THE Westonia gold mining area lies on the western side of the Yilgarn Goldfield and six miles north of Carrabin, a station on the Perth-Kalgoorlie railway. The principal mines are situated in a narrow belt of gneiss. This belt is about 500 ft. wide at its maximum, and appears to taper at each end, though this is not actually proved. It underlays north at an average angle of about 45, and strikes nearly east and west. This gneissic belt is bounded on both sides by greenstone rocks, which appear to lie conformably with the gneiss. Both gneiss and greenstones are intruded with numerous granite dykes derived from the granite masses which form a considerable portion of the adjacent country. In the gneiss, basic dykes of amphibolite also occur.