Castell mine, west Wales. This fine example of hydraulic brecciation is made up of host rock, quartz, and ferroan dolomite. The brecciation is the result of inwardly exploding walls in deep fractures. A sudden decrease of pressure causes shards of wall rock to be injected into the hot mineral rich fluids which are circulating through the fracture. Larger pieces of the ferroan dolomite can easily be found on the spoil heaps at this long abandoned mine. Along with quartz, the ferroan dolomite would be considered as one of the gangue minerals at the mine.