Fig. 8. In any one dyke or sheet swarm, most of the dykes/sheets do not reach the surface, but rather become arrested, commonly at layer contacts, at depth in the crust. Arrest is encouraged if there is a large elastic mismatch at the contact, that is, the adjacent layers have very different mechanical properties. Here the arrest can be attributed to rotation of the maximum principal compressive stress σ1 at the upper contact so as to become unfavourable to dyke propagation (but favourable to sill injected provided the magmatic overpressure is high enough to propagate the sill along the contact).