The state of Western Australia covers about one-third of the Australian continent, that is, almost a million square miles. As far as known not less than one-third of this area is underlain by sedimentary rocks, not including the pre-Cambrian. All the major geological systems are represented, with the exception of the Silurian, but the occurrence of Triassic is doubtful. The stratigraphical sequences and the distribution of sedimentary rocks are described according to periods, beginning with the Cambrian and proceeding to the Pleistocene. Eventually, the sedimentary areas are classified according to their significance and grouped in the following categories: (1) scattered outcrop areas, (2) large, comparatively thin sheets, (3) coastal basins, (4) major basins. Thicknesses in (1) and (2) do not exceed 2,000 feet. Basins of group (3) are little known, except for the southwest coastal basin with sediments probably 6,000-7,000 feet thick. There are three major basins of type (4), with thicknesses exceeding 10,000 feet, but one of them has only been recently discovered and is as yet almost unknown. The other two are: the Desert basin, with Devonian to Jurassic stratigraphy and thicknesses probably up to 14,000 feet, and the North-West basin, with Permian to Tertiary strata, with a known maximum thickness of more than 14,000 feet. These two basins are geosynclinal in type. They are idiogeosynclines in Umbgrove's, or paralic basins in Tercier's, terminology. The economic significance of these facts is briefly discussed. In both basins there is an abundance of po ential source, reservoir, and cover rocks.