Summary: The preparation of thin rock-sections using a modified high-speed surface grinding machine is described. The machine is fitted with a new type of jig designed to hold ¼ in. thick glass slides to which the rock slices are mounted for grinding. The machine operates automatically once a cut is set and the large size of the jig enables fifteen 3 in. × 1 in. sections to be produced at one grinding operation without the constant attention of an operator.