Sentence:  On certain days, when hot currents shimmered off Oyster's Reef, we would detect the chalk-dust of the <WSD>mullock</WSD> heaps, acrid; or, from the opal mines themselves, the ghastly fug of the tunnels and shafts. 
Meaning of the word marked with <WSD> tags:  Waste rock from which the wanted gold, minerals, opal, etc., has been extracted; waste material generated while searching for minerals or while mining, such as when sinking a shaft.