Sentence:  The gold, both in the quartz and <WSD>mullock</WSD>, is very fine, and, owing to the soft and ferruginous nature of the stuff, specks can but very rarely be seen during working. 
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.