This type of brecciation occurs when hydrothermal fluids within a fracture exert terrific pressure on the walls of the fracture, which pushes the walls apart If the contained fluid finds an escape route there is an intantaneous release of pressure on the fracture walls, which shatter and implode fragments of wall rock into the fluid. The fluid eventually cools and solidifies, creating examples such as this specimen. The shattered fragments don't move a great distance as can be observed in the inset picture. This specimen was collected from Dylife mine in Powys.