Examination of a contact between Late Devonian intrusive acid igneous rocks and Siluro-Devonian Mathinna hornfels. and the unconformity between these rocks and the overlying Permian beds, has revealed the relative ages of the joints developed. The steep joints of the acid igneous rocks are demonstrably of pre-Permian age although they form a pattern identical to those of the Permian beds, and each joint set is correlated with approximately parallel trending sinuous normal faults. The repeated development of the steep joints is attributed to reactivation of a fundamental mechanism. Some of the joints in the igneous rocks occur between compositional layers which have inherited their shape from the form of the joint-governed contact with the hornfels.