Page:Cyclopedia of Western Australia, volume 1.pdf/46

ships- and mode of occurrence, fall naturally into two main classes:—(a) Ores associated with the crystalline schists and other allied rocks and (b) superficial deposits of limonite (laterite), which occupy extensive areas in many and widely-separated portions of the State; and the soft porous deposits of bog ore of comparatively recent origin. The deposits associated with the crystalline schists, which are by far the most important, appear to be very largely developed in the Murchison district. The most important deposits are those at the Wilgie Mia (Weld Range), Mounts Hale, Taylor, and Matthews, and Gabanintha. In addition to these places, however, iron-bearing schists are found almost all over the Murchison goldfield and at numerous other centres in Kimberley, Pilbara, Ashburton, Gascoyne, Peakhill, East Murchison, Mount Margaret, North Coolgardie, Yilgarn, and Phillips River fields, as well as from other localities outside the limits of any legally-defined mining field. The deposits of this type consist of highly-inclined beds, bands and lenses of almost pure hematite (and magnetite), or admixtures in all proportions of hematite and quartz, usually laminated, some of which seem to be the result of the chemical alteration of highly-foliated and crushed belts of greenstone. The siliceous bands very often attain a width of as much as 10 chains, and can often be followed across country for a great many miles.

The superficial iron deposits comprise the laterite ores and the bog iron ores.

The laterite ores, together with the gravel resulting from their denudation, are the most widely-distributed ores in the State, but unfortunately they vary greatly in their composition, ranging from a ferruginous bauxite to an almost pure limonite or turgite. The ores are developed mostly on the tops of hills or ranges; in depth they pass gradually without any distinct line of demarcation into the underlying rock, and nowhere do they attain any great thickness. The composition of the ores naturally varies within very wide limits, and is very largely determined by the nature of the underlying rocks. When these belong to the basic series (amphibolites, dolerites, etc.) the overlying ores are relatively rich in iron; but when the underlying rocks are of the acidic series (granite, etc.) the ores are poorer in iron and proportionately richer in alumina.

The ores of this class have been used principally for fluxing purposes, for which 57,270 tons valued at £36,148 have been raised.

The bog iron ores consist of soft porous deposits of hydrated oxide of iron of comparatively recent formation, and are known to occur at one or two places along the southern and western coastline. So far no deposits of this class have been worked in any way.

Coal—The Collie coalfield is the only one upon which any active operations are being carried on, though there are other districts in the State in which lignites and brown coals occur. The Collie coalfield lies to the east of Bunbury and south of Perth, near the north-western edge of the tableland which succeeds the Coastal Plain. The area occupied by the Collie River coal measures is approximately 50 square miles. The beds consist of alternations of shales, sandstones, and grits of Permo-Carboniferous age, which rest directly upon granite, schists, and other crystalline rocks. The boundary of the field is, with one exception, everywhere defined by faults. On the south-western side of the field the boundary fault has been estimated to have a downthrow to the north-east of at least 2,000 ft. There are several coal seams on the field; these are of variable thickness and consist, in descending order, of:–

In addition to these beds a further series of seams in the north-east corner of the field has been discovered, but which cannot at present be correlated with any of those referred to. The seams from this new locality consist, in descending order, of (omitting all those under 3 ft. in thickness):—Coal, 3 ft.; coal, 4 ft.; coal, 6 ft.; coal, 6 ft. 4 in.; coal, 5 ft. 4 in.; coal, 4 ft.; and coal, 5 ft.