Arrays of mineral-filled, echelon, sigmoidal fractures in cobbles of sandstone from the Crackington Formation of north Cornwall were examined by cutting serial sections. In three dimensions each array consists of a set of blade-like, mineral-filled fractures. These blades project as a fringe from the single but composite fracture that forms through their linkage. Linkage takes place as the strips of rock (bridges) between blades fail through cross-fracture. Cross-fractures give rise to the steps that mark the margins of the composite fractures so formed. Bridges show little penetrative deformation. Arrays are not zones of high ductile strain. Displacement is a consequence of dilatation. Echelon fractures were formed and opened by intruding hydrothermal fluid.