The Mount Samson gabbro-granite complex, northwest of Brisbane, has intruded a low-grade metamorphosed argillite causing the formation of a contact olivine-bearing hornfels. The presence of a transition from hornfels to argillite and the absence of a transition from gabbro to hornfels substantiate the sedimentary origin of the hornfels. The composition of the argillite is rather unusual, with a high Mg and Fe content, almost equivalent to that of the hornfels, so that a minimum of supplementary metasomatic activity would have been necessary to produce the hornfels. It is concluded that basic hornfelses must have a parent rock of suitable basic composition. Chemical analyses and petrographic descriptions are included.