstone-filled cracks in the dolerite were produced by >intrusions> of detritus mush into contraction and dislocation fissures and not by normal sand sedimentation. Previous to the dolerite intrusion the sandstone had suffered some compaction, but little silica cementation. Evidence is not sufficient to decide whether the dolerite is a sill intruded between any particular lithological units of the sandstone series.