A study of the nature and origin of the receptacles for valuable mineral. deposits is not only of interest, but of the highest importance to the miner, who thus gains an idea of how to work any given deposit to the best advantage, and; to a certain extent, enables him to form an opinion as to the probable size of the deposit in question, with the view of ascertaining the amount of capital advisable to spend in opening it out.Although the filling of receptacles is another subject to that of their formation, still an ore cannot be deposited unless a place is found for it ; at the same time we must bear in mind that the nature of a receptacle does.not govern the richness of a deposit if the mineral is not available for deposition ; but should sufficient valuable mineral matter be present, then the size and shape of the receptacle may greatly affect the result. By speaking of receptacles, we do not wish to restrict ourselves to the filling of open spaces only, but also desire to include replacement deposits, where certain ores, e.g. iron oxides, take the place of limestone.Receptacles may be conveniently classed, according to their origin, into (A) Chemical ; (B) Mechanical.